Effect of Cyclone Assisted Milling on Legume Flour

Effect of Cyclone Assisted Milling on Legume Flour Characteristics and

Functionality in Selected Food Products

by

Mark Jarrard Jr.

(Under the direction of Yen-Con Hung)

Abstract

Legumes (cowpea and soybean) were milled using a cyclone-assisted attrition mill to

produce fine legume flour. Milling variations consisted of alterations in mill design

and operating parameters in addition to preconditioning treatments of the seed.

Legume flour from cowpea seed was used to prepare moin-moin (steamed cowpea

paste) with variations in particle size, water, solids concentration, and cooking time.

Legume flour from soybean was incorporated into an “instant beverage mix” with

other common ingredients found in soy milk. Milling parameters were found to have

a greater effect on legume flour particle size and yield, whereas seed conditioning

showed little to no affect. Moin-moin prepared from attrition milled flour was found to

be denser than a traditional product; additionally it was found that longer cooking

times of attrition milled cowpea flour produced sticky products. Soy mix prepared

from attrition milled flour was shown to have slightly greater suspension stability than

non-attrition milled soy flour; however, the presence of adjunct ingredients did not

allow for a clear delineation.

INDEX WORDS: Cyclone, attrition mill, cowpea, soybean, cowpea flour, soy flour,

moin-moin, soy beverage, soy milk



by

Mark Jarrard Jr.

B.S., University of Tennessee, Knoxville, TN 2004

Thesis Submitted to the Graduate Faculty of the University of Georgia in Partial

Fulfillment of the Requirements for the Degree

Master of Science

Athens, Georgia

2006

© 2006

Mark Jarrard Jr.

All Rights Reserved



by

Mark Jarrard Jr.

Major Professor: Yen-Con Hung Committee: Robert D. Phillips Manjeet S. Chinnan Susan K. McWatters Electronic Version Approved: Maureen Graso Dean of the Graduate School The University of Georgia August 2006

v

Dedication

This thesis is dedicated to my parents, Mark and Christine Jarrard, who have

always supported me in any decision that I have made. Secondly, to my close

friends who have always been my sanity that has allowed me to keep the course.

vi

Acknowledgments

I would like to thank Dr. Hung for always being the ever patient mentor with

me. During my time with Dr. Hung, he has always shown me the guidance

necessary to allow me to complete my tasks while leaving enough unanswered

questions for me to figure things out on my own. Thank you for all that you have

shown me and for playing “Devil’s Advocate” when necessary.

Thank you to Ms. K. McWatters for allowing me the extended privilege to

formulate various peanut products in your kitchen in addition to baking cowpea

crackers which I have grown to love. Dr. Phillips, your personal blueberry hobby was

quite a treat from time to time, yet those times always seem to find me at the end of

a long day. Dr. Chinnan, thank you for allowing me to understand one of the more

crucial aspects of my research and without this fundamental understanding I would

not have completed my research in the time frame that I had planned.

I would like to give special thanks to Mr. Glen Farrell and Ms. Sandra Walker

who assisted me when needed and tolerated me when I had moments of free time.

This study was supported by the Bean/Cowpea Collaborative Research

Support Program (Grant no. DAN-1310-G-SS-6008-00), U.S. Agency for

International Development, and by state and Hatch funds allocated to the University

of Georgia Agricultural Experiment Station-Griffin Campus.

vi

Table of Contents

Page

ACKNOWLEDGMENTS .................................................................................................. vi

LIST OF TABLES ............................................................................................................ ix

LIST OF FIGURES........................................................................................................... xi

CHAPTER

1 INTRODUCTION .......................................................................................1

2 LITERATURE REVIEW .............................................................................4

Soybean..................................................................................................5

Soy Products ..........................................................................................7

Soymilk ...................................................................................................8

Cowpea.................................................................................................11

Akara ....................................................................................................12

Moin-moin .............................................................................................13

Milling....................................................................................................16

Dry Milling Equipment ...........................................................................18

Cyclone Air Classifiers ..........................................................................20

Particle Size Measurement ...................................................................24

Particle Size Analysis ...........................................................................29

Particle Size in Food Systems ..............................................................30

References ...........................................................................................34

3 MILLING OF COWPEA FLOUR USING CYCLONE ASSISTED

ATTRITION MILLING ..............................................................................46

Abstract.................................................................................................47

vii

Introduction ...........................................................................................48

Materials and Methods .........................................................................51

Results and Discussion.........................................................................57

Conclusions ..........................................................................................62

Acknowledgements...............................................................................63

References ...........................................................................................64

4 APPLICATION OF ATTRITION MILLED SOY FLOUR IN A

BEVERAGE APPLICATION ....................................................................75

Abstract.................................................................................................76

Introduction ...........................................................................................77

Materials and Methods .........................................................................79

Results and Discussion.........................................................................86

Conclusions ..........................................................................................90

Outlook .................................................................................................91

Acknowledgements...............................................................................91

References ...........................................................................................92

5 EFFECT OF MILLING METHOD N THE PHYSICAL PROPERTIES

OF COWPEA FLOUR AS AN INGREDIENT IN MOIN-MOIN

(STEAMED PASTE) ................................................................................98

Abstract.................................................................................................99

Introduction .........................................................................................100

Materials and Methods .......................................................................102

Results and Discussion.......................................................................106

Conclusions ........................................................................................113

Outlook ...............................................................................................114

Acknowledgments...............................................................................114

viii

References .........................................................................................115

6 SUMMARY AND CONCLUSIONS ........................................................124

ix

List of Tables

Page

Table 2.1: Names, general descriptions, and utilization of traditional non-fermented

oriental foods .................................................................................................39

Table 2.2: Names, general descriptions, and utilization of fermented oriental foods ......40

Table 2.3: Nutrient content of soymilk, cow’s milk, and human breast milk ....................41

Table 2.4: Particle size classification by number and mass ............................................42

Table 3.1: Particle diameters at selected percentiles of “wet” and “dry” cowpea flour ....67

Table 3.2: Coefficient of variance of wet cowpea flour particle size measurements a

specific percentiles ........................................................................................70

Table 3.3: Averaged data of factorial design for cyclone assisted attrition milling of

cowpea flour as affected by milling parameters.............................................71

Table 3.4: Effect of milling parameters on cowpea flour .................................................72

Table 3.5: Averaged data of factorial design for cyclone assisted attrition milling of

cowpea flour as affected by seed conditioning..............................................73

Table 3.6: Effect of seed conditioning on cowpea flour...................................................74

Table 4.1: Trypsin inhibitor content of thermally treated defatted soy flake ....................94

Table 4.2: Averaged responses of processing regimens on soy flour.............................95

Table 4.3: Effect of suspension temperature and anti-caking level on soy beverage .....96

Table 4.4: Averaged responses for separation rate, viscosity, and water holding

capacity of soy flour .......................................................................................97

Table 5.1: Moin-moin formulations ................................................................................117

Table 5.2: Sensory evaluation descriptors ....................................................................118

x

Table 5.3: Calculated and measured proximate composition of moin-moin and akara.119

Table 5.4: Texture profile analysis of moin-moin...........................................................120

Table 5.5: Averaged texture profile analysis of moin-moin ...........................................121

Table 5.6: Objective measurements of moin-moin........................................................122

Table 5.7: Sensory ratings for moin-moin .....................................................................123

xi

Figure List

Page

Figure 2.1: Example of a cumulative percentile particle size distribution by mass..........43

Figure 2.2: Particle size distribution on a non-logarithmic scale .....................................44

Figure 2.3: Logarithmic probability graph ........................................................................45

Figure 3.1: Original and modified configuration of a Super Wing Mill DM-200................66

Figure 3.2: Cluster plot of “wet” vs. “dry” cowpea flour particle size distribution

measurements to demonstrate a relationship................................................68

Figure 3.3: Cumulative undersize distribution by mass of cowpea flour (wet and dry) ...69

1

Chapter 1

INTRODUCTION

2

Cowpea (Vigna unguiculata ) and soybean (Glycine max) are both highly

nutritious food sources that provide the staple parts of diets for many nations across

the world, mainly in locations that cannot find sufficient sources of animal protein.

Traditional processes that are used to prepare popular consumer products from

either of these raw legumes often require relatively large amounts of consumable

resources (e.g. water) and energy that may not be readily available to many of these

populations. Some of the processing technologies that are used to produce

consumer products from these legumes do so at the cost of sacrificing the nutritional

quality of the products, mainly through the loss of water soluble compounds.

Development of convenient starting materials (cowpea or soy flour) can significantly

reduce consumable resource dependency while reducing the overall energy needed

to produce many traditional products.

Moin-moin is a popular Nigerian dish prepared from a steamed, un-aerated

cowpea paste. Generally this product is mixed with oil, salt, onions, peppers, and

other seasoning agents before it is steamed in either a metal/glass mold or banana

leaves. The product is served either as a cool gel or as a warm pudding with either a

grain or cereal. Soymilk is traditionally the filtered, aqueous extract of soaked and

ground soybean. The okara that is filtered from this process contains relatively large

amounts of fiber and some proteins. Instant beverages created from nutritious food

sources (e.g. powdered milk) provide a nutrient dense beverage to populations that

are located in areas that cannot support refrigeration or ultra high temperature

processing.

Particle size of food ingredients has been shown to play significant roles in

ingredient functionality as well as in the sensory perception of food. Generally

speaking, these differences are noticed to a greater extent as the particle size of the

3

food ingredient approaches the micron and submicron levels whereas at larger sizes

the differences in functional properties tend to be comparatively minimal. Legume

proximate composition in conjunction with current milling technologies creates a

dynamic difficulty for producing legume flours with extremely small particle sizes.

However, modifications in legume crop or milling technologies can be implemented

to efficiently achieve these fine particle sizes.

The main objective of this study was to produce legume flour using a cyclone

assisted attrition mill with a final geometric particle mean diameter less than 20 μm.

By creating a relatively small particle size with the intended purpose of a

convenience food product, the functional properties of the legume flour would be

greatly altered and potentially exemplify certain aspects of traditional products

created from the respective legumes.

In the latter two objectives, moin-moin and an “instant beverage mix” were

prepared from either cowpea or soybean, respectively. These products were

compared to traditional products by using objective measurements to identify how a

reduction of particle size would alter the functional properties of the convenience

starting material.

4

Chapter 2

LITERATURE REVIEW

5

Soybean

The soybean (Glycine max) has been cultivated and consumed by the human

species for centuries with the popularity of the crop rapidly increasing in Western

countries in recent years, primarily due to recently identified health benefits. Detailed

information about the soybean, component functionality in food systems, health

benefits, health concerns, as well as soybean uses other than as a food source can

be found in the extensive and detailed literature presented by Liu (1997) and

Friedman and Brandon (2001). The general proximate composition of soybean (wet

basis) under general storage conditions is 13% water, 35% protein, 17% oil, 4.4%

ash, and 31% carbohydrates.

Soy proteins are perhaps the most valuable component of the soybean and

have received much attention in recent decades; however, soy proteins are mainly

utilized for animal feed with the refined soy oil dominantly consumed by humans (Liu

1997). Protein composition is mainly comprised (80%) of conglycinin (11S globulin)

and β-conglycinin (7S globulins) which are the storage proteins of the soybean.

Through genetic manipulation of soybeans, the content of these two storage proteins

varies greatly in transgenic soybean lines with each protein having different

functional, nutrient, and biological properties (Kinney 2003). Soybeans contain all of

the essential amino acids required for human growth, but are limited in methionine

and tryptophan. Chymotrypsin and trypsin inhibitors along with lectins limit the

consumption of the protein source unless properly heat treated to eliminate these

components. The common processes of necessary thermal treatments to eliminate

these anti-nutritional factors can improve protein quality to a point (Friedman and

Brandon 2001).

6

The carbohydrate fraction (second largest component on a dry weight basis)

is of little economic value to the soybean processing industry other than for caloric

value in ruminant animal feed; additionally the bulk of the carbohydrate fraction is

comprised of fiber. The main reason why the carbohydrate fraction of soybeans

receives attention is for the elimination or degradation of oligosaccharides raffinose

and stachyose. These two saccharides cannot be digested by humans due to the

lack of α-galactosidase, which is the enzyme needed to hydrolyze the α-galactosidic

bond of the oligosaccharides structure. The undigested form of the carbohydrate

proceeds to the lower intestine of the human gastrointestinal tract where microflora

that do contain the enzyme digest the oligosaccharides producing byproducts of

carbon dioxide, hydrogen, nitrogen, and methane resulting in flatulence and other

undesirable side effects to the host (Cristofaro and others 1974; Liener 1994).

Genotype variations of soybean differ greatly in fatty acid composition.

However, nearly 75% of the lipid content of most soybean varieties is comprised of

linolenic and oleic fatty acids making soybean oil ideal for the food processing

industry (Liu 1997; McGee 2004). Oil extraction from soybeans is primarily done

using solvent-based extraction yielding crude soybean oil. Crude soy oil is then

further refined to remove impurities such as lecithin, free fatty acids, and pigments.

After the impurities are removed, the oil is then subjected to additional processing

depending on the final product usage (Liu 1997).

In summary, the soybean has many limiting factors as a food source for

humans: oligosaccharides and protein-related limitations. Despite these limitations,

the soybean is a vital food source to many populations throughout the world as it

does provide key nutrients that would otherwise be unattainable (mainly as a protein

source). Continued soybean research in the industrialized nations of the world

contributes to the increased demand for soy-based foods as well as to the nutritional

7

improvements of the populations that are dependent on the soybean as a staple of

the diet.

Soy Products

The soybean is presently utilized on every continent of the world with China

leading the world production for human consumption of nearly 5 million metric tons of

soybean grown on an annual basis (FAOSTAT 2005). Soybeans are a nutritious

food source that provides vital nutrients to populations that would be pressed to find

other sources of such nutrients, primarily proteins. Development and manipulation of

Glycine max cultivars can produce significant variations in the nutrient composition of

the bean; however, the greatest variation comes from the processing of the raw bean

into a consumable human food source. In addition to the macronutrients provided by

the soybean, emerging research on isoflavone content and benefits provided to

humans has lead to the significant increase in consumer demand for soy based

products in many developed countries within the past decade (Liu 1997).

Tables 2.1 and 2.2 summarize traditional non-fermented and fermented soy

foods, their native oriental names, generalized production methods, and common

uses as a food source (Liu 1997). Development of soy processing technologies for

the transformation of the raw soybean to a usable consumer food source has lead to

the development of new soy food items; however, these new items are often variants

of the production methods used for the traditional foods. Many of the production

methodologies are designed to optimize protein content while reducing or eliminating

anti-nutritional factors associated with soybean.

8

Soymilk Soymilk is a thermally processed beverage that is produced from either the

aqueous extract of soybean slurry or a fine emulsion of soy flour. A crude version of

soymilk was first thought to have been made in China around the second century

B.C. and since then has developed to a wide variety of industrialized processes

across the world (Liu 1997). Soymilk has a nutrient profile (table 2.3) that is

analogous to cow’s milk with specific benefits of no lactose or cholesterol, greater

content of unsaturated fatty acids and proteins, and able to be produced in areas

where cow’s milk cannot be produced due to economic or geographic constraints.

Advances in soymilk processing technology, transgenic lines of soybean, and

consumer awareness have lead to the recent popularity and demand for soymilk and

product derivatives (e.g. tofu) throughout the world. Major advances in soymilk

include the identification of beany flavor compounds and anti-nutritional factors along

with the associated processing and genetic technologies to yield a nutritionally and

organoleptically acceptable product for consumers.

Beany Flavors The characteristic beany flavor associated with soymilk is caused by the

activation of lipoxygenases (LOX) during the grinding phases of soymilk production.

These off-flavors are commonly caused by n-hexanal and n-hexanol with the latter

considered to be one of the major factors in off-flavors (Mizutani and Hashimoto

2004). In order for enzymatic action to occur, two conditions must be satisfied 1) the

enzyme and substrate must be released from the cotyledon (i.e. the cotyledon must

be damaged), and 2) there must be excess moisture content in the system ( > 13%)

(Liu 1997). Presently, there are a handful of transgenic varieties of soybeans lacking

9

some of the LOX, but most still contain one or two LOX complexes. The American

Soy and Tofu Corporation (Macon, GA) has recently marketed a specific variety of

soybean (L-Star®) that lacks all three LOX complexes in addition to having

exceptionally high levels of tocopherol. Several products have been created from

the defatted soy products of the L-Star® soybean variety and are presently being

tested on American consumers for acceptability.

Processing temperatures have been shown to play a vital role in both the

formation as well as the retention of the volatiles generated through the lipid

oxidation process. Research shows that grinding soybeans at low and high

temperatures (3o and 80oC, respectively) can control the off-flavor content during the

soymilk manufacturing process (Mizutani and Hashimoto 2004). Mizutani and

Hashimoto (2004) suggest that grinding soybeans at lower temperatures (i.e. 3oC) to

be used for soymilk manufacturing requires less energy, causes less protein

degradation, provides a beneficial system for further processed soy products (e.g.

tofu), and an overall reduction in oxidized byproducts.

Anti-nutritional factors Trypsin and chymotrypsin inhibitors (TI), Kunitz (KST) and Bowman-Birk

(BBI), are the primary anti-nutritional factors of concern in soybean culitvars. Other

anti-nutritional factors, such as lectins, are present in soybean; however, criteria for

thermal treatments to sufficiently reduce KST and BBI inhibitory activity (TIA < 20%

activity) exceed the criteria for reduction of other anti-nutritional factors. Extensive

empirical research has been conducted on the thermal inactivation kinetics of KST

and BBI in soymilk and defatted soy flour, as well as analytical enzymatic studies to

identify how these TI are eliminated from the soy system. It is generally agreed that

the degradation of TI exhibits two separate first-order reactions with different reaction

constants for each TI over various ranges of moisture and temperature treatment

10

regimens (Kwok and others 1993; Rouhana and others 1996; Liu 1997; Kwok and

Liang 1998; van den Hout and others 1999; Kwok and others 2002). In addition to

reducing TIA to safe levels for human consumption, the necessary thermal treatment

processes parallel improvement of soy protein quality until a TIA value of 90% or

greater is achieved at which point the protein quality is adversely affected. Rouhana

and others (1996) identified that a continuous HTST pasteurization process for

soymilk to reduce TIA to 20% of native soybean should consist of a 77s residence

time at 140oC with a thermal conductivity value of 500 ( )KmW

o throughout the

heated sections of the HTST unit. This thermal treatment exceeds processing

criterion for dairy milk to the point of producing a sterile soymilk beverage without

adversely affecting the protein quality of soymilk. Kwok and Liang (1998) determined

that 30-40% of cystine is contained within the TI enzymes, thus explaining why the

inhibition levels to 10-20% activity improve protein quality as they liberate the

disulfide linkages of the TI enzymes making them available to the host for nutrient

adsorption. Kwok and Liang (1998) also identified that a holding time and

temperature relationship of 143 and 154oC for 62 and 29s, respectively, will reduce

TIA to 10% of native soybean activity.

Liu (1997) reviewed methods of using defatted soy flour to produce soymilk

with preferable flavor when compared to soymilk produced from fresh soybean. The

primary reason for consumer preference is due to the low levels of fatty acids in the

soy product used to create the soymilk. The primary problem with this method is

processing the defatted soy flake/flour so that the TIA is 10-20% of native soybean in

the final soymilk product. Inhibition of KST and BBI must occur in the presence of

moisture, otherwise sufficient dry thermal treatments would completely denature soy

protein making proteins unavailable for nutrient adsorption (DiPietro and Liener

11

1989; Liu 1997). Van den Hout and others (1999) examined the enzyme kinetics of

KST and BBI in defatted soy flour at various moisture contents. It was concluded

that from a kinetics standpoint, 30% moisture content of the defatted soy product

was sufficient to reduce TIA to an acceptable level and that moisture contents

greater than 30% showed negligible benefits. Based on the kinetics model for two TI

groups with different reaction rates (eqn 1) a 5 min processing time at 110oC with a

30% moisture content would be needed to reduce TIA to 10% of native soybean (van

den Hout and others 1999).

( ) tktkt eAAeCC

21 10

−− −+= (1)

Where Cx is the measured trypsin inhibitor activity (mg (gds)-1), A is the activity

fraction of one of the two TI groups at t = 0, k is the reaction rate constant ((mg (gds)-

1)1-n s-1), and t is time in seconds.

Cowpea (Black-eyed pea, Crowder pea, Southern pea)

Cowpea (Vigna unguiculata L. Walp.) has been extensively reviewed and

reported upon for nearly three decades (Phillips and others 2003; Plahar 2004). The

cowpea is a grain legume that is nutrient dense, stress tolerant, and an adaptable

crop that is grown on every continent of the world with Africa leading in production of

nearly 35 million metric tons per year (Phillips and others 2003; FAOSTAT 2005).

Research groups such as The International Institute of Tropical Agriculture and

collaborative research support programs between host universities in Africa and The

United States of America have lead to significant advances in breeding lines,

growing methodology, improvement of nutrient composition, and improvements in

consumer products made from cowpea (Singh and Rachie 1985; Ehlers and Hall

1997). The research group at The University of Georgia-Griffin Campus (Griffin, GA)

12

has specifically focused upon the improvement of cowpea starting materials used by

West African populations for preparation of staple food items with the intent of

reducing energy, manual labor, and consumption of resources (i.e. water) needed for

food preparation; in addition, an overall improvement in consumer acceptability of

traditional cowpea-based foods has been identified for several products (e.g. Akara)

(Phillips and others 2003; Singh 2003; Plahar 2004).

Benefits of cowpea when compared to other legumes of similar nutrient

composition include a lower content of anti-nutritional factors (including flatulence-

producing oligosaccharides), similar health benefits with respect to the

cardiovascular system, a valuable source of dietary fiber, and exhibit tolerance to

harsh growing conditions (i.e. low moisture climates) making this an ideal staple food

for developing countries in dry climates (Phillips and others 2003). The dry bean

(common form for West African consumers) contains on average 11% moisture,

23.85% protein, 2% lipids, 3.39% ash, 10.7% dietary fiber, and 48.94%

carbohydrates (HealtheTech 2005). General processing of dry cowpea seed by

traditional means involves the removal of the seed coat which reduces the fiber

content considerably. The removal of the seed coat allows for consumers to produce

more acceptable products from this legume; however, production of cowpea meal

and flours that retain the seed coat during processing yields a higher fiber content for

consumer products (Phillips 1982; Phillips and others 2003).

Akara Akara is popular in many West African countries and is made by whipping

cowpea paste mixed with bell /hot pepper, onion and salt then deep fried in either

peanut or vegetable oil in a ball shape form (Dovlo and others 1976). Sensory

studies conducted by McWatters (1990) and McWatters and others (1997) focused

on U.S. consumer acceptance of Akara and found that there is potential for such

13

West African products. Interestingly, U.S. populations that indicated acceptance of

Akara are also populations that tend to be familiar with indigenous U.S. foods that

have similar textural characteristics to Akara, such as the hushpuppy (fried corn meal

fritter).

Development of a starting material (cowpea meal) was shown to produce

consumer acceptable Akara when compared to a traditionally prepared product;

additionally, use of this starting material eliminates the excessive use of water and

manual labor for preparation of Akara (Singh 2003; Patterson and others 2004;

Plahar 2004; Singh and others 2004). Singh and others (2005) found that the

hydration properties of milled cowpea meal and flours played an important role in the

formation of a paste that has ideal characteristics for akara preparation. It was found

that the foaming capacity is related to the degree of water holding capacity and

swelling capacity of the paste; therefore, these two hydration properties can be used

as a control measure for production of cowpea paste that requires formation of foam

structures for ideal product qualities.

Moin-moin

Moin-moin is a popular Nigerian food product made from cowpea paste; small

amounts of oil, salt, tomato, egg, chili peppers, onion, or cooked meats can be added

after formation and dilution of the paste. The mixture is homogenized with minimal

incorporation of air and then steamed to form a gel or gel-like structure. The cooked

product can be served as a snack or an entrée with rice or cereals (Dovlo and others

1976). Moin-moin originated in West Africa, but it is strongly based on the Nigerian

culture with little documentation on the proper form of the product. Many of the

traditional recipes referenced for moin-moin and summarized by Dovlo and others

(1976) state that the cowpea paste, “…be of good consistency with no air

14

incorporated into the paste.” Traditional cowpea paste prepared for moin-moin is

done using decorticated, soaked cowpea seeds which are then manually pulverized

into a paste using a mortar and pestle or in modern times a food processor. The

removal of the seed coat alters the resulting paste in three ways 1) before genetic

variations in the seed color this eliminated the grey color of the paste, 2) the fibrous

seed coats disrupt the protein and starch matrices, and 3) significantly reduces the

dietary fiber of the resulting product. Subsequent studies on moin-moin, discussed

below, consider moin-moin to be a protein-starch gel that is semi-solid when warmed

and solidifies upon cooling. As such, factors that can affect these types of gels were

varied to study the effects on moin-moin.

Early research into moin-moin formulation optimization indicated that the form

and size of cowpea particles can play a significant role in the final form of the

product. Ngoddy and others (1986) evaluated moin-moin with a 1:3 ratio of cowpea

solids to water using cowpea meal/flour that had geometric particle mean diameters

(dgw) of 69.34, 86.66, 111.48, 128.47, and 184.44 μm. They concluded that an

optimum moin-moin formulation, based on an expert sensory panel, should produce

a product that has optimum surface sogginess, is moderately homogeneous with

respect to surface structure, and has an optimum texture. This sensory data does

little to identify what an ideal moin-moin product is, but the apparent viscosity of the

moin-moin paste for the “ideal” samples was between 110 and 113.3 cP with the

control sample (made from traditionally prepared cowpea paste) having an apparent

viscosity of 119.2 cP, and cowpea flour used to make the “ideal” moin-moin had dgw

ranging from 111 to 129 μm.

Additional ingredients in the moin-moin formulation can have a significant

affect on the organoleptic properties of moin-moin, but it is unclear as to the affects

15

on the texture of the final product. Moin-moin can be comprised of many different

ingredients, but oil, egg, and salt commonly appear in many traditional recipes and

all three can have an effect on the functional properties of cowpea as they relate to a

protein/starch gel. Ossai and others (1987) evaluated the effects of oil, egg, and salt

on moin-moin using dry decorticated cowpea seeds at a 1:5 ratio of cowpea solids to

water in addition to low, medium, and high contents of the evaluated ingredients.

They concluded that the rupture force of the moin-moin sample can vary from 2.8 to

3.5 N with vegetable oil fractions ranging from 5.5 to 8.5% of the total formulation.

Secondly, rupture force of the moin-moin sample can vary from 2.9 to 4.88 N with

egg fractions ranging from 3 to 4.5% of the total formulation. Salt was shown to have

no effect on the rupture force or deformation of rupture in this study. This result

suggests that moin-moin is more like a starch gel due to the salt having no significant

affect on the protein in the cowpea significantly.

A considerable amount of attention towards the modeling and development of

thermal process (canning) to prolong the shelf life of moin-moin has been conducted

since the shelf life of traditional moin-moin is short (24 hr under tropical conditions)

(Okechukwu and others 1991a; 1991b; 1991c; 1992). The thermal processing

research summarized that particle size of cowpea flour did not affect the thermal

heating and cooling curves. However, the rheology studies did show that

gelatinization onset temperature and peak rigidity both increased with decreases in

the water to solids ratio of cowpea slurries (Okechukwu and others 1991b).

Kerr and others (2000; 2001) observed similar physicochemical and

functional properties of cowpea flour with decreasing particle size in both cowpea

pastes and when used in a snack chip formulation. Snack chips prepared from

cowpea flour showed an increase in snapping peak force from 2.68 to 59.2 N/cm3 as

the dgw decreased from 385 to 90 μm. It was also observed that with increasing

16

starch concentrations, a lighter and less brown color was measured in the snack

chip. Lastly, a decrease in particle size produced cowpea flour with higher

measurable starch content and an increase in protein solubility. This was attributed

to the methodology of starch measurement which required that the starch be

extracted using water, thus smaller particle size yielded a greater amount of

extractable starch.

The paradigm for cowpea paste functionality for akara and moin-moin

production thus far is that foaming has the most significant effect on the texture of

the end product. Secondly, most cowpea studies involve removal of the seed coat

prior to milling or pulverization of the dry seed. Kethireddipalli and others (2002a;

2002b) examined the role of cell wall material on cowpea paste functional properties.

Cell wall material (CWM) included in the wet milling of cowpeas produced a highly

functional paste with improved hydration values and protein solubility. These two

improvements gave way to increased foaming capacity allowing for a deep fried

product (akara) to have a lighter and fluffier texture. Production of cowpea meal

through dry milling produced a poor quality end product which was attributed to low

protein solubility and the presence of a high proportion of heavy, gritty tissue

particles. Intense dry milling of dry cowpea for the production of cowpea flour greatly

enhanced protein solubility but adversely affected the hydration properties due to the

complete destruction of the cell wall material.

Milling

The earliest notion of milling dates back to the prehistoric era when grains were

processed by removing the tough protective layers in a crude manner and the inner

portions of the grain were consumed (McGee 2004). Since this prehistoric

development, the basic concept of milling is the same; however, current milling

17

processes have become relatively complex. Current grain milling technologies

consist of breaking grain kernels into pieces of various sizes so that a consumable

product can be easily utilized. The resulting products of a milling operation are often

referred to as flour or meal with the physical and nutritional compositions varying

greatly depending on the crop, growing conditions, milling procedure used, and

refining processes that may be implemented after milling (Potter and Hotchkiss 1995;

Chiang and Yeh 2002; Dijkink and Langelaan 2002; Kethireddipalli and others

2002b; Haros and others 2003; Miralbes 2004; Salmenkallio-Marttila and others

2004; Singh and others 2005; Velu and others 2006). In the general milling process,

the germ and bran are often removed in the first steps of the operation. These

portions of the germ generally account for most of the fiber, a significant portion of

oil, B-vitamins, and in some cases as much as 25% of the grain protein. Removal of

the germ and bran is desirable because the consumer generally prefers the refined

form of a cereal or legume (endosperm) as it is easier to work with in home kitchens

along with producing consumable goods with a more appealing color. Additionally,

removal of the bran allows for a significant reduction in oil content which if otherwise

present in the flour can lead to lipid degradation effects in the final product (Potter

and Hotchkiss 1995; McGee 2004).

Wet and Dry Milling

There are many types of milling devices and tools used for size reduction of

the grain kernel. Two common methods of milling used alone or in combination with

these devices are the wet and dry methods. The primary difference in the two

processes is a wet milling process involves either soaking or moistening the product

in a particular form at a specific time and temperature which are dependent upon the

characteristic of the desired end product (Dupont and Osman 1987). Wet-milling of

18

grains can be done in several ways but generally involves either soaking the whole

grain kernel or a milled portion of the grain. In the case of corn, the kernels are

soaked in dilute sulfurous acid for 10 - 58 hr at temperatures ranging from 45 - 55oC.

After this phase, the soaked kernels are processed through a series of grinding and

filtration steps while in a slurry state. Wet milling of wheat is typically done on milled

flour fractions creating a dough-like product and filtering off specific fractions that are

water soluble (Sayaslan 2004). In either form of wet milling, the primary benefit is

the specific separation of grain components for specific functional purposes such as,

corn starch, vital wheat gluten, and wheat starch (Dupont and Osman 1987;

Sayaslan 2004).

However, some disadvantages that exist with wet milling when compared to

dry milling are the costs associated with processing equipment, the drying process,

waste water, and a greater potential for microbial contamination (Phillips 1982).

Overall improvement of wet milling technology has lead to a reduction in these

problems; however, they are still present and of great concern if the process is not

controlled properly (Dupont and Osman 1987; Sayaslan 2004).

Dry milling is simply the pulverization and/or grinding of a grain or legume to

a particular particle size without the use of water and is often the preferred milling

methodology as energy consumption and water usage is considerably less (Phillips

1982; Potter and Hotchkiss 1995) when compared to wet milling.

Dry Milling Equipment

Milling equipment commonly used in both grain and legume processing

include roller (conventional), hammer, stone, disc, and jet milling. Each of these

processes relies on a different form of processing methodology for size reduction

and separation of grain particles into flour or meal.

19

Roller (Conventional) Milling

Roller milling involves the evolution of particle disintegrations combined with

sieving steps. As the milling process continues, the rollers move progressively

closer so as to reduce the particle size of the milled grain with sieves located in

between the vertically stationed rollers. This sieving action removes particulates,

such as bran, from the milled product allowing for the desired end product to vary in

composition from whole grain flour to refined flour free of bran. As the product is

milled finer and finer it becomes characteristic of the preferred consumer flour in that

it is light in color and fine in texture (Potter and Hotchkiss 1995; Culinary Institute of

America. 2002).

Hammer Milling

Hammer milling uses a series of rotating hammers to both impact and throw the

material to the exterior of the mill housing. Impacting forces on the grain caused by

either surface result in the gradual reduction in particle size of the product. Control

of particle size can be achieved by changing the rotation speed of the hammers,

changing the pore size of the screen used to retain the particulates in the milling

region, or multiple passes of the milled product through the mill (Kethireddipalli and

others 2002a; Singh 2003).

Stone and Disc Milling

Stone and disc milling operations are essentially the same in that mechanical

shearing and compression on the material are the main grinding forces. In each

case, the grinding surface exposed to the material is varied with the disc mills having

more geometric versatility than stone mills. Counter rotational speeds of the two

milling surfaces, distance between surfaces, and surface textures all impact the

20

performance of the mill and functional properties of the milled product. Yield is

generally low in comparison to other milling operations, yet these are often the mills

of choice for many developing areas because of low costs and high durability (Singh

and others 2004; Bayram and Oner 2005; Singh and others 2005; Mohapatra and

Bal 2006; Velu and others 2006).

Jet Milling

Jet milling is a relatively novel idea for the milling of fine particles, yet it is an

overall highly inefficient method as energy consumption is relatively high. Jet milling

occurs within a circular milling chamber that has air nozzles (jets) around the

perimeter. Compressed air is supplied to the milling chamber in a single direction at

a specific pressure (velocity) which creates a cyclone or vortex within the milling

chamber. Material is fed into the milling chamber at a specific location and is thrown

against the walls of the chamber causing pulverization (impact forces) of the

material. This continual impact milling eventually reduces the particle size/mass to a

range which then becomes characteristic of attrition (particle to particle collisions)

milling. This specific particle size range is dependent on the population balance

function of the milling system as described by Gommeren and others (2000). The

continual microparticulation of particles from these two forces makes this style of

milling ideal for situations where extremely small particle size is desirable, such as

pharmaceutical products, fat substitutes or beverage applications (Hayakawa and

others 1993; Guinard and Mazzucchelli 1996).

Cyclone Air Classifiers

In the simplest of terms, cyclones are devices that remove particles of various

sizes from a gas or fluid stream. They are simple in design, cost efficient, and can

21

be operated under various environmental conditions (Buttner 1999). The basic

operation of a cyclone involves a particle-laden fluid stream entering the cyclone

structure through a tangent inlet opening generating a vortex along the outer wall of

the cyclone structure. The centrifugal force generated by the vortex causes particles

to exit the fluid stream into a collection device. Particle separation depends largely

on the velocity of the particle laden fluid, particle loading, and the natural vortex

length. A third opening (vortex finder) located directly above the collection opening

allows a second vortex, located within the bounds of the outer vortex, to carry fine

particles out of the cyclone separator, assuming any particles remain in the fluid

medium. Buttner (1999) extensively reviewed previous studies relating to cyclone

separation and functionality based on various geometrical proportioning of the

cyclone, operating temperatures, and flow rates in an attempt to identify a predictive

means of identifying a predetermined particle size cut off point. It was concluded

that for Re <105 the following dimensionless collection efficiency relationship is:

3

2

31

21

Re ⎟⎟⎠

⎞⎜⎜⎝

⎛=

o

i

dd

StkE (2)

Where E is the grade efficiency (‘S’ shaped curve), Stk is the Stokes number, Re is

the Reynolds number of the particle-laden fluid, di is the cyclone inlet diameter (m),

and do is the cyclone outlet diameter (m). This relationship is valid for high

operational temperatures and when the body diameter of the cyclone (db) does not

influence the collection efficiency. This study did mention that further studies need to

be conducted on the relationship of cyclone height and the effect of high particle

concentrations that can be subjected to aggregation, agglomeration, and attrition

effects.

22

Xiang and others (2001) examined the affects of cone bottom diameter on

cyclone efficiency and noted that as the cone base opening changes other various

geometric proportions change, such as cone area and vortex turn diameter, thus

altering the collection properties. Zhu and Lee (1999) demonstrated that as the cone

became more narrow, the tangential velocities at the base of the cyclone increased

proportionally. Their experiment focused on three cyclones with bottom diameters of

19.4, 15.5, and 11.6 mm and gas flow rates ranging from 30 to 60 L/min. The

experiment demonstrated that the smaller the do the greater the collection efficiency,

yet the overall shape of the efficiency curve did not change with changes in cone

geometry. The structural modification allows for the removal of smaller particles due

to greater centrifugal forces, but if the environment within the cyclone became

unbalanced, (i.e. if the vortex touches the cyclone wall) this would allow for particle

re-entrapment and a decrease in overall collection efficiency. The experiment did

show that under the operational conditions of this particular cyclone, the reduced

volume of the cyclone structure allowed for the generation of greater centrifugal

forces near the do thus increasing particle collection efficiency.

Presently, a prediction model that can encompass various cyclone sizes, flow

rates, and particle loading has yet to be agreed upon due to the innate complexities

of the dynamic nature within the cyclone structure. A new insight to this problem is

the observation by Avci and Karagoz (2003) that identifies three types of fluid flow

regimes (laminar, turbulent, or transitional) can exist within a cyclone at the same

time. This observation renewed ideas about cyclone behavior/performance as these

flow regimens have never been accurately identified. The aim of their study was to

adapt a prediction model for particle cut-off size and collection efficiency to account

for the various flow regimens in both large and small cyclones while accounting for

friction coefficients of particles. General assumptions about particle homogeneity

23

were taken into account along with an assumed homogeneous tangential inlet fluid

velocity. The conclusion of the study did state that a model was identified and

validated experimentally; however, specifics to cyclone operation, geometry, and

performance as identified in earlier research were general and vague. It was noted

that particle friction coefficients were assumed in this experiment and when applied

to theoretical models a negative response in cyclone performance was generated;

that is, the rougher the surface the more negative the performance of the cyclone.

The general understanding of the vortex end (natural vortex end) has been a

relatively large hindrance in previous attempts at understanding the behavior of the

cyclone. Common thought dictated that if the vortex end significantly touches the

cyclone wall at any point this will cause previously separated particles to re-enter the

inner vortex thus decreasing collection efficiency or even destabilizing the entire

cyclone flow (Xiang and others 2001; Avci and Karagoz 2003). Peng and others

(2005) expanded on the studies by examining the nature of a vortex end via

measurement of the vortex core frequency through the use of a stroboscopic and

high-time-resolution pressure sensing. The natural phenomenon of the vortex end

was shown to bend towards the wall of the cyclone; traditionally, this was assumed

to be a break down of the vortex, but in reality it is the natural behavior of the system.

Data from the pressure transducers in the cyclone-based experiments confirmed the

notion that the known inlet velocity is either close to or the same as the wall velocity

of the cyclone body (Peng and others 2005) if the inlet is rectanglular in shape. Early

on, Buttner (1999) noted the need to alter the shape of the inlet as this will cause for

variations in particle distribution across the inlet opening and in turn to the centrifugal

force generated by the particles within the cyclone body section. Qian and Zhang

(2005) identified a polynomial quadratic model (second-order response surface

model) for the prediction of the natural vortex length utilizing response surface

24

methodology and computational fluid dynamic techniques. Similar to previous

prediction model experiments, the model was created with certain variables

unaccounted for; namely wall roughness, particle load, and shape of the inlet. It was

noted that inlet shape was evaluated in this study, but only variations in the

dimensions of a rectangular inlet were examined. The recent advancements in

understanding the inner vortex behavior has lead to the design and utilization of

more efficient cyclone collectors for the removal of various particle sizes (Peng and

others 2005; Qian and Zhang 2005). However, practical application still yields the

most reliable and cost effective results for identification of cyclone operation,

performance, and functionality.

Particle Size Measurement

Mesh Sieving

Mesh sieve analysis is the process of taking a known weight of material and

sifting the material using a series of sieves with known mesh pore diameters for the

establishment of a particle size distribution (PSD) by mass. Sieve size standards,

established by the American Society of Testing Materials (ASTM 2004), have a

range of mesh sizes from 38 μm – 4.75 mm in pore diameter with smaller, non-

standard mesh sizes as low as 20 μm in pore diameter. The sieves are stacked

starting with a collection pan and then a series of sieves in ascending order of pore

diameter. The meal/flour sample to be measured is sifted for a designated amount

of time with the weights of each sieve recorded before and after sieving. Applying

the mass distribution of the sieves to equations 3-5 (ASABE 2003) produces the

geometric mean particle diameter by mass (dgw) and standard deviation of mean

particle diameter by mass (sgw). Additional percentile values, particle surface area,

25

and number of particles in a sample can be extracted from this method when

appropriate values are plotted along a logarithmic probability graph.

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

=

∑

∑

=

=−n

ii

n

iii

gw

W

dWd

1

11)log(

log (3)

( )3.2

loglogln

21

1

1

2

logS

W

ddWs n

ii

n

igwii

=

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡−

=

∑

∑

=

= (4)

( )[ ]1log

1log

1 loglog21 −−− −≈ SSds gwgw (5)

Where:

di is nominal sieve aperture size of the ith sieve, mm

id is (di x di+1)1/2

di+1 is nominal sieve aperture size in next larger than ith sieve, mm (just

above in a set)

dgw is geometric mean diameter or median size of particles by mass, mm

slog is geometric standard deviation of log-normal distribution by mass in

ten-based logarithm, dimensionless

sln is geometric standard deviation of log-normal distribution by mass in

natural logarithm, dimensionless

sgw is the geometric standard deviation of the particle diameter by mass,

mm

Wi is mass on ith sieve, g

n is number of sieves+1 (pan)

There are several disadvantages to using sieve analysis as outlined by Rawle

(2002):

26

• Measurement of dry particles smaller than the 400 mesh (38 μm) threshold is

difficult and results tend to misrepresent the true particle size below 400

mesh.

• Dry particles can create a static charge thus effecting the particle weight

distribution.

• Resolution of the analysis is strongly based upon the composition of the

material being measured.

• Particles that agglomerate, exhibit a cohesive property, or are non-spherical

in geometry cannot be accurately measured by sieve analysis. Sieve

analysis relies on particles to freely pass through the mesh openings and

assume that the particles are separate from one another.

• Often standardized methodology requires a mechanical action in addition to

shaking for particles to pass through the sieve mesh openings, such as

tapping, ultra sonic sieving, or sieving aids that break apart agglomerated

particles.

As mentioned in the summary, methods such as tapping, ultra sonic sieving, or

addition of a sieving aid are often employed to aid in the dispersion of particles so

that they can freely fall through the mesh openings. These additional methods can

be costly and require specific procedures that would result in the misrepresentation

of the mass distribution. The primary reason why sieving is not an acceptable

means for measuring fine, dry particles is due to the general agglomeration and

aggregation effects that occur due to moisture accumulation or electrostatic charge

during the sieving process. It is recommended that for particles that are speculated

to have a dgw < 38 μm other measurement techniques, such as laser or optical

27

analysis, be utilized for an accurate PSD measurement of the sample (ASABE

2003).

Laser Diffraction

Laser diffraction, also known as light scattering, is the preference for particle

size analysis since 1999 when the ISO approved guidelines for such procedures with

particle size ranges of 0.1 – 3000 μm (ISO13320 1999). The new standard states

that the Mie Theory of light diffraction is to be used for particle size measurement for

particles < 50 μm. Prior to the present standard, the Fraunhofer diffraction theory

was the calculation of choice due to insufficient computing resources needed to

support the Mie theory (Dodds and others 2004; Wilson and Foster 2005). The

current models of particle size measurement using laser diffraction (Mie Theory) are

based on the principle, “diffraction angle is inversely proportional to particle size”

(Rawle 2002). The measurement procedure must have the particles suspended in a

fluid medium, pass the particles through a perpendicular laser beam with a known

wavelength (λ), and use a number of detectors (16 – 32) to measure and record the

angle of diffraction. Numerous factors are taken into account for the calculation of

the exact or estimated particle size as reviewed by Rawle (2002).

Key parameters to an accurate measurement procedure are 1) establishment

and adherence to measurement methodology, 2) appropriate particle suspension to

ensure particle separation, and 3) an appropriate fluid medium and delivery method

of the medium to carry the particles. In the case of dry powders, there are options

for suspending the particles in a liquid medium or using a blower to pass the particles

in front of the laser beam. The disadvantages to using the liquid medium are

potential changes in particle structure (specifically with food items) and not knowing

the refractive index of the liquid medium. If a blower is to be used to carry the

28

particles, the particles often pass through the laser path in a shorter period of time

and are seldom recirculated through the laser beam. This delivery method can skew

the measurement of the true PSD of the sample, whereas a liquid medium can be

recirculated for a given amount of time and speed thus increasing the accuracy of

the PSD measurement (Ward-Smith and others 2002).

Lastly, when dealing with particles that are predominantly < 7 μm the problem

of particle aggregation has to be considered. It may become necessary to use

chemical dispersion agents to effectively separate the particles in this range so that

an accurate representation of the PSD can be viewed by the laser detectors (Rawle

2002).

The establishment of a representative PSD measurement procedure for a

sample using laser diffraction is no different than other scientific-based

measurements. Questions such as appropriate sample size, sampling rate, or

delivery method should all be established on a sound scientific justification (Ward-

Smith and others 2002). The statistical results produced from a PSD measurement

data set will indicate the precision and reproducibility of a measurement method.

These results will be dependent upon the measurement methodology but can be

similar when compared to other measurement protocols (i.e. different dispersion

agent or medium to suspend the particles) depending on the material to be

measured. As such, conducting a verification procedure for any new sample

material is a requirement of laser particle size measurement. Most software

packages offer the statistical analysis functions to verify a procedure, provided that

appropriate parameters about the material and fluid medium are inputted (i.e. particle

density and refractive index) (Ward-Smith and others 2002).

29

Particle Size Analysis

Particle size analysis is of importance to the cereal and legume processing

industries as both a quality control standard and as an identifier of physicochemical

and functional properties. Various methods are employed to measure the particle

size with laser diffraction measurement and mesh sieve analysis being the most

utilized methodologies. Each of the measurement methods offers various benefits

as well as drawbacks depending on the composition of the product, particle size

range, and intended use of the product. Regardless of the method used to measure

particle size the ultimate question is how to represent the particles measured in a

measurement method. Table 2.4 demonstrates the difference between counting the

number of particles in a size range versus quantifying the particles by mass.

Rawle (2002) illustrates that 99.3% of the total particles have an estimated

diameter below 1.0 cm; however, as depicted in the final column, 99.96% of the

particles have a diameter less than 1000 cm when they are quantified on a mass

distribution basis. Neither of the two quantification methodologies is inadequate, but

this example does illustrate that the same particle size data can be represented in

two very different ways with two equally different conclusions.

Generally particle size distributions are represented by either histograms or

using logarithmic scales with a cumulative percentage of particles by mass or volume

(Rawle 2002). Figure 2.1 illustrates how a logarithmic particle size distribution (PSD)

curve compares to a non-logarithmic PSD curve (fig 2.2). In a graphical

representation, it is easier to analyze the logarithmic scale due to the entire PSD

being viewable, whereas in the non-logarithmic scale the majority of the curve is too

condensed to make any use of the data. The percentiles of the curve demonstrate

30

how the majority of the particles by mass occur below the respective micron value

(e.g. 60% of the particles have a diameter of 23.464 μm or less).

On a visual representation basis, it can be seen that the logarithmic scale is

easer to view; however, drawing mathematical conclusions from the logarithmic

curve is quite difficult when compared to the latter graphical representation. In the

latter PSD curve, a simple linear slope can be derived in an attempt to predict the

probability of a particle’s size at a given point on the curve. However, such a slope

would not be appropriate as a significant portion of the curve extends past 700 μm

and would give a false probability of a particle’s size. Identification of the probability

of particle sizes that occur below or at certain percentiles requires the use of either a

logarithmic probability graph (fig 2.3) for probability estimation or a log-log probability

function (eqn 6) to obtain an exact value.

( ) ( )( )50loglog1 ixbecdcy −+

−+= (6)

Where:

y is the percentile probability of a specific particle size

c & d are the upper and lower asymptotes of a logarithmic PSD

curve (fig x)

x is the particle size at a specific probability

i50 is the particle size at the 50th percentile probability value

Particle Size in Food Systems

Sensory Perception Texture, as defined by Szczesniak (2002), “is the sensory and functional

manifestation of the structural, mechanical and surface properties of foods detected

31

through the sense of vision, hearing, touch and kinesthetics.” In earlier work,

Szczesniak (1963) classified particle size, shape, and orientation as a geometric

characteristic with potential descriptors listed as gritty, grainy, and coarse. A later

classification (Tyle 1993) mentions that the minimum particle size range that humans

can perceive these texture-based sensations is between 10 - 20 μm depending on

the food composition. The study tested three materials (garnet, micronized

polyethylene, and mica particles coated with titanium dioxide) differing in shape,

hardness, and particle size. Four particle sizes of each material were suspended in

flavored syrup solutions and evaluated by 25 panelists in a single-blind taste trial.

The study showed that particles that are soft and rounded can be as big as 80 μm

before the suspension is perceived as gritty; however, particles that are hard and

angular are perceived as gritty if the particle size range is above 11-22 μm. Relating

the descriptions of the materials used in the study to food applications, seed coats of

legumes and grains can be viewed as relatively hard angular shapes whereas starch

granules can be classified as a soft, rounded material (Tyle 1993). Guinard and

Mazzucchelli (1996) state that oral mechanoreceptors can detect globules ranging

from 5 - 25 μm in particle diameter as well as discriminate between various sizes

when the distances between globules is between 0.5 – 3.0 μm, depending on the

food and/or beverage composition. The summation of these earlier studies on the

texture sensory perception of liquid-based food systems indicates that particles of

hard angular shapes need to have a dgw < 11 – 22 μm and soft rounded particles

should have a dgw < 80 μm in order for the beverage system to be perceived as

smooth and free of negative textural attributes. Additionally, distance between

particles can impact the sensory perception relating to particle size; therefore, the

32

concentration of the particles in the food composition needs to be accounted for to

ensure an appropriate distance between particles.

Ingredient Functionality

Sensory perception due to particle size is just one component of food

formulations that is taken into consideration during the formulation process, but

ingredient functionality is altered depending on particle size and the means by which

the particle size is created. A series of studies on the milling of cowpea and

production of snack chips revealed that the finer milled product with a mean particle

size ranging from 40 – 70 μm did have different functionality when compared to

larger counterparts with a mean particle size greater than 300 μm (Kerr and others

2000; Kerr and others 2001). It was concluded that the finer milled product yielded a

higher extractable starch value and a lighter color snack chip; in addition, the peak

force necessary to snap the crackers was correlated with increasing levels of starch

in the formulation. Decreases in gelatinization temperatures and generally hydration

properties along with increases in protein solubility were observed with decreasing

particle size (Kerr and others 2000). These changes in ingredient functionality can

be negative in products where a spongy texture is preferred because they will not

create the necessary particle structure (foam) resulting in a dense and somewhat dry

structure (McWatters 1983; Ngoddy and others 1986; McWatters 1990; Singh and

others 2004; Singh and others 2005).

Food particles milled using common methods (e.g. roller, stone, or hammer

mills) produce particle size ranges that are comparatively larger when compared to

particles created via microparticulation, also known as attrition milling.

Microparticulation offers various benefits for certain food systems as the reduced

particle size can improve dispersibility, nutrient bioavaibility, sensory perception, and

33

in some cases mimic tactile sensations of food components (Hayakawa and others

1993; Park and others 2001). Hayakawa and others (1993) found that the

microparticulation of casein and egg white increased the hydrophobic nature of the

particles which was related to the significant increase in particle surface area; the

intent of the research was to create a food additive with a lipid-like mouth feel without

the nutrient content of a lipid. A study on the development of green tea powder

processed via microparticulation concluded that as the particle size decreased due to

increasing the microparticulation process (time and rpm) the dispersibility of the

powder mix increased, overall solution stability increased, sedimentation rates

decreased, and higher concentrations of the powder could be incorporated into

solution at lower temperatures without adverse effects on solution stability (Park and

others 2001).

34

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39

Table 2.1: Names, general description, and utilization of non-fermented traditional oriental foods. Native names

Soy foods Chinesea Japanese Korean Othersb General Description Uses Soymilk Doujiang Tonyu Kong kook Heated water extract of

soybeans after grinding and filtration. Resembles dairy milk.

Served hot or cold, as breakfast, beverage, or with other foods.

Dou nai Doo goo Dou ru Soybean curd Doufu Tofu Doo bu Tahu (In.) White protein curd precipitated

from soymilk with a salt or acid. Bland taste. Can be dried, frozen, or fried.

Cooked with or without meat, vegetable, and seasonings, and served as a main dish or soup.

Tofu (Toufu) Tau foo (Ma.) Tokua (Ph.) Soy sprouts Huang dou

ya Daizu no moyashi Kong na

mool Germinated soybeans in dark,

yellow cotyledons with white sprouts.

Cooked as vegetable or in soup.

Dao dou ya Soy pulp Dou zha Okara Bejee Tempeh gembus (In.) Insoluble residue after filtration

of soymilk. Rich in dietary fiber. Made into dish, salted as pickle, or fermented into a temphe (In.)

Okara Soy film, Yuba Do fu pi Yuba Kong kook Fu chok (Ph.) Creamy, yellowish protein-lipid

film formed from surface of boiling soymilk. Sheets, sticks, or flakes.

As delicacy, cooked with meat or vegetables, or in soups.

Fu zhu Immature soybeans Qing dou Edamame Put kong Green immature soybeans.

Prefer large seeded cultivar. Cooked in pod or pod removed. Served as snack or vegetable.

Sweet beans Mao dou Roasted soybeans Chao da dou Iri-mame Dry roasted soybeans,

seasoned or non-seasoned. As snack or made into powder.

Toasted soy powder Chao dou fen Kinako Kong ka au Bubuk kedelai (In.)

Dou fen

Dry roasted then ground soybeans, Yellow powder, and nutty flavor.

Coating for rice cakes or sprinkled over cooked rice.

a Mandarin Chinese (or Cantonese) b In.= Indonesian, Ma. = Malaysian, and Ph. = Philippine Table adapted from Liu (1997)

40

Table 2.2: Names, General description, and utilization of fermented traditional oriental foods. Native names

Soy foods Chinesea Japanese Korean Othersb Fermentation Organisms General Description Uses Soy sauce Jiang you Shoyu Kang jang Kecap (In. Ma.) Aspergillus, Pediocuccus,

Torulopsis, and Zygosaccharomyces.

Whole soybeans (or defatted soy flake) and wheat fermented. Dark brown liquid, salty and meaty.

All purpose seasoning for dishes or soups.

(Chiang yu)

Shoyu Tayo (Ph.)

Soy Paste Dou jiang Miso Tauco (In. Ma.) Aspergillus, Pediococcus, Zygosaccharomyces, Torulopsis and Streptococcus.

Whole soybeans with wheat flour, rice, or barley, and fermented. Light yellow to dark paste, salty, and meaty.

All purpose seasoning for dishes or soups.

Miso Jiang Tao si (Ph.) Natto Na dou Natto Bacillus natto Cooked whole soybeans that

are fermented. Soft beans covered by viscous, sticky polymer, distinct aroma.

Seasoned and eaten with cooked rice.

Shui dou chi

Tempeh Tian bei Tempe Tempe Kedelai (In.)

Rhizopus oligosporus Cooked and dehulled soybeans that are fermented. Soft beans are bound by white mycelia, cake like, nutty flavor.

Fried or cooked as part of meal, snack, or in soups.

Tempeh (Ma.)

Fermented tofu Doufu ru

Actinomucor or Mucor Tofu fermented producing a creamy cheese like, salty, distinct aroma.

A condiment, served with or without further cooking.

Chinese Cheese Fu ru

Sufu (Tou fu ju) Soy nuggets Dou chi Hamanatto Tao si (Ph.) Aspergillus Whole soybeans and wheat

flour fermented producing soft beans with a black color, salty, and meaty taste.

Cooked with vegetable and meat or served as seasoning.

Black beans (Toushih) a Mandarin Chinese (or Cantonese) b In.= Indonesian, Ma. = Malaysian, and Ph. = Philippine

Table adapted from Liu (1997)

41

Table 2.3: Nutrient content of soymilk, cow's milk, and human breast milk. Nutrient / 100 g Soymilk Cow's Milk Human Milk Calorie 44 59 62 Water 90.8 88.6 88.2 Protein 3.6 2.9 1.4 Fat 2 3.3 3.1 Carbohydrates 2.9 4.5 7.1 Ash 0.5 0.7 0.2 Minerals (mg) Calcium 15 100 35 Phosphorous 49 90 25 Sodium 2 36 15 Iron 1.2 0.1 0.2 Vitamins (mg) Thiamine (B1) 0.03 0.04 0.02 Riboflavin (B2) 0.02 0.15 0.03 Niacin 0.5 0.2 0.2 Saturated fatty acids (%) 40-48 60-70 55.3 Unsaturated fatty acids (%) 52-60 30-40 44.7 Cholesterol (mg) 0 9.24-9.9 9.3-18.6 Source: Liu (1997)

42

Table 2.4: Size classification by number and mass

Size / cm Number of Objects

% by Number % by Mass

10 - 1000 7000 0.20 99.96 1.0 - 10 17500 0.50 0.03 0.1 - 1.0 3500000 99.30 0.01

Total 3524500 100.00 100.00 Data Source Rawle (2002)

43

Figure 2.1: Example of a cumulative percentile particle size distribution by mass

Cumulative Percentile PSD

20%, 7.992

80%, 35.995

40%, 15.254

60%, 23.454

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

0.1 1 10 100 1000

di (μm)

Cum

ulat

ive

Perc

enta

ge

44

Figure 2.2: Particle size distribution curve on a non-logarithmic scale

Particle Size Distribution Curve

20%, 7.992

80%, 35.995

40%, 15.254

60%, 23.454

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

0.1 100.1 200.1 300.1 400.1 500.1 600.1 700.1 800.1 900.1

di (μm)

Cum

ulat

ive

Perc

enta

ge

45

Figure 2.3: Logarithmic probability graph of two PSD curves demonstrating establishment of particles size probability and identification of PSD curve slopes.

46

Chapter 3

MILLING OF COWPEA FLOUR USING CYCLONE ASSISTED MILLING

47

Abstract: A two part factorial design was used to evaluate the effects of mill

configuration and operational parameters of a cyclone assisted impact/attrition mill,

and preconditioning treatments of cowpea seed on the geometric mean particle

diameter (dgw) and yield of cowpea flour. Cowpea flour was milled using a Super

Wing Mill DM-200 cyclone assisted impact/attrition food mill. Milling parameters (mill

configuration, turbine speed, airflow restriction, sample feed, and milling time) and

sample preconditioning treatments (moisture and temperature) were systematically

altered to examine the dynamic response on the dgw and yield of cowpea flour. A

two part factorial design (25 and 33) showed that the modified wing mill configuration

produced a larger dgw along with a significant increase in yield; turbine speed and

airflow restriction within the mill offered additional control of dgw and yield.

Preconditioning of cowpea seed showed that temperature preconditioning had no

effect on the dgw or yield, but preconditioning of moisture content below the

monolayer had a significant effect on dgw and yield of cowpea flour. Parameters for

the milling of fine cowpea flour (dgw ≤ 17μm; ≥ 90% yield) were identified as using

preconditioned cowpea seed at 4% moisture (20oC) milled using the modified wing

mill configuration with a turbine speed of 7200 rpm at a 90o airflow restriction for 5

min of milling time.

Keywords: cyclone, particle size, cowpea, flour

48

Introduction:

Cowpea (Vigna unguiculata) is grown on every continent in the world with

Africa leading production at an average of 35 million metric tons per year (FAOSTAT

2005). Like most legumes, the main drawbacks to preparing cowpea for

consumption are the intensive labor processes and utilization of consumable

resources (i.e. water) that accompany these nutrient dense food sources (Dovlo and

others 1976). In recent years, the research group at the University of Georgia, Griffin

Campus, funded through USAID, has improved on ease of use of cowpea by

developing cowpea meal for use as a starting material for consumer applications

which require less water and processing time when compared to whole dry seeds

(Phillips and others 2003). Current milling research related to legumes primarily

focuses on stone, hammer, jet/turbo, and disc milling methods or combinations

thereof (Phillips and others 2003). Under normal circumstances, the geometric

mean particle diameter by mass (dgw) of legume meal or flour is often 60 to 150 μm

or greater (Kethireddipalli and others 2002; Phillips and others 2003; Singh 2003;

Singh and others 2004). It has been shown that acceptable consumer products, like

akara, can be produced from cowpea flour/meal at these dgw ranges; however, a

smaller dgw could result in greater consumer acceptability for foods like gels,

couscous, beverage mixes, and fry batter applications (Labensky and Hause 1995;

McGee 2004).

Particle-to-particle interactions (attrition milling) at the micron level have been

shown to exert abrasion/erosion forces contributing to the continued reduction in

particle size after primary milling (Gerhards and others 2004). Milling of food items

or biological materials at cryogenic or cryogenic-like temperatures has been shown

to decrease particle size in addition to retaining compositional components that may

49

be lost due to heat generated by the milling operation (i.e. volatile oils or moisture)

(Singh and Goswami 1999; Goswami and Singh 2003). In a standard cryogenic

milling operation, a sub-freezing temperature is maintained throughout the milling

operation. The cryogenic temperatures needed for successful milling of spices have

to be below the brittle point of the lipid molecules of the spice so that maximum

particle fracture can be achieved (Singh and Goswami 1999; Manohar and Sridhar

2001; Goswami and Singh 2003). Control of attrition milling parameters (i.e.

residence milling time or sample temperature) can lead to a certain degree of control

on the particle size of milled legume flour in an appropriate milling operation.

Presently, a milling operation to produce legume flour with a small dgw would require

longer or additional milling and sieving steps than the traditional milling process

currently utilizes.

The Super Wing Mill DM-200 (wing mill) can be related to a jet milling

operation in that the primary milling zone is similar in operational principle. The

milling operation consists of two types of milling forces: impact and attrition. Impact

milling occurs in the turbine housing immediately after the sample is introduced to

the wing mill. When the particle size/mass within the turbine milling chamber

becomes too small for impact milling, the particles are then subjected to attrition

milling throughout the remainder of the wing mill system. The wing mill differs from

the common jet milling operation in that it is a closed loop system that exposes food

particles to a greater frequency of particle collisions until either a particle size range

is achieved or the fluid stream alters velocity causing particles to exit out of the

system via the cyclone exit point (Gommeren and others 2000).

50

Cyclone Theory

Cyclone or gravity separators are currently used in the heating ventilation and

air conditioning (HVAC), pharmaceutical, and mineral processing industries as a

means of semi-selective particle separation/filtration (Buttner 1999; Funk and others

2003; Zhao 2005). Cyclones are easily manufactured, economically preferable for

particle filtration when compared to other methods, and have been shown to operate

over a wide range of processing and environmental conditions (Zhu and Lee 1999;

Avci and Karagoz 2003; Funk and others 2003; Zhao 2005). Cyclone applications

have been shown to be an overall efficient means of particle separation; specifically

cyclones with body diameters less than 120 mm allow for relatively efficient micron

and sub-micron particle separation without excessive modifications to current

manufacturing systems (Buttner 1999).

Operational principles of cyclones have been effectively reviewed and

elaborated upon in recent literature (Barletta and Barbosacanovas 1993; Buttner

1999; Avci and Karagoz 2003; Zhao 2005). However, a sound explanation for the

exact behavior of a cyclone has yet to be agreed upon due to the dynamic nature of

the system. In general, the inlet-tangential velocities of the particles cause

acceleration towards the wall of the cyclone body generating a centrifugal force

which forms an outer vortex along the cyclone structure wall. As the velocities of the

particles decrease from various forces, they begin to flow downward towards the

base of the cyclone where particles of a specific mass exit the system into a

collection vessel. With the application of a vortex finder located at the top of the

cyclone structure, a second internal vortex is created in which the particles that

remain in suspension escape out the top of the structure. Since the wing mill is a

51

closed system, the inner vortex causes the returning particles to reenter the central

column of the wing mill where they are exposed to additional attrition and/or

agglomeration effects (Barletta and Barbosacanovas 1993).

The aim of this study was to utilize a cyclone assisted milling operation

similar in principle to a jet mill for the production of cowpea flour with a dgw ≤ 20 μm.

Specific objectives were 1) to identify the effect of mill configuration and operational

parameters on the geometric particle mean diameter and yield of cowpea flour, and

2) identify the effects of preconditioning treatments of cowpea seed on the geometric

particle mean diameter and yield of milled cowpea flour.

Materials and Methods Cowpea (Inland Empire Foods, Riverside, CA) of the California Cream variety

(breeding line UCR 97-15-33) was used as the model crop for the milling of cowpea

flour. Cowpeas were received as dry (9.6% moisture) whole seeds with a cream

seed coat and cotyledon color and sporadic darker colored beans. The dry seeds

were manually graded to remove any stones or foreign matter. After grading, the

seeds were further dried in a forced air oven (model no. 8107; The Electric Hotpack

Co., Inc., Philadelphia, Pa., U.S.A) at 55oC to a final moisture content of 4 ±0.1%.

Calculated proximate composition of the raw product prior to milling was

approximately: 4% water, 22.9% protein, 1.99% total lipid, 3.25% ash, 10.27% total

dietary fiber, and 57.59% carbohydrate (HealtheTech 2005). The cowpea seeds

were then stored in a seed cooler (5 ± 1oC) until further processing.

A Super Wing Mill DM-200 (Sanwa Engineering Co., LTD., Japan) was used

for all milling operations in this study (fig 3.1a) with design modifications (fig 3.1b)

based on preliminary experiments (data not shown). Wing mill design modifications

consisted of repositioning of the airflow restriction valve downstream of the vortex

52

finder. The cyclone utilized in this experiment had a body diameter of 98 mm and a

cyclone height of 245 mm. A sample size of 180g of cowpea seeds was used for

each milling run; cowpea flour collected was coded and stored in Ziploc® bags at

ambient conditions.

During the experiment, the wing mill was cleaned in between millings using a

4 gal, 5.0 hp, wet/dry vacuum (model SG400; RIDGID, St. Louis, MO) which in turn

was cleaned after every 4 millings. Complete disassembly of the wing mill was

conducted after every 12 millings and the components were thoroughly cleaned

according to manufacturer’s recommendations.

Particle Size Analysis Particle size analysis was conducted using a Malvern Mastersizer S laser

diffraction system with a QSpec small volume sample dispersion unit operating at

3000 rpm (Malvern Instruments, Worcestershire, U.K.). The Mastersizer S uses the

Mie theory of light diffraction for particle size measurement which assumes an

equivalent sphere size of particles and performs particle size calculations based on

the fact that the angle of light diffraction is inversely proportional to particle size

(Rawle 2002).

To establish a standard operating procedure (SOP) for particle size

measurement of cowpea flour, 10 replicate measurements each of a dry flour and

wet flour preparation methods were conducted using a cowpea flour sample. Dry

cowpea flour sample measurements involved applying the dry flour to the sample

dispersion unit of the Mastersizer and conducting the measurement procedure per

manufacturer protocol. Wet cowpea flour sample measurement consisted of creating

three 10% suspensions (w/w) of cowpea flour and deionized water. Cowpea flour

was dispersed into deionized water using a magnetic stir plate and bar, and then

53

allowed to hydrate at ambient conditions for 10 minutes prior to application to the

dispersion unit of the Mastersizer using the same measurement protocol as the dry

flour. The raw measurement data obtained from the Mastersizer consisted of the

particle size distribution (PSD, di = 0.05 – 865 μm) of the respective sample

expressed as volumetric diameter of particles; additionally, particle size percentiles,

volumetric mean diameter, summary statistics of the PSD, and sample surface area

of particles were calculated using the Mastersizer data acquisition software package

(Mastersizer S-Long Bed v2.19).

A cluster plot of dry vs. wet percentile values was used to identify the particle

size relationship between the two preparation methods. Cumulative size distribution

of wet flour and dry flour measurements were also plotted on a logarithmic probability

graph to further explain the relationship between the two preparation methods.

Percentile data of dry flour vs. wet flour measurements were used to identify the

variability and significance between the two preparation methods by calculating the

coefficient of variance (CV) to determine the least number of particle size

measurements needed to statistically represent the PSD of a cowpea flour sample

(Ward-Smith and others 2002). PSD data of both flour sample measurements were

applied to ASABE standard S319.3 (ASABE 2003) to calculate the geometric mean

diameter (dgw, eq1) and geometric standard deviation of particle diameter by mass

(sgw, eq 3).

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

=

∑

∑

=

=−n

ii

n

iii

gw

W

dWd

1

11)log(

log (1)

54

( )3.2

loglogln

21

1

1

2

logS

W

ddWs n

ii

n

igwii

=

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡−

=

∑

∑

=

= (2)

( )[ ]1log

1log

1 loglog21 −−− −≈ SSds gwgw (3)

Where:

di is nominal sieve aperture size of the ith sieve, mm

id is (di x di+1)1/2

di+1 is nominal sieve aperture size in next larger than ith sieve, mm (just

above in a set)

dgw is geometric mean diameter or median size of particles by mass, mm

slog is geometric standard deviation of log-normal distribution by mass in

ten-based logarithm, dimensionless

sln is geometric standard deviation of log-normal distribution by mass in

natural logarithm, dimensionless

sgw is the geometric standard deviation of the particle diameter by mass,

mm

Wi is mass on ith sieve, g

n is number of sieves+1 (pan)

Mill Configuration and Operational Parameters Parameter variables of a 25 factorial design were used to identify the effects

of mill configuration and operational parameters on the dgw and yield of cowpea flour.

Variables included: mill configuration (original vs. modified), turbine speed (5400 rpm

vs. 7200 rpm), airflow restriction (15o vs. 90o), sample feed method (batch vs.

continuous), and milling time (1 min vs. 5 min). Design modifications to the wing mill

were chosen based on preliminary empirical performance data (data not shown).

The primary difference in the two mill configurations was the repositioning of the

airflow restriction valve. The design modification (fig 1b) placed the airflow restriction

55

valve downstream from the vortex finder of the cyclone; in the original design (fig 1a),

the airflow restriction valve was located immediately after the turbine housing.

Repositioning the valve impacted the mill by 1) eliminating clogging of the piping

system immediately exiting the turbine housing, and 2) in theory, allowed for indirect

control of airflow into the cyclone thus controlling the amount/size of particles drafted

into the cyclone.

The factory default turbine speed was 3600 rpm and maximum safe operating

turbine speed was 7200 rpm. Turbine speeds of 7200 rpm and 5400 rpm (midpoint

between base line and maximum speed) were selected for the experiment. Airflow

restriction (valve positioning) was in reference to the orientation of a directional valve

contained within the piping system where 90o positioning allowed for maximum

airflow, 15o positioning allowed for minimum allowable airflow, and 0o positioning

eliminated airflow in the local piping system of the valve. Batch sample feed method

consisted of loading the cowpea seed sample (180g) in 3-4 batches at 5400 rpm and

5-6 batches at 7200 rpm. Continuous feeding consisted of a constant feed of

cowpea seed into the wing mill so that the seed was consistently introduced to the

mill. Milling time values were chosen based upon preliminary work with the 5 min

milling time exhibiting no perceivable milling after the 180g of sample was fed into

the wing mill. Milling time was started after the last of the cowpea seed sample was

completely fed into the wing mill. The mill ran for the predetermined time and at the

end of the milling time the shutdown procedure began; this took 0.5 min to 2 min

depending upon the rpm setting of the wing mill.

Preconditioning Treatments of Cowpea Seed A milling procedure based on results from the milling parameter study was

identified and used for the seed preconditioning treatment study in addition to further

56

examination of airflow restriction effects on the end product. Cowpea seeds were

preconditioned to different temperatures (-80 oC, -20 oC, and 20oC) and moisture

contents (0.5%, 1.5%, and 4%) in addition to airflow restriction variation of the wing

mill (30o, 60o, and 90o). Moisture content of the cowpea seed was initially

conditioned to 4% using a forced air oven at 55oC. Saturated salt solutions of

potassium hydroxide (KOH; aw = 0.1234), potassium acetate (CH3COOK; aw

=0.2338), and potassium carbonate (K2CO3; aw =0.4314) were used to control the

relative humidity of an enclosed plastic container at room temperature to further

reduce seed moisture content and/or maintain equilibrium moisture content (EMC) at

0.5, 1.5, and 4%, respectively (Hung 1989; Ayranci and Duman 2005). EMC is “the

moisture content that a material would reach if exposed to air at a given relative

humidity and temperature” (Hung 1989). In preliminary experiments with the wing

mill (data not shown), it was observed that at seed moisture levels ≥6% the cowpea

flour would coat the interior of the mill, clog the milling operation, and/or cause a

pasting in the turbine housing to occur. A moisture content of 4% prevented the

clogging and pasting problems. Airflow restriction variations were chosen based on

the results from the study described in the previous section; these suggested that

control of the airflow exiting the cyclone could allow for a certain degree of control on

the particle-to-particle interactions. Controlling the airflow could provide a means of

controlling the yield and particle size of the cowpea flour based on the

preconditioning treatments utilized in this study.

Statistical Analysis

Data were analyzed using the general linear model (GLM) procedure; mean

separation tests were performed using Duncan’s multiple range test (SAS 2003).

57

Prediction models were generated for the prediction of dgw and yield of cowpea flour

under the various conditions evaluated in these studies.

Results and Discussion

Particle Size Measurement

Percentile data generated from the Mastersizer software were used to identify

a relationship, if any, between dry flour and wet flour measurements and secondly to

identify the least number of measurements needed to statistically represent the PSD

of a cowpea flour sample. Table 3.1 shows the averaged percentiles (20th, 40th, 60th,

and 80th) of 10 replicated PSD measurements of the wet and dry flour preparation

methods along with the calculated dgw. The only significant difference (α = 0.05)

between the two flour preparation methods was at the 40th percentile where wet

sample particles have a mean particle diameter less than 14.76 μm verses dry

sample particles having a mean particle diameter less than 15.25 μm. This particle

size difference was not observed at other percentiles, and there was also no

difference in calculated dgw.

A cluster plot of the 20th, 40th, 60th, and 80th percentiles of dry flour vs. wet

flour particle size measurements was used to identify a relationship between the two

preparation methods. Linear regression (R2=0.9869) of the cluster plot (fig 3.2)

illustrates the relationship that the dry flour particles were 93.26% the size of the wet

flour particles. Figure 3.3 shows the cumulative distribution by mass for wet flour

and dry flour preparation methods on a logarithmic probability graph. The key

element in the logarithmic probability plot is that the slope of the dry flour sample is

steeper than the wet flour sample (12.401 vs. 13.647, respectively) supporting the

linear regression of the cluster plot that the dry flour particle size was smaller

(6.74%) than the wet flour particle size (α = 0.05). Based on the linear regression

58

model (fig 3.2), overall percentile means and dgw (table 3.1), and that cowpea flour

consumer utilization would be in a hydrated state, the wet flour preparation method

was chosen for particle size analysis of cowpea flours milled in this experiment.

The CV of the mean percentile diameters from Mastersizer particle size

measurements of the wet sample preparation method were used to identify the

minimal number of replicated measurements needed to statistically represent the

PSD of a cowpea flour sample (table 3.2). In general measurement of biological

materials, a CV < 15% is an acceptable degree of precision. Overall the smallest CV

for all percentiles occurred at 10 measurement replications for the 60th percentile

(3.25) and the largest CV values occurred at three measurement replications with the

20th percentile (12.09). Gommeren and others (2000) explain that the population

balance of particles within a jet mill exhibit increasing levels of stochastic behavior as

the particle size is reduced. The wing mill relies on the same principles of particle

population balance in the attrition milling operation; as such, the CV of particle

diameter increases with decreasing particle size and reduction of measurement

repetition (table 3.2). CV values were smallest at the 60th percentile and increased

from 3.25% at 10 replicated measurements to 5.6% at 3 replicated measurements.

CV values were highest at the 20th percentile and increased from 7.86% at 10

replicated measurements to 12.09% at 3 replicated measurements. Even at 3

replicated measurements, CV values from all percentiles were still less than 15%.

Hence, 3 replicated particle size measurements of a 10% (w/w) cowpea flour

suspension was selected to accurately represent the PSD of a cowpea flour sample.

Mill Configuration and Processing Parameters

Table 3.3 shows the treatment regimens created in the factorial design and

the effects on the dgw and yield of cowpea flour. Note that feed method is not shown

59

due to this treatment level showing no significance (α = 0.05, table 3.4). Identifying

how a particular treatment regimen affects the dgw of the cowpea flour is not obvious

from this table, but table 3.3 can aid in the identification of sample yield trends based

on various regimens. The general trend for milling time shows that 1 min of milling

produced a smaller sample yield than a 5 min milling. The only exception was for the

treatment regimen of the modified configuration operating at 7200 rpm with a 15o

airflow restriction which showed a higher collection yield for 1 min than for 5 min.

However, the overall difference in milling time was significant for a 5 min milling time

to produce a higher yield of cowpea flour (table 3.4). All airflow restriction treatments

showed that the 15o restriction consistently produced a smaller yield when compared

to the counterpart of the treatment with a 90o restriction. In principle, reducing the

airflow/volume at any point in the system reduces the amount of particles that can be

retained within the fluid medium, thus contributing to a lower yield when the

airflow/volume is restricted at the 15o positioning of the airflow restriction valve

(Gommeren and others 1996; Buttner 1999). Operating the wing mill at 7200 rpm

with the restriction valve open at 90o for a 5 min milling time produced the highest

yield (94.3% to 95.5%, table 3.3).

Statistical analysis demonstrated that mill design, turbine speed, and airflow

restriction all had a significant effect on the dgw of cowpea flour; all three parameters

in addition to milling time had a significant effect on yield (table 3.4; α=0.05). The

modified wing mill configuration produced cowpea flour with a larger dgw and yield

(17.13 μm, 69.05%) than the original wing mill configuration (16.84 μm, 59.88%).

Turbine speed at 7200 rpm produced cowpea flour with a smaller dgw and greater

yield (16.80 μm, 74.45%) than a speed of 5400 rpm (17.18 μm, 54.44%). Restriction

of the airflow to 15o produced cowpea flour with a smaller dgw and yield (16.67 μm,

60

49.98%) whereas the 90o restriction produced cowpea flour with a larger dgw and

greater yield (17.30 μm, 78.95%). The only significant difference (α=0.05) in feed

method and milling time was that yield was higher at 5 min (69.55%) than for 1 min

(59.38%). Identifying a parameter that has the greatest effect on the end product is

not practical due to the dynamic nature of the system. Overall, the dgw for cowpea

flour produced in this portion of the experiment was less than 18 μm for the various

treatment regimens (table 3.3) with the exception of a modified configuration

operating at 5400 rpm using a 90o restriction for 5 min of milling producing the

largest dgw of 17.92 μm. Milling yield was significantly affected by mill configuration

and processing parameters. Table 3.4 shows that using the modified configuration

at a turbine speed of 7200 rpm with the restriction valve opening at 90o for a 5 min

milling time had a higher yield than the other corresponding conditions. Hence, the

modified configuration at a turbine speed of 7200 rpm for a 5 min milling time using

the continuous feed method was selected for the milling condition used for the seed

preconditioning study.

Preconditioning Treatments of Cowpea Seed

Table 3.5 outlines the preconditioning treatment levels of temperature (T0),

moisture, and airflow restriction along with the dgw, yield, and temperature of the

collected cowpea flour (Ti) milled in this portion of the experiment. The only apparent

trend in the various treatment regimens is that as the airflow restriction increased

(less airflow), sample yield decreased slightly. Interestingly, preconditioning the

seeds to 1.5% moisture produced a higher yield range (92.11% - 95.13%) when

compared to the yield ranges for 0.5% and 4% treatment regimens (85.82% -

95.69% and 85.15% - 90.84% respectively). Regardless of initial temperature of

61

cowpea (T0), final cowpea flour temperature (Ti) did not differ significantly (25.2 to

28.5oC).

Significant affects (α = 0.05) on the dgw and yield of cowpea flour due to the

preconditioning treatments and airflow restriction are shown in table 3.6. As

identified from findings reported in table 3.5, no cryogenic milling temperature effect

was observed in this experiment as shown by the dgw or yield of cowpea flour. Two

possible explanations for this lack of cryogenic temperature effect could be 1) the

total cowpea sample used for each milling (180 g) and its heat capacity were

relatively small compared to the thermal energy generated by the milling operation,

and/or 2) heat transfer rates to small particles are high when compared with other

operation using whole seeds.

Conditioning cowpea seed to 0.5% moisture caused the cowpea flour to have

a larger dgw (17.7 μm ±0.02) than that of the 4% moisture samples (17.4 μm ±0.02)

with 1.5% moisture samples being intermediate (17.48 μm±0.02). Preconditioning

cowpea seed to 1.5% moisture produced cowpea flour with the highest yield of

93.68%, whereas 4% moisture produced cowpea flour with the lowest yield of

89.51%. The 30o airflow restriction produced a lower cowpea flour yield of 89.63%

than the 60o and 90o openings which produced higher cowpea flour yields at 92.48

and 93.12%, respectively. Fennema (1996) stated that when moisture content is

below or at the BET monolayer, water is tightly bound and incapable of being

involved in most reactions. In this experiment, the moisture content was below the

monolayer of cowpea (4.23% at 200C) (Ayranci and Duman 2005) and this helped

minimize the potential impact of thermal energy and agglomeration associated with

milling operations of flour.

62

Conclusions

The particle size analysis of cowpea flour produced in this study can be

completed using either of the two sample preparation methods (dry flour or wet

flour). The main advantage of using the wet preparation method is based on the

intended use by the consumer in which cowpea flour would be in the hydrated form;

thus a hydrated sample measurement would be an appropriate means of particle

size measurement.

The results from this study demonstrated a reasonable degree of control, with

respect to dgw and yield, using the Super Wing Mill DM-200 for milling cowpea flour.

The modified wing mill configuration resulted in a substantial increase in cowpea

flour yield with minimal effect on the dgw. Turbine speed, airflow restriction, and

preconditioning cowpea seed moisture content exerted influential forces on the dgw

and yield of cowpea flour. Preconditioning of cowpea seeds at the selected

temperatures in this study showed no affect on the dgw or yield of cowpea flour.

Results from this study demonstrated that efficient milling of cowpea flour produced

under minimal sample treatment conditions (i.e. 4% moisture at 20oC) was

comparable to cowpea flour produced under extensive sample treatment conditions

(i.e. 0.5% moisture at -80oC).

Additional study of moisture content effects on the dgw of cowpea flour should

be conducted to explain why moisture content different than that of the monolayer

contributes significantly to particle size and yield. Moisture levels above the BET

monolayer of cowpea, in addition to a true cryogenic milling operation, could lead to

further control of the dgw and yield of cowpea flour.

63

Acknowledgements This study was supported by the Bean/Cowpea Collaborative Research

Support Program (Grant No. DAN-1310-G-SS-6008-00), U.S. Agency for

International Development and by State and Hatch funds allocated to the University

of Georgia Agricultural Experiment Station, Griffin Campus. The authors of this

paper would like to thank Mr. Glenn Farrell for his generous assistance in the design

modifications of the wing mill, Dr. Manjeet Chinnan for assistance in understanding

cyclone theory, and Dr. Nepal Singh for assistance in laser particle size analysis.

64

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control of a jet mill plant. Powder Technology 108(2-3):147-54. Goswami TK, Singh M. 2003. Role of feed rate and temperature in attrition grinding

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roasted peanuts. Transactions of the ASABE 32(3):968-72.

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Kethireddipalli P, Hung YC, McWatters KH, Phillips RD. 2002. Effect of milling method (wet and dry) on the functional properties of cowpea (Vigna unguiculata) pastes and end product (akara) quality. Journal of Food Science 67(1):48-52.

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Sakyi-Dawson E, Ngoddy P, Nnanyelugo D, Enwere J, Komey NS, Liu KS, Mensa-Wilmot Y, Nnanna IA, Okeke C, Prinyawiwatkul W, Saalia FK. 2003. Utilization of cowpeas for human food. Field Crops Research 82(2-3):193-213.

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66

Figure 3.1: (a) Original configuration of a Super Wing Mill DM-200 (b) Modified configuration of the Super Wing Mill DM-200 (a) (b)

1) Hopper (sample inlet) 2) Turbine housing 3) Control Panel 4) Airflow restriction valve (0, 15, 30, 45, 60, 75, and 90o positioning) 5) Cyclone particle separator 6) Sample collection vessel 7) Vortex Finder (airflow exit point from cyclone)

67

Table 3.1: Particle volumetric diameters (μm) at selected percentiles and calculated geometric mean diameter (dgw).1 Dry2 Wet3 20% 7.99 ± 0.36 a 7.54 ± 0.59 a

40% 15.25 ± 0.28 a 14.76 ± 0.66 b

60% 23.45 ± 0.47 a 23.26 ± 0.76 a

80% 36.00 ± 1.49 a 37.29 ± 1.57 a

dgw 15.91 ± 0.3 a 16.06 ± 0.78 a

a,b Mean values in a row not followed by the same superscript letter are significantly different (α = 0.05) 1 Values represent the average of 10 replicated particle size measurements 2 Dry cowpea flour PSD measurement is the application of the dry flour directly to the QSpec dispersion unit of the Mastersizer S 3 Wet cowpea flour PSD measurement is the application of a hydrated 10% cowpea flour solution (w/w) to the QSpec dispersion unit of the Mastersizer S.

68

Figure 3.2: Cluster plot of 20th, 40th, 60th, and 80th percentiles of dry flour vs. wet flour PSD measurements from a cowpea flour sample.

wetdry dd 9326.03591.1 +=

69

Figure 3.3: Cumulative undersize distribution by mass for wet flour and dry flour preparation methods. The steeper slope of the dry flour indicates smaller particle size.

( )( )xwet eP ln1462.1*647.13=

( )( )xdry eP ln1099.1*401.12=

70

Table 3.2: CV at specific percentiles (μm) for particle size measurement replications from the wet sample preparation method. Replicated Measurements

Percentile 10 8 6 4 3

20% 7.86 8.79 10.07 10.56 12.09 40% 4.48 5.07 5.99 6.71 8.03 60% 3.25 3.66 4.24 4.57 5.60 80% 4.21 4.68 4.94 5.19 5.99

71

Table 3.3: Averaged results1 of mill configuration, turbine speed, valve positioning, and milling time effects on geometric mean (dgw) and yield.

Config RPM Valve Time dgw (μm) Yield

1 16.92 37.17% 15o 5 16.69 58.26% 1 16.86 49.87%

540090o

5 17.92 80.79% 1 17.21 74.40% 15o 5 16.92 69.89% 1 17.29 87.62%

Modified

720090o

5 17.26 94.39% 1 17.24 37.84% 15o 5 16.90 39.19% 1 17.07 60.14%

540090o

5 17.82 72.26% 1 15.69 37.03% 15o 5 15.78 46.07% 1 17.13 91.00%

Original

720090o

5 17.08 95.55% 1Sample feed method was not included as no significant difference was observed for geometric mean or yield (table 3.4). Results from feed method were averaged under the respective milling time treatment levels.

72

Table 3.4: Effect of mill configuration, turbine speed, airflow restriction, feed method, and milling time on the geometric mean (dgw) and yield of cowpea flour milled in a Super Wing Mill DM-200. dgw (μm) Yield

Mill Design Modified 17.13 a 69.05% a Original 16.84 b 59.88% b

RPM 7200 16.80 b 74.45% a 5400 17.18 a 54.44% b

Valve Opening 90o 17.30 a 78.95% a 15o 16.67 b 49.98% b

Feed Method Batch 17.04 a 64.15% a

Continuous 16.93 a 64.79% a Milling Time

5 min 17.05 a 69.55% a 1min 16.93 a 59.38% b

a,b Mean values followed by the same superscript letters in the same column within each variable are not significantly different (α = 0.05)

73

Table 3.5: Averaged results of moisture content and temperature (T0) preconditioning of cowpea seed in addition to wing mill valve position affects on geometric mean (dgw), yield, and final temperature (Ti) of cowpea flour milling. Moisture Content

T0 (oC)

Valve Position

dgw (μm)

Yield Ti (oC)

30o 17.54 90.12% 27.0 60o 17.74 95.69% 27.1 -80 90o 17.67 94.07% 27.2 30o 17.98 85.82% 27.8 60o 17.14 92.79% 27.1 -20 90o 17.59 93.76% 27.2 30o 17.56 91.33% 27.3 60o 18.05 91.64% 27.5

0.50%

20 90o 18.03 93.12% 26.6 30o 17.34 92.11% 27.0 60o 17.46 94.06% 27.3 -80 90o 17.32 95.13% 28.0 30o 17.16 92.50% 28.0 60o 17.58 93.60% 27.6 -20 90o 18.04 94.91% 27.3 30o 17.65 93.07% 27.8 60o 17.60 93.60% 28.1

1.50%

20 90o 17.21 94.19% 27.5 30o 17.12 88.70% 27.7 60o 17.38 89.56% 27.7 -80 90o 17.65 90.46% 28.0 30o 17.18 87.87% 26.3 60o 17.44 90.78% 26.6 -20 90o 17.44 91.58% 25.2 30o 17.36 85.15% 28.3 60o 17.45 90.63% 28.5

4.00%

20 90o 17.55 90.84% 28.0

74

Table 3.6: Geometric mean (dgw) and yield of cowpea flour produced in a Super Wing Mill DM-200 as affected by preconditioning treatments of temperature and moisture along with airflow restriction variations.

dgw (μm) Yield Sample Temp (oC)

-80 17.47 ± 0.02 a 92.21% a -20 17.51 ± 0.02 a 91.51% a 20 17.61 ± 0.02 a 91.51% a

Moisture 0.50% 17.70 ± 0.02 a 92.04% b 1.50% 17.48 ± 0.02 ab 93.68% a 4.00% 17.40 ± 0.02 b 89.51% c

Valve Position 90o 17.61 ± 0.02 A 93.12% a 60o 17.54 ± 0.02 A 92.48% a 30o 17.43 ± 0.02 A 89.63% b

a,b,c Mean values in a row not followed by the same superscript letter are significantly different (α = 0.05)

75

Chapter 4

APPLICATION OF ATTRITION MILLED SOY FLOUR IN A BEVERAGE

APPLICATION

76

Abstract

A soy beverage mix was created using thermally treated, attrition milled soy

powder, anti-caking agent (1, 1.5, and 2%), and adjunct ingredients (salt, sugar,

carrageenan, calcium phosphate) commonly found in a soy beverage. Thermal

processing of defatted soy flake at elevated moisture levels (30, 35, and 40%) from

two different sources was evaluated for effective denaturation of the soy trypsin

inhibitor complexes. The best thermal treatment condition was then used to treat soy

flakes before milling in a cyclone assisted attrition mill. Soy beverage mixes were

homogenized with a handheld blender at solution temperatures of 4, 30, and 70oC.

Separation rates were highest at all treatment levels consisting of the 70oC solution

temperature (1.67 mL/min) and lowest for all treatment levels at 4oC (0.55 mL/min).

Thermal treatment of defatted soy flake resulted in solutions with lower viscosities

(87.32 and 151.90 cPoise) and faster separation rates (0.47 and 10.33 mL/min)

within respective treatment regimens. Results from this study demonstrated that

milling of defatted soy flake resulted in reducing particle size and hence lowering

viscosities as well as slowing the suspension separation of the soy beverage.

77

Introduction

Soymilk is a traditional Asian food produced from either the filtered aqueous

extract of ground, soaked soybeans or from a fine emulsion of defatted soybean

product (soy flour or protein concentrate) and water (Liu 1997). The most common

method of manufacturing soymilk involves the soaking of soybeans for specific times

and temperatures to optimize flavor and texture attributes of the final product,

grinding the soaked beans with an appropriate amount of water to give a specific

water:bean ratio, and then filtering insoluble material (okara) so that the aqueous

extract can be further processed and packaged. Consumer preference for soymilk

varies greatly for various reasons such as flavor, texture, viscosity, and cultural

background.

Thermal processing of soymilk to reduce trypsin inhibitor (TI) content (Kunitz

and Bowman-Birk) to 10% of native soybean has been shown to produce parallel

nutritional improvement of soy protein (Hackler and others 1965). Animal feeding

studies have shown adverse effects resulting in pancreatic hypertrophy and growth

suppression (Grant 1989; Gumbmann and others 1989; Friedman and Brandon

2001) which have caused soy food processors to continue the thermal processes

that reduce TI content. Reseland and others (1996) conducted a study using 6

human participants to identify the so-called anti-nutritional effects of TI and observed

that in humans, the trypsin and chymotrypsin secretion systems have a different

feedback mechanism than in rats or chicks commonly used in previous studies.

Their observation suggests that the anti-nutritional effects of soy may not be as

detrimental to human growth and health as previously thought. Additionally, a review

on the health benefits of soy foods identified recent studies and suggests that the

Bowman-Birk inhibitor has anticarcinogenic effects (Friedman and Brandon 2001).

78

Soymilk production using the traditional process involves the removal of

insoluble particulates from the beverage before further processing. On a household

level, this is achieved by using a cloth sack whereas in industry, the filter is generally

the equivalent of a 400 mesh screen (38 μm) (Liu 1997). The relatively large (25 –

37 μm) residual particulates left in the extract can cause problems with

sedimentation or, depending on the size and quantity of the particulates, either a

gritty or grainy mouth feel (Tyle 1993). Another drawback to the traditional method is

that macronutrients, mainly fiber and some protein, are removed with the okara

which could be otherwise utilized if they were incorporated in the extract. Despite

these potential negative factors, the popularity of soymilk in the United States has

increased greatly in recent years with the approval of the FDA health claim that

consumption of 25 g of soy protein a day as part of a diet low in saturated fat and

cholesterol may reduce the risk of heart disease (USFDA 2001).

The use of soy in U.S. consumer markets is mainly limited to soy oil with the

resulting defatted soy product going into livestock feed as the primary protein source

(Liu 1997). Many consumer products utilize soy flours such as soy protein isolates

or soy flour in the health food consumer market or in the meat processing industry as

a means of improving water and fat retention (Riganakos and others 1994; Reema

and others 2004; Tripathi and Misra 2005). Soy beverage processing methods using

these defatted products have been developed; however, due to insufficient

manufacturing processes, the success was limited (Steinkraus 1973). There are

currently several products on the market aimed at creating a powder-based soy

beverage, but these products are primarily the “protein shake” style of beverage.

Microparticulation, also known as attrition milling, is often utilized in

combination with air classifiers when a small and controlled particle size of the final

79

product is desired. Microparticulation of food particles has been shown to offer

various benefits in food systems and in some cases altering the functional properties

of ingredients that would otherwise be unobtainable with the larger particle sizes

(Hayakawa and others 1993). Alteration of particle size can have a number of

effects upon a food system with the primary effects being the physicochemical

changes due to the increase of a particle’s surface area (Schubert 1987). In

beverage systems, reducing the particle size of an ingredient can improve

dispersibility, improve the overall stability of a solution, alter viscosity, and reduce

gritty/grainy tactile sensations when compared to a larger particle size (Park and

others 2001).

The aim of this study was to identify the effects of attrition milling of defatted

soy flake and incorporate attrition milled soy flour into a soy beverage-like system

that would resemble “instant soy milk”. A simulated thermal treatment of defatted

soy flake for eliminating or reducing anti-nutritional factors of soybean was also

evaluated.

Materials and Methods

Materials

Two types of defatted soy flake were used to produce the soy flours used in

this study. The first defatted soy flake was derived from L-Star® soybean (American

Soy & Tofu Corporation of America, Macon, GA) which is free of all three

lipoxygenase enzymes and has a higher content of tocopherols (5.4% vs. 1.8%)

when compared to standard soybean (America 2006). The L-Star® defatting

process involves using propane as an extraction solvent under a mild pressurized

environment resulting in less thermal energy generated during the solvent removal

process. The second defatted soy flake sample was obtained from USDA #2 soy

80

beans (Archer Daniels Midland, Decatur, IL) which was created using a flash solvent

extraction process for the removal of hexane-based solvents; this resulted in

comparatively higher thermal energy when compared to the oil extraction process of

the first soy flake sample. Moisture content of both soy flours was determined using

a vacuum oven (25mmHg) at 80oC for 18 hrs.

Thermal Treatment of Soy Flake

Van den Hout and others (1999) identified kinetics parameters at different

moisture contents of soy flour to reduce or eliminate Kunitz and Bowman-Birk TI. A

model was proposed (eqn 1) that accounts for two different first-order reaction rate

constants of the two TI complexes in order to predict the thermal treatment reduction

of soy TI.

( ) tktki eAAeCC

21 10

−− −+= (1)

Where Cx is the measured or estimated TI activity at time t (mg (g ds)-1), A is the

activity fraction (%) of the Kunitz TI at t=0, k1 is the reaction rate constant for the

Kunitz TI ((mg (g ds)-1)1-n s-1),, and k2 is the reaction rate constant for the Bowman-

Birk TI ((mg (g ds)-1)1-n s-1), and t is time in seconds. Establishment and

determination of reaction rate constants as well as activity fractions (A) for each of

the TI groups are discussed in detail by van den Hout and others (1999). In this

experiment the reference temperature used for thermal modeling was 110oC.

Soy flakes from each source were conditioned to moisture levels of 30, 35,

and 40% using a food processor (GE® 4-speed food processor, Model 106622F,

General Electric, Fairfield, CT) by addition of appropriate amounts of water and soy

flake and blending briefly without the generation of heat. The conditioned samples

were equilibrated in sealed plastic bags at 10oC for 48 hrs before thermal

81

processing. Thermal processing consisted of placing partially hydrated soy flake in

an aluminum pan with an average thickness of 5-8 mm, heating the product to 110oC

(verified with a digital thermometer equipped with thermal couples) for 5 min, then

rapidly cooling the product in a walk-in cooler (4oC). After the product was

completely cooled, thermally treated soy flake was then dried to 4% moisture in a

vacuum oven (25 mmHg, 50oC, 18 hrs).

Milling of Soy Flour

Milling of non-thermally and thermally treated soy flake (40% moisture,

110oC, 5 min) was done using a Super Wing Mill DM-200 (Sanwa Engineering Co.,

LTD., Japan) with modifications as described in Jarrard and Hung (2006).

Operational parameters for the milling of soy flour consisted of a modified mill

configuration with a turbine speed of 7200 rpm, a 90o airflow restriction valve

position, and a milling time of approximately 6 min. The predicted geometric particle

mean diameter (dgw) of milled soy flour under these parameters is less than 20 μm

(Jarrard Jr. and Hung 2006). Particle size distribution of the milled soy flour was also

measured using a Mastersizer S laser diffraction measurement system with a QSpec

small volume sample dispersion unit operating at 3000 rpm (Malvern Instruments,

Worcestershire, U.K.) as described in Jarrard and Hung (2006). Geometric mean

particle diameter (dgw) was calculated according to ASABE standard S319.3 (ASABE

2003).

Trypsin Inhibitory Assay

Thermally treated soy samples were assayed for residual TI based upon the

hydrolysis of Nα-benzoyl-DL-arginine 4-nitroanilide hydrochloride (BAPNA) by trypsin

adjusted for use in a multi-plate spectrophotometer as described in van der Ven and

82

others (2005). Trypsin (2x crystalline, salt free, 1610 BAEE units/mg) and Nα-

benzoyl-DL-arginine 4-nitroanilide hydrochloride (BAPNA) were purchased from

Sigma-Aldrich (St. Louis, MO). A SpectraMax Plus384 multi-plate reader (Molecular

Devices, Sunnyvale, CA) with data acquisition and preliminary kinetic analysis

performed by operating software SoftMax® Pro (Molecular Devices, Sunnyvale, CA)

was used to measure the trypsin hydrolysis of BAPNA. An increase of TI content in

soy flake sample will result in a reduction of hydrolysis as indicated by a lower

millioptical density (mOD) reading. A high mOD reading indicates lower contents of

TI.

TI complex was extracted from milled soy samples as described by AACC

method 22-40 (AACC 2000) by mixing 0.95 to 1 g of defatted, ground soy sample in

50 mL of 0.01N NaOH and slowly stirring with a magnetic stir bar for 3 hours. Soy

extracts were subsequently centrifuged (5 min at 18,000 g) and the supernatant was

used for the assay. Reference and calibration of the assay was measured using

purified Kunitz soybean trypsin inhibitor (KSTI) suspended in Tris buffer (0.05M

containing 20 mM CaCl2, pH 8.2) at a concentration of 1mg/mL; trypsin was

suspended in 0.001N HCl at a concentration of 0.4mg/mL. A total of 150 μL

consisting of 50 μL buffer and 100 μL trypsin was added to the first well as the blank.

Subsequent wells contained 150 μL of buffer, trypsin, and TI (soy extract or KSTI) in

a 1:3 dilution scheme. The microtiter plate was incubated for 10 min at 37oC to allow

for binding of TI and trypsin. Fifty μL of BAPNA (4 mM dissolved in 50:50 dimethyl

sulfioxide: Tris buffer [v:v] room temperature) was immediately added to each well.

The plate was immediately scanned for BAPNA hydrolysis using a multi-plate

spectrophotometer at 405 nm for 5 min with a temperature chamber set at 37oC and

83

scanning every 15 sec (3 sec plate shake prior to scan). Reaction kinetics were

measured and then reported as an arbitrary endpoint in mOD.

The IC50 for KSTI was calculated by obtaining the linear regression (mOD vs.

μg of inhibitor) over the linear portion of the reaction curve to obtain vmax (y-intercept).

Concentration of KSTI at half of vmax (vmax/2) was used as the IC50 for KSTI; IC50

values for soy samples were calculated in a similar manner. The IC50 was used to

represent the concentration of TI needed to inhibit 50% of the trypsin used under the

experimental conditions of the assay. The TI content per gram of soy sample was

then calculated and expressed as mgTI/gsample.

Water Holding Capacity

Water holding capacity (WHC) for ADM and L-Star® soy flake and powders

was determined before incorporation of adjunct ingredients as described by

Kethireddipalli and others (2002) with modifications. Soy sample in triplicate (0.2 to

0.25 g) was combined with 20 mL of deionized water (25oC) in 20 x 125 mm screw-

cap test tube and vortexed (Vortex-Genie™, model K-550-G; Scientific Industries,

Inc., Bohemia, NY, USA) for 1 min at a speed setting of 10. The samples were

allowed to hydrate at room temperature for 10 min and then vacuum filtered through

Whatman No. 50 filter paper until all residual water was filtered off. The wetted filter

paper and hydrated sample were weighed together as is. WHC was calculated as

defined in eqn 2 and reported as grams of water held per gram of dry sample.

84

(2)

Soy Flour Mixtures

Five soy flour mixture variables were used in this study and consisted of

starting material (ADM soy flake vs. L-Star® soy flake), with or without additional

milling of the soy flake, thermal (40% moisture, 110oC, 5 min) and non-thermal

treatment of soy flake, level of anti-caking agent (1, 1.5, and 2%), and temperature of

deionized water (4, 30, or 70oC for soy drink preparation. Calcium phosphate was

purchased from Fischer Chemical (Fairlawn, NJ), a carrageenan/maltodextrin mix

ideal for cold water solubility was purchased from TIC Gums (TIC Pretested® Colloid

775 Flour, TIC Gums, Inc., Belcamp, MD), anti-caking agent was purchased from

Degussa (Sipernat 500ls, 4.5 μm, Degussa, Akaron, OH), and salt and

confectioner’s sugar were purchased from a local retailer (Griffin, GA). Commercial

instant soy beverage powder “Fat Not! Crème it!” was obtained from Dixie USA, Inc.

(Tomball, TX). Pre-weighed amounts of each ingredient were mixed together and

then hand sifted 3 times using a mesh flour sifter and stored in sealed glass jars at

room temperature.

Soy Drink Preparation

Soy suspensions were created using 15.75 g of soy flour mixtures and

132.25 g of deionized water at the temperatures previously listed. The level of the

soy flour added to the suspension was calculated as having 6.25 g of soy protein per

serving (based on 240 mL serving) to comply with the FDA health claim for soy

protein and the reduced risk of cardiovascular disease (USFDA 2001). The soy flour

mix was placed into a 400 mL beaker and then water (at a specified temperature)

(Filtered Sample with paperg) – (wet paperg)

dry sampleg WHC =

85

was added to the beaker. The two components were mixed with a handheld blender

(Bamix® Mono, Mettlen, Switzerland) and then homogenized on low speed while

minimizing foaming of the suspension.

Separation Rates One hundred mL of soy suspension was transferred into a 100 mL graduated

cylinder. At predetermined time increments the level of suspension separation was

recorded and reported as milliliters of separation. After 30 minutes, the suspensions

were shaken by hand for 10 seconds and then observed for suspension separation

for an additional 10 min. Slopes of the initial separation phase (t=10 min) and

separation time after shaking (t=10 min) were calculated and reported as rate of

separation (mL/min).

Viscosity Measurement

Apparent viscosity of soy suspensions (10 mL) was measured in triplicate, at

room temperature with a Brookfield Viscometer (Model LVDV –II+, Brookfield

Engineering Laboratories, Inc., Stoughton, MA) using a small sample adapter and a

number 31 cylinder spindle at 60 rpm. Results of triplicate measurements were

averaged and reported in centipoises (cP).

Statistical Analysis

Data were analyzed using the general linear model (GLM) procedure, and

mean separation tests were performed using Duncan’s multiple range test (SAS

2003).

86


Trypsin Inhibitor Assay

The addition of BAPNA in the TI assay is a time-sensitive step such that a

difference of a few seconds can cause a difference in mOD readings within the same

sample set. As such, the values obtained for TI (mg/g) were not analyzed for

statistical significance but taken as is (table 4.1). Kwok and others (1993) state that

TI content of foods intended for human consumption should have 10 – 20% residual

TI when compared to native soybean. This paradigm of TI was originally intended to

create safe food for consumers; however, it has also been observed that

improvement of the nutritional quality of soy proteins parallels TI inhibition up to 90%

(Hackler and others 1965). The thermal treatment process used in this study

showed little to no reduction of TI content when processed samples were compared

to the respective unprocessed samples (e.g. ADM soy flake with no thermal

treatment had 49.91 mgTI/g vs. ADM soy flake at 40% moisture thermal treatment

had 52.17 mgTI/g) with the exception of the 40% moisture treatment of L-Star® soy

flour showing a reduction of 66.73 mgTI/g from the non-thermally processed L-Star®

soy flour. TI content of ADM soy flake and flours compared to native soybean

showed virtually no change due to thermal processing with the overall residual TI

content being approximately 30% of native soybean. The relatively low initial TI

content of ADM soy flake could be attributed to either the solvent removal process or

changes in USDA #2 soybean growing practices causing this initial reduction. L-

Star® soybean is a specific strain of soybean developed to contain no lipoxygenase

enzymes along with higher levels of tocopherols. A comparison of L-Star® to raw

soybean (Liu 1997) shows that the TI content is relatively similar to the common

soybean but appears to be more susceptible to thermal treatment when compared to

87

ADM soy flake at 40% moisture content vs. the 30 or 35% moisture levels in terms of

TI reduction. However, additional study should be conducted to accurately validate

this observation.

Milling of Soy Flour

The dgw of soy flour milled under the conditions in this study are reported in

table 4.2. Statistically there was no difference in dgw or sgw (α = 0.05, data not

shown) based on the type of soy flake or if the soy flake was thermally treated at

40% moisture. In a study using the Super Wingmill DM-200 for the milling of cowpea

flour, similar dgw’s (16 – 19 μm) were produced using similar processing parameters

(Jarrard Jr. and Hung 2006). Overall, the L-Star® powders had a smaller dgw under

the same treatment conditions when compared to the ADM powders; for example,

milled (“yes”) and thermally treated (“yes”) L-Star® flour had a dgw of 18.70 μm

whereas ADM had a dgw of 21.40 μm. Results of particle size analysis indicate that

the thermal treatment of soy flake at 40% moisture did not affect the final dgw of

attrition milled soy flour.

Soy Suspensions

Soy suspension studies were conducted under the premise of an “instant

beverage” use. Suspension separation measurements (mL) are reported for 1, 5,

and 10 min intervals as well as the rate of separation for the initial 10 min and the

rate of separation after suspensions were remixed for 10 seconds for a 10 min time

period (table 4.3) for suspension temperature and anti-caking treatment levels.

The level of anti-caking agent used showed no significance (α=0.05) for level

of separation at selected time intervals or for separation rates before and after

suspension remixing. Anti-caking usage at 1.5% yielded the most viscous

88

suspensions (144.94 cP) with 1% being the least viscous (111.79 cP). In an

unrelated study (data not shown) the apparent viscosity of Silk® brand soy milk (5.1

cP, WhiteWave Foods Co., Boulder, CO) was measured using the same viscometer

but with a #16 cylinder spindle operating at 60 rpm. Generally speaking, the

apparent viscosity for a representative commercial brand of soy milk has a lower

viscosity than the suspensions prepared in this study. Measurement of the

commercial beverage under the same conditions used in this study could not be

accomplished due to the reading being outside the measurement scale.

Initial suspension temperatures of 4o and 30oC at the time of homogenization

(t=0) showed no significant difference (α=0.05) for level of separation or separation

rates. The initial suspension temperature of 70oC was significantly different

(α=0.05) from 4 and 30oC for all levels of separation at 1, 5, and 10 min in addition

to both separation rates before (1.67 mL/min) and after (1.05 mL/min) remixing the

suspension. Under the conditions of this experiment, it is apparent that the 70oC

suspension temperature caused rapid separation of the suspension. Additionally,

the separated solid mass appeared to resemble a gel-like structure. This could be

attributed to either soy proteins aiding in this formation or the carrageenan forming

this network. Viscosity of the soy suspensions was highest for 30oC at 193.93 cP

with 70oC showing the lowest viscosity at 80.46 cP. However, this data is unreliable

based upon visual appearance of the suspension at the time of measurement. If a

gel structure formed upon cooling to room temperature, the remixing of the

suspension would create smaller gel-like structures giving the appearance of

uniformity but allowing for greater slippage at the surface of the cylinder surface; thus

a false reading would be obtained.

These results indicate that the soy suspensions prepared in this study would

be ideally suited for cold suspensions. Due to the rapid separation observed with the

89

70oC suspension temperature and the unusually low viscosity measurements, further

data analysis did not include the 70oC treatment.

Table 4.4 illustrates the averaged treatment level effect upon the suspension

separation at specific time intervals (1, 5, and 10 min), the rate of suspension

separation before and after remixing (mL/min), viscosity (cP), and WHC (gwater/gsolid)

of soy flake or soy powder. WHC was measured at room temperature in triplicate

and reported as grams of water held per gram of dry solid (gwater/gsolid). The origin of

soy flake, milling of soy flake, and thermal treatment process had no significant affect

(α=0.05) upon the WHC. Although no significant difference in WHC was shown, a

general trend of thermal treatment and milling of soy flake yielded a lower WHC than

a soy flake that was non-thermally treated. The WHC of soy material was

considerably lower than that of cowpea cellular material (5.33 – 19.18 gwater/gsolid), but

similar to that of cowpea pastes(1.42 – 3.80 gwater/gsolid) as reported by Kethireddipalli

and others (2002). Kerr and others (2000) observed a similar trend in cowpea flour

intended for use in snack chips where decreasing the particle size of cowpea solids

was shown to have lower water holding capacity values. The dgw of the soy flour

from this experiment had an average size of 18 μm indicating that low WHC values

were related to reduced particle sizes.

The separation trends as affected by treatment were relatively similar with the

exception of the thermally treated L-Star® soy flake that had been milled. A possible

explanation for this would be that the protein structures had a greater extent of

denaturation due to the thermal treatment process at 40% moisture. The TI assay

showed that this particular treatment had the only notable reduction in TI content; TI

reduction has also been associated with protein nutritive quality improvement which

translates to a mild degree of protein coagulation (Hackler and others 1965). The

possible protein denaturation associated with this treatment level would limit or

90

completely inhibit the functionality of the soy flour in suspension (i.e. remain in

suspension). Separation rates were notably higher for the thermally treated L-Star®

and commercial samples compared to other treatment levels. The ADM non-

thermally treated and milled sample was not significantly different in separation rate

(initial slope) from the other treatments, but this could be attributed to a small

experimental error as the value is close to that of other treatment levels.

Conclusions

The sensitivity of the TI assay is strongly dependent upon concentration of all

reagents used, specifically the time that the BAPNA reagent is in contact with the

enzyme system. TI content of thermally treated soy flake at 30, 35, and 40%

moisture content showed minimal to no reduction of TI when compared to the

respective untreated samples. Initial TI content of ADM soy flake showed an

average of 30% residual TI for all samples analyzed when compared to common raw

soybean as reported by Liu (1997). Cyclone assisted attrition milling of soy flake

using a Super Wingmill DM-200 produced a final product with an average dgw of 18

μm, which is consistent with the dgw for similar operational parameters used for

production of cowpea flour in earlier studies. Hydration properties (WHC) for this

range of dgw are in agreement with the paradigm that smaller particle sizes reduce

WHC. Creating suspensions from soy flour that had been subjected to thermal

treatment (40% moisture, 110oC, 5 min) and attrition milling produced suspensions

with an overall low viscosity. The temperature of the water used for homogenizing

the suspensions showed that relatively high temperatures produced unstable

suspensions whereas lower temperatures produced stable suspensions. In most

cases, the separation rates of the soy flour suspensions after remixing for 10 sec

were generally lower than the initial separation rates prior to remixing. The levels of

91

anti-caking agent used in this study showed no effect on the rate of suspension

separation, but did show differences in viscosity with the highest viscosity value

associated with 1.5% concentration of anti-caking agent.

Results from this study establish the basis for an instant soy beverage

system. Low temperature suspensions, using the reduced dgw soy flour and either

form (ADM or L-Star®) of soy flake, can produce a stable suspension within a 10 min

period without suspension agitation; however, optimization of the formulation should

be further explored.

Outlook

Future study should be conducted on the thermal treatment of soy flake prior

to attrition milling with the intent of reducing TI content to less than 15% of native

soybean. Health-related issues are debatable for the reduction of TI, but protein

nutritive values do increase in a parallel manner with the inhibition of TI due to

thermal treatment. As such, the protein nutritive value as affected by these thermal

studies should be measured to confirm previous findings. Additional suspension

studies should further be explored so as to identify how and why the observed gel-

like structures formed over the course of this experiment with specific focus on the

hydrocolloid interactions that appear to be the primary contributor to this effect.

Acknowledgements

The researchers of this paper would like to acknowledge Ky-anh The Nguyen

for his generous assistance with calibrating and conducting the TI assay.

92

References

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ASABE. 2003. Method of Determining and Expressing Fineness of Feed Materials by

Sieving (S319.3). ASAE Standards. p 589. Friedman M, Brandon DL. 2001. Nutritional and health benefits of soy proteins.

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Nutrition Science 13(3-4):317-48. Gumbmann MR, Dugan GM, Spangler WL, Baker EC, Rackis JJ. 1989. Pancreatic

response in rats and mice to trypsin-inhibitors from soy and potato after short-term and long-term dietary exposure. Journal of Nutrition 119(11):1598-609.

Hackler LR, Vanburen JP, Steinkra.Kh, Rawi IE, Hand DB. 1965. Effect of heat

treatment on nutritive value of soymilk protein fed to weanling rats. Journal of Food Science 30(4):723-728.

Hayakawa I, Yamada Y, Fujio Y. 1993. Microparticulation by jet mill grinding of

protein powders and effects on hydrophobicity. Journal of Food Science 58(5):1026-9.

Jarrard Jr. M, Hung Y-C. 2006. Milling of cowpea flour using cyclone assisted milling.

Applied Engineering in Agriculture (Submitted). Kerr WL, Ward CDW, McWatters KH, Resurreccion AVA. 2000. Effect of milling and


Kethireddipalli P, Hung YC, Phillips RD, McWatters KH. 2002. Evaluating the role of


Kwok KC, Qin WH, Tsang JC. 1993. Heat inactivation of trypsin-inhibitors in soymilk

at ultra-high temperatures. Journal of Food Science 58(4):859-62. Liu K. 1997. Soybeans: Chemistry, Technology, and Utilization. New York: Chapman

& Hall. xxvi, 532 p. Park DJ, Imm JY, Ku KH. 2001. Improved dispersibility of green tea powder by

microparticulation and formulation. Journal of Food Science 66(6):793-8.

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Reema, Hira CK, Sadana B. 2004. Nutritional evaluation of supplementary foods prepared from germinated cereals and legumes. Journal of Food Science and Technology-Mysore 41(6):627-9.

Reseland JE, Holm H, Jacobsen MB, Jenssen TG, Hanssen LE. 1996. Protease

inhibitors induce selective stimulation of human trypsin and chymotrypsin secretion. Journal of Nutrition 126(3):634-42.

Riganakos KA, Demertzis PG, Kontominas MG. 1994. Water sorption by wheat and

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particulate food systems. Journal of Food Engineering 6(1):1-32. Steinkraus KH, inventor, Cornell Research Foundation, Inc., assignee. 1973. Method

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94

Table 4.1: Trypsin inhibitor content of soy samples (mg/g) at respective moisture levels1 and percent residual inhibitor when compared to a standard raw soybean.

TI (mg/g) %Residual Raw Soy2 171.00 -

L-Star No Treatment 123.54 72.24%

30% 122.44 71.60% 35% 130.75 76.46% 40% 56.81 33.22%

ADM No Treatment 49.91 29.19%

30% 54.86 32.08% 35% 54.47 31.85% 40% 52.17 30.51%

1 Moisture levels are in reference to moisture content at time of thermal processing (110oC for 5 min.) 2 Data obtained from Liu (1997)

95

Table 4.2: Experimental regimen effects on geometric particle mean diameter (dgw) and standard geometric deviation (sgw) of soy powder milled using a Super Wingmill DM-200.

Soybean Thermal Milleddgw

(μm) sgw

(μm) yes yes 21.40 0.03 ADM no yes 16.66 0.01 no no 58.86 0.10 yes yes 18.70 0.02 L-star no yes 15.66 0.02

96

Table 4.3: Soy suspension properties of viscosity and separation at 1, 5, and 10 min in addition to separation rates before and after remixing of suspensions. Suspension Separation1 Viscosity

1 min (mL)

5 min (mL)

10 min (mL)

Initial Slope

(mL/min)

Recovery Slope

(mL/min) (cP)

Suspension Temp 4o 0.07b 1.37b 4.93b 0.55b 0.12b 109.66b

30o 0.00b 2.28b 7.09b 0.79b 0.82b 193.93a 70o 3.56a 12.25a 19.84a 1.67a 1.05a 80.46c

Anti-Caking Level 1.0% 0.90a 4.17a 8.63a 0.86a 0.63a 111.79c 1.5% 1.27a 5.90a 11.10a 1.09a 0.69a 144.94a 2.0% 1.70a 6.47a 11.27a 1.05a 0.52a 127.31b

a,b Values in the same column within the same treatment level followed by different letters are statistically significant (a = 0.05). 1 Suspension separation was recorded every minute for the first 10 min and then at 20 and 30 min time intervals. After remixing, the separation was recorded every 2 min.

97

Table 4.4: Averaged data for soy suspension separation, viscosity, and WHC as affected by treatment. Suspension Separation1 Viscosity WHC

Starting Material

Thermal Treatment Milled 1 min

(mL) 5 min (mL)

10 min (mL)

Initial Slope

(mL/min)

Recovery Slope

(mL/min) cP (gwater/gsolid)

No yes 0.00 1.08b 5.50abc 0.58ab 0.58a 181.00a 2.75a ADM

Yes yes 0.00 0.33b 5.08bc 0.47b 0.31a 87.32c 2.05a no 0.00 1.50b 4.08c 0.41b 0.45a 173.68ab 2.82a

No yes 0.00 0.00b 4.50c 0.42b 0.58a 165.08ab 1.91a L-Star

Yes yes 0.17 5.75a 10.33ab 1.13a 0.54a 151.90b 1.97a Crème No no 0.00 5.00a 10.50a 1.07a 0.28a - -

1 Suspension separation was recorded every minute for the first 10 min and then at the 20 and 30 min time intervals. After remixing, the separation was recorded every 2 min. a,b,c Values followed by different letters within the same column are significantly different (α=0.05)

98

Chapter 5

EFFECT OF MILLING METHOD ON THE PHYSICAL PROPERTIES OF

COWPEA FLOUR AS AN INGREDIENT IN MOIN-MOIN (STEAMED

PASTE)

99

Abstract

Moin-moin is steamed cowpea paste native to Nigeria. This product is

classified as a protein/starch gel with the dominate portion of the gel network

consisting of cowpea starch. Moin-moin was prepared from starting materials

(cowpea meal and cowpea flour) and compared to traditional moin-moin prepared

from dry, whole, undecorticated cowpea seeds. Texture profile measurements

showed that moin-moin made from cowpea flour (small particle size) formed a firmer

structure when compared to moin-moin made from either cowpea meal or whole, dry

cowpea seeds. Starting materials with smaller particle sizes and longer cooking

times produced stickier moin-moin. Generally, color was unaffected across

treatment regimens as identified by an expert sensory panel, although instrumental

color measurements showed significant differences for hue angle and chroma. This

study demonstrated that the particle size of a food ingredient contributes significantly

to its functionality in food formulations.

100

Introduction

Cowpea (Vigna unguiculata), more commonly known in the United States as

the black-eyed pea, is a staple food source for many populations throughout the

world, primarily in East and West African countries (Ehlers and Hall 1997;

Kethireddipalli and others 2002a). Fresh cowpea that is processed into a paste,

provides the basis for many traditional West African dishes; however, the traditional

process by which the paste is produced often involves large quantities of water and

manual labor (Dovlo and others 1976). Several studies have evaluated the use of

milled cowpea for production of a traditional West African product that is essentially

deep fried balls made from aerated cowpea paste (Akara); this product is consumed

in a similar manner as a hushpuppy, a fried cornmeal fritter popular in the

Southeastern United States (Davidson 1999; Singh and others 2004; Singh and

others 2005; Plahar and others 2006). The successful production of an aerated

paste from cowpea meal or flour signifies that convenience food products can be

used in place of traditional methodologies for preparation of popular foods without

sacrificing consumer quality attributes.

Moin-moin is a popular Nigerian dish prepared from steamed cowpea paste

as described by Dovlo and others (1976). The exact form of this product can be

highly variable as paste consistency, mixing temperature, cooking (steaming) time,

and cooking container or mold shape can all affect the properties of the final

structure. The general formulation of the product consists of cowpea, water, oil, and

salt with optional ingredients consisting of egg, chopped shell fish, chili peppers,

tomato puree, onion, and green pepper (Dovlo and others 1976). It is often served

with either cereals or over grains (e.g. rice) in a pudding-like fashion in either hot or

cold form.

101

Previous research on West African consumer acceptability of moin-moin

found that cowpea particle size played a significant role and that the ideal form of dry

milled cowpea starting materials should have 70-80% of the particles distributed

between 45 – 150 μm (Ngoddy and others 1986). Okechukwu and others (1991b)

found that cowpea slurries (1.8 – 3.4 water:solids ratio) had a starch rigidity onset

occurring slightly above 70oC with final gelatinization temperature of the slurry

system (8% oil, 1% salt, and cowpea solids) generally around 86oC. Ossai and

others (1987) found that deformation force curves of cowpea gels decreased with

increasing oil content but did not observe any significant response from salt

concentrations as noted in a later study by Okechukwu and others (1992).

Each of these studies on moin-moin focused on a particle size range as

identified by Ngoddy and others (1986). Kerr and others (2001) found that in the

production of cowpea snack chips, cowpea flour with reduced particle size

distributions (PSD) decreased the onset temperature of starch gelatinization (To).

Okechukwu and others (1991b) reported that textural variations of cowpea gels are

expected to reflect the distribution of maximum temperature attained in the course of

gelation. The general consensus of the previously described studies was that a

greater content of damaged starch granules occur in finer milled cowpea flour which

allows for greater extraction and unfolding of starch molecules which can then

contribute to a more extensive gel network. This gel network has also been shown

to have peak force values that will diminish at prolonged exposures to moist heat at

temperatures in excess of pasting temperature, which is typical of starch-based gels

(Okechukwu and others 1991b; Fennema 1996).

In this study, moin-moin based on the formulation of Ngoddy and others

(1986) was used to evaluate how the form of cowpea, particle size of a convenient-

to-use milled starting material, and concentration of cowpea solids affect the final

102

product properties using a combination of objective measurements and sensory

analysis.

Materials and Methods

Materials

Cowpea of the California Cream variety (breeding line UCR 97-15-33, Inland

Empire Foods, Riverside, CA) was used in this study. Whole, dry cowpeas to be

used as imbibed seeds were immersed in tap water for 18 hrs in a sealed container

at room temperature. Cowpea to be milled into flour or meal was conditioned to 4

±0.1% moisture in a forced air oven (model no. 8107; The Electric Hotpack Co., Inc.,

Philadelphia, Pa., U.S.A) at 55oC based on results from a previous study on cyclone

assisted attrition milling of cowpea (Jarrard Jr. and Hung 2006). Cowpea flour was

milled using a Super Wing Mill DM-200 (Sanwa Engineering Co., LTD., Japan) as

described by Jarrard Jr. and Hung (2006). Cowpea meal was produced using a

Hammer mill with a 2.30 mm screen (Champion, Model no. 6X14, Champion

Products, Inc., Eden Prairie, MN, USA) as described by Singh and others (2005).

Measurement of dry milled cowpea (meal and flour) particle size was done using a

Mastersizer S laser diffraction measurement system with a QSpec small volume

sample dispersion unit operating at 3000 rpm (Malvern Instruments, Worcestershire,

U.K.) in the wet sample preparation method described by Jarrard Jr. and Hung

(2006). Geometric particle mean diameter was calculated as described in previous

studies with cowpea attrition milling (Jarrard Jr. and Hung 2006). It should be noted

that all cowpea used for the preparation of moin-moin was undecorticated (seed coat

remained intact). Salt, vegetable oil, tomato puree, liquid smoke, and onion were

purchased from a local grocery store (Kroger, Griffin, GA).

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Moin-moin Preparation

Moin-moin was prepared as previously described by Ngoddy and others

(1986) using a handheld blender (Bamix® of Switzerland, Model “Mono” 122/133,

Mettlen, Switzerland) to properly mix ingredients (table 5.1).

Moin-moin prepared from either cowpea meal or flour was partially hydrated

with 37% of the total formulation water at 70oC for 2 min before addition of the

remaining water; all water added to the moin-moin batter was heated to 70oC as

suggested by Ngoddy and others (1986). Moin-moin prepared from dry, whole

seeds used a hand held blender to puree the imbibed seeds along with all other

ingredients prior to the addition of 70oC water. Moin-moin batter (75g) was portioned

into glass ramekins (9 cmid x 3.5 cmh) coated with non-stick spray, loosely covered

with a lid, and then steamed in a household stovetop steamer (headspace

temperature was approximately 80oC) for either 20 or 40 min at ambient pressure.

Moin-moin samples to be evaluated by an expert sensory panel were removed from

the steamer and allowed to rest for 5 min uncovered and then removed from the

ramekin and allowed to rest uncovered for an additional 2 min to ensure handling

safety by the panelists. Moin-moin samples to be evaluated by objective

measurements were allowed to cool to room temperature uncovered (approximately

40 min) and then removed from the ramekin for measurement. Nutrient composition

of moin-moin samples was calculated using Master Cook v.8.0 (ValueSoft 2005).

Objective Measurements

Texture measurement

Texture Profile Analysis (TPA) and puncture tests were conducted using an

Instron Universal Testing Machine (Model 5544, Instron, Inc., Canton, MA) fitted with

a 2000 N load cell. TPA tests were conducted on duplicated cored samples (18 mmd

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x 12 mmh) of moin-moin (total of 6 measurements per treatment regimen) using a

stainless steel plunger with a 58 mm diameter to compress the cored sample twice in

reciprocating motion each time to 25% of the original height at a cross head speed of

50 mm/min (Ossai and others 1987). Peak heights (mm), firmness (N), and energy

required for each compression (mJ) were measured from the force-deformation

curve. Firmness and energy required for compression were calculated up to the

maximum force measured during the first compression cycle. Cohesiveness,

gumminess (N), springiness (mm), adhesiveness (mJ), and chewiness (N*mm)

values of the moin-moin sample were calculated as described by Bourne (1978).

Puncture tests were performed in triplicate per moin-moin sample (total of 9

measurements per treatment regimen) in sequence across the equator of the sample

using a Magness-Taylor probe with a diameter of 8 mm at a crosshead speed of 60

mm/min to an extension of 8 mm into the sample. Probe extension (mm) at peak

force (N) and energy of the force curve (mJ) were obtained from the puncture test

curves.

Viscosity

Apparent viscosity of the moin-moin batter prior to cooking was measured

using a Brookfield Viscometer (Model HATD, Brookfield Engineering Laboratories,

Inc., Stoughton, MA) with a #3 disc spindle operating at 100 rpm after the sample

had been equilibrated to room temperature. Results are an average of triplicate

measurements and are reported in centipoise (cP).

Color measurement

Color measurements of moin-moin samples were made using a Minolta

colorimeter (Model CR-200, Osaka, Japan) calibrated with a white reference tile (L*=

94.50, x = 0.3144, y =0.3206). Three color measurements for each moin-moin

105

sample in a set of three samples (total of 9 measurements) were averaged to obtain

Hunter color vector coordinates of L*, hue angle ( ( )ab1tan−=θ ), and chroma

( ( ) ( )22 ** baC += ).

Water Holding Capacity

Water holding capacity (WHC) was measured on cowpea flour and cowpea

meal as described by Kethireddipalli and others (2002b) with modifications. A “wet

filter paper” constant to be used in the calculation of WHC was established by

averaging the weights of 10 filter paper discs that were vacuum filtered with 20 mL of

deionized water for 2 min. WHC sample measurement was conducted in triplicate by

weighing 0.2 to 0.25 g of sample combined with 20 mL of deionized water (25oC) into

a 20 x 125 mm screw-cap test tube and mixed (Vortex-Genie™, model K-550-G;

Scientific Industries, Inc., Bohemia, NY, USA) for 1 min at a speed setting of 10. The

samples were allowed to hydrate at room temperature for 10 min and then vacuum

filtered through Whatman No. 50 filter paper until all residual water was filtered off.

The filtered, hydrated sample and filter paper were weighed as is. WHC was

calculated as defined in eqn 1 and reported as grams of water held per gram of dry

sample.

(1)

Sensory Analysis: Expert Panel

Sensory analysis of moin-moin samples was conducted using an expert

sensory panel as described by Stone and Sidel (1985). Four panelists were selected

based upon either their professional knowledge of cowpea-based products or cultural

dietary choices consisting primarily of legume-based products that are similar to

(Filtered Sample with paperg) – (wet paperg)

dry sampleg WHC =

106

moin-moin. During the evaluation session, panelists were instructed to sample the

product and evaluate the sample in reference to the control (dry, whole, imbibed

seeds) which was available to all panelists during the evaluation session. Hedonic

ratings (5 or 9-point scales) or intensity ratings (150 mm line scales) were used to

evaluate the product as outlined in table 5.2.

Nine-point hedonic scales were labeled 1 as “dislike extremely” and 9 as “like

extremely” with the midpoint being “neither like nor dislike”. The evaluation

parameter of size used a 5-point hedonic scale to compare the size of a moin-moin

sample in reference to the size of the control and labeled 1 as “very small” and 5 as

“very large” with the midpoint being “same size”. Intensity scales consisted of a 150

mm line with anchor points at either end of the line. The panelists were instructed to

mark an “x” on the line proportional to the intensity of a particular evaluation

parameter as it applied to the sample being evaluated; the left portion of the line

indicated “low intensity” and the right indicated “high intensity”.


Moin-moin Composition

Nutrient composition of moin-moin with 3.25:1 water to cowpea solids ratio

was calculated using MasterCook v8.0 (ValueSoft 2005) and then compared to akara

nutrient composition (Plahar 2004). Akara was chosen to compare nutrient profiles

due to formulation similarities, and consumption of akara in West African countries is

similar to that of moin-moin. Table 5.3 lists the proximate composition of the two

products with total fat, total carbohydrates, protein and ash reported on a dry weight

basis. Overall, akara has a greater fat content compared to moin-moin (19.23% vs.

9.12%, respectively) which is expected as akara is a deep-fat fried product whereas

107

moin-moin is a steamed product. On a dry weight basis, moin-moin has a higher

concentration of carbohydrates (63.85%) and protein (24.32%) compared to akara.

Although the nutrient profiles of akara and moin-moin are fairly similar as

reported in this study, empirical verification should be conducted under the

justification that alterations of cowpea material particle size may not be properly

factored into computer database calculations. Additionally, the moin-moin prepared

in this study focused upon texture of the product and not upon the organoleptic

properties. Formulation optimization of moin-moin to improve consumer preference

could significantly alter the present nutrient profile.

Objective Measurement: Results

Texture Profile Analysis

Geometric particle mean diameters (dgw) of cowpea flour (dgw = 17 μm) and

cowpea meal (dgw = 376.7 μm) were shown to significantly alter the textural structure

of moin-moin prepared in this experiment. Table 5.4 shows the significant

differences (α=0.05) for the TPA of moin-moin samples at room temperature based

upon starting material used, water:cowpea solids ratio, and cooking time. The

overall treatment effect of using cowpea flour produced a firmer product as indicated

by the firmness and energy of the first compression (27.93 N, 69.59 mJ).

Additionally, the cohesiveness (0.28), gumminess (7.39 N), springiness (2.47 mm),

and chewiness (19.26 N*mm) of moin-moin prepared from cowpea flour were

significantly higher (α=0.05) than moin-moin prepared from imbibed cowpea seed or

cowpea meal. The adhesiveness of moin-moin prepared from cowpea flour was the

lowest (0.65 mJ) suggesting that the significantly smaller particle size of cowpea flour

108

had a greater degree of amylose-lipid complex formation as observed in earlier

studies with moin-moin (Ossai and others 1987; Okechukwu and others 1992).

The significant differences (α=0.05) of water:cowpea solids ratio are better

observed in table 5.5, but table 5.4 shows that a 3.75:1 ratio (mid range) produced

higher values for gumminess (5.95 N), springiness (2.10 mm), and chewiness (15.73

N*mm) with the dilute system (4.25:1) having the lowest values (2.58 N, 1.53 mm,

and 4.68 N*mm, respectively). Cooking the moin-moin samples for a shorter period

of time (20 min vs. 40 min) produced an overall firmer product (24.99 N, 64.45 mJ).

However, 40 min cooking times yielded products that were “stickier” as indicated by

cohesiveness (0.22) and adhesiveness (1.27 mJ) values.

Table 5.5 identifies the specific measurement values for TPA measurements

as affected by treatment levels. The general trends are the same as observed in

table 5.4, but specific trends are better represented in this format. Cowpea material

with smaller particle sizes (cowpea flour) had overall higher force values (e.g.

firmness and gumminess) when compared to either the cowpea meal or imbibed

cowpea seeds under similar treatment regimens. Overall, the lower concentrations

of water:cowpea solids (3.25:1 and 3.75:1) had similar and higher force values than

the dilute system (4.25:1) indicating that firmer gel structures were obtained at these

lower concentrations.

Cohesiveness values for cowpea flour moin-moin with a cooking time of 40

min had a higher range (0.30 – 0.42) than the 20 min cooking time samples (0.20 –

0.24) for the same treatment regimens; a similar response was also observed for

adhesiveness. The same trends observed for cowpea flour were not observed for

cowpea meal. This difference can be explained due to the larger particle size of the

meal which retained more intact starch granules vs. damaged starch granules; this

109

would require greater amounts of time, temperature, and moisture to achieve similar

responses as observed for cowpea flour.

WHC, Color, Viscosity, and Puncture Test The WHC of cowpea meal (2.14 g/g) and cowpea flour (1.66 g/g) prepared

under the conditions of this experiment were significantly different (α = 0.05). These

values are similar to WHC found in similar studies for the preparation of cowpea

starting materials intended for production of cowpea snack chips and paste (Kerr and

others 2000; Kethireddipalli and others 2002b).

Table 5.6 lists color vector values, viscosity, and puncture tests for moin-moin

samples that had been cooled to room temperature. The most significant (α=0.05)

color finding showed that chroma values for moin-moin sample prepared from

cowpea flour at a 3.25:1 concentration yielded a product with a more intense,

saturated color than a product made from cowpea meal at a 4.25:1 concentration.

Even though hue angles were significant for the various treatment levels, the

differences (at best) were only 0.03o.

The significantly (α=0.05) higher apparent viscosity of moin-moin prepared

from imbibed cowpea seeds (733.33 cP) could be the result of interference of pieces

of intact seed coat material at the interface between sample and spindle.

Kethireddipalli and others (2002b) state that reducing particle size reduces the

viscosity of cowpea pastes, which is supported in the present study by cowpea flour

samples having a lower viscosity than cowpea meal systems (404.78 vs. 522 cP).

As moin-moin batter became more dilute the viscosity decreased (631.33 to 356.50

cP).

Puncture tests showed that increasing the particle size of starting material

significantly (α=0.05) decreased peak force of moin-moin samples (2.26 N to 1.67

110

N). Increasing the ratio of water:cowpea solids significantly (α=0.05) decreased

peak force values (2.21 N to 1.28 N). Water:cowpea solids ratios of 3.25 and 3.75

had similar puncture energies (7.98 and 8.30 mJ, respectively) whereas the dilute

system of 4.25 required the least amount of energy to puncture (4.62 mJ). Peak

force extension values were highest for cowpea flour (7.26 mm) and at a 3.75:1 ratio

(6.92 mm) suggesting that this particle size range and concentration produced the

firmest gel structure. Lastly, cooking time showed that a firmer product was

produced from moin-moin cooked at 20 min than 40 min (2.21 N, 7.42 mJ vs. 1.67 N,

6.72 mJ, respectively).

Objective Measurements: Discussion

Kerr and others (2000) reported that as particle size of cowpea flour/meal

mixtures decreased, a reduced WHC was observed. In the same study, they also

reported higher values of extractable starch as the particle size of the cowpea

mixtures decreased. Based upon this observation, the dgw of cowpea flour produced

from attrition milling would have a significantly greater amount of damaged or free

starch when compared to the cowpea meal milled using the Hammer mill. In

general, at the temperatures used to measure the WHC of the cowpea starting

materials starch is relatively insoluble, thus lower WHC values were measured due

to the dgw of the starting material decreasing.

Okechukwu and others (1991a; 1991b) reported that the gelatinization onset

temperatures (To) for cowpea slurries decreased with increasing water:solids ratios

and with decreasing particle size. Kerr and others (2000) reported To of 74oC for a

61% moisture cowpea slurry with the cowpea flour particles less than 148 μm. A

typical Brabender Visco/amylo/graph curve (Fennema 1996) for starch shows that

shortly after To the viscosity of the starch solution rapidly increases with increasing

111

temperature due to the swelling of the starch granules. Continuation of heating,

either due to time or increasing temperature will result in rupture of swollen starch

granules causing the release of amylose and decreasing the strength of typical

effects observed upon cooling of a cooked starch paste.

Higher peak force values (TPA and puncture tests) were observed for moin-

moin samples prepared from cowpea flour (smaller particle size), 3.75:1 water to

cowpea solids ratio, and shorter cooking time. These three trends suggest that the

cooking treatment produced maximum starch swelling at these conditions and upon

cooling maximum binding of amylose chains. The transition from a firm to a sticky

gel between the 20 and 40 min samples suggests that starch is fully gelatinized near

the 20 min cooking time, but relatively prolonged cooking times causes deformation

of starch granules resulting in a reduction of junction zone formation (Okechukwu

and others 1992; Fennema 1996; Kerr and others 2000). The cowpea flour used in

the present experiment had a dgw of 17 μm and moin-moin samples were cooked in a

stovetop steamer with a headspace temperature range of 75 – 80oC. Under the

experimental conditions used for the preparation of moin-moin from cowpea flour, it

is suggested that the To was achieved slightly before or at 20 min of cooking time

with maximum starch swelling occurring near 20 min of cooking (depending on

product formulation); rupture of starch granules probably occurred after this point,

based upon the observations from TPA and puncture tests and previous

observations of moin-moin and cowpea particle size (Okechukwu and others 1991b;

1992; Kerr and others 2000; Kethireddipalli and others 2002b).

Expert Sensory Panel

The results from the expert sensory panel (table 5.7) are subjective ratings

from panelists who were familiar with legume-based products from either a research

112

or cultural dietary standpoint. Overall, there was no significance (α=0.05) for the

visual appeal, surface sogginess, surface moisture, size, chewiness, starchy mouth

feel, beany flavor, and overall liking of the moin-moin samples prepared under the

conditions of this study (data not shown). The individual ratings recorded for many

of these attributes had highly variable responses; however, upon averaging the data

these differences became much less with many of the averaged results occurring in

the midrange of respective scales.

The cohesiveness intensity ratings from the sensory analysis show that upon

mastication moin-moin samples prepared from cowpea flour had a greater intensity

(more cohesive structure) than moin-moin prepared from either imbibed cowpea

seeds or cowpea meal (92.08 vs. 66.63 or 70.30). A similar trend was shown in the

TPA of moin-moin made from cowpea flour having the greatest cohesiveness ratio

(0.24) vs. imbibed cowpea seeds (0.12) or cowpea meal (0.14) samples.

Springiness intensity ratings of moin-moin were greatest for samples prepared from

cowpea flour (83.82) whereas the lowest values were recorded for imbibed cowpea

seed samples (47.37); the same trend was observed in the TPA of moin-moin made

from cowpea flour (2.47 mm) having the greatest value and that from imbibed

cowpea seeds (0.88 mm) the lowest. Lastly, color intensity of moin-moin was shown

to be the greatest for samples prepared from cowpea flour (58.56); chroma values

also reflected greater color intensity (24.25) for cowpea flour moin-moin.

The surface smoothness of moin-moin prepared from imbibed cowpea seeds

received a significantly lower intensity rating (28.75) than moin-moin prepared from

either of the other two starting materials. This can be attributed to inefficient

homogenization of the cowpea seed coats from the imbibed seeds during the

blending phase of moin-moin preparation; in the case of either of the other starting

materials, this was not a problem as the size of the seed coat was reduced during

113

the milling operations. Similarly, the texture rating was lower for the imbibed cowpea

seed moin-moin (3.75) when compared to moin-moin prepared from either of the

other two starting materials. Although chewiness showed no significant difference, it

is interesting to note that moin-moin prepared from imbibed cowpea seeds had a

higher value (16.00) than either of the other two starting materials which had similar

values (10.18, 10.94). Furthermore, cooking times were shown to produce a stickier

product at longer cooking times based on objective measurements, yet the panelists

found that the firmer produces required fewer chews before comfortably swallowing

the sample. The general handling ratings showed significant difference (α=0.05) for

starting material and cooking times, yet all values were within the “neither dislike nor

like” range suggesting that improvement at all levels is necessary.

Conclusions

The conclusions from this study support the paradigm that reducing particle

size alters the functional properties of a food ingredient. Moin-moin prepared from

cowpea flour was shown to produce an overall firmer product than that prepared

from imbibed seeds or meal. The ratio of water to cowpea solids show that the mid

range used in this study (3.75:1) created firmer products than either of the other two

ratios (3.25:1 or 4.25:1) suggesting that the amount of water in the 3.75:1 system is

the ideal amount necessary to maximize gelatinization of cowpea starch. A 20 min

cooking time produced a firmer product than at 40 min whereas the 40 min cooking

time produced a stickier product. Moin-moin prepared from cowpea flour with dgw of

17μm showed significantly smaller adhesiveness values (TPA) than that made from

either the imbibed cowpea seeds or cowpea meal suggesting that amylose-lipid

complexes were easily formed due to the smaller particles sizes (increased surface

area). The objective measurements from this study accurately identified how

114

cowpea starting material, the water:solids ratio, and cooking time affected the texture

of the final product. Subjective sensory evaluations from this study vaguely identified

preference based upon various treatment conditions; however, conclusive results

would require a larger sensory panel.

Outlook

It is suggested that future research focus upon consumer preference as it

relates to firmness and stickiness-related attributes of moin-moin or similar products.

Objective measurements can provide a means of initial indicators of consumer

preference as it relates to firmness and stickiness-related attributes during

formulation trials once an ideal consumer texture has been identified. Flavor of

moin-moin should be evaluated as this study focused only on the textural aspects of

the product.

Acknowledgements

This study was supported by the Bean/Cowpea Collaborative Research

Support Program (Grant No. DAN-1310-G-SS-6008-00), U.S. Agency for

International Development and by State and Hatch funds allocated to the University

of Georgia Agricultural Experiment Station, Griffin Campus. The authors would also

like to thank Mr. Glenn Farrell for his assistance with procedural development of

texture measurements for moin-moin samples.

115

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Table 5.1: Moin-moin formulations used for objective and sensory analysis. Water:Cowpea ratio

Ingredient 3.25:1 3.75:1 4.25:1 Cowpea1 21.32% 19.06% 17.24% Water2 69.31% 71.47% 73.28% Tomato paste 2.06% 2.06% 2.06% Onion puree 2.06% 2.06% 2.06% Salt 1.44% 1.44% 1.44% Oil 3.71% 3.71% 3.71% Liquid smoke 0.21% 0.21% 0.21% 1 Calculated as cowpea solids, moisture free 2 Calculated as total water (added water and water in the cowpea fraction)

118

Table 5.2: Evaluation parameters, descriptions, and scoring method used to evaluate moin-moin samples by expert panel.

Evaluation Parameter Description Scoring Method

Visual appeal General visual appeal of the external/internal structure

9 point

Color Intensity of the color of the sample 150 mm

Surface sogginess Tactile sensation of surface texture as it relates to moisture absorption

150 mm

Surface smoothness Intensity of surface smoothness 150 mm

Surface moisture Intensity of moisture accumulation on the surface 150 mm

Size Comparison of sample size to the control 5 point

Texture Overall textural feel of the product during the first three chews of the sample compared to the product

9 point

Chewiness Number of chews during mastication required before samples can be comfortably swallowed

Count

Springiness Degree to which sample returns to the original height when compressed between the tongue and palate

150 mm

Starchy mouth feel Intensity of mouth coating and gummy sensation associated with starchy products during mastication

150 mm

Cohesiveness Degree to which sample deforms rather than breaks apart upon mastication

150 mm

Beany flavor Intensity of beany flavor (aroma and taste) of the moin-moin product

150 mm

General handling Overall ease of handling 9 point

Overall liking Overall liking of the sample 9 point

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Table 5.3: Calculated proximate composition of Moin-moin1(3.25:1 water to cowpea solids ratio), and reported composition of Akara2. Moin-moin Akara

Moisture 56.15% 48.75% Total Fat3 9.12% 19.23%

Total Carbohydrates3 63.85% 55.84% Protein3 24.32% 19.25%

Ash3 2.69% 5.67% 1 Proximate compositions calculated using MasterCook v8.0.03.01 (ValueSoft, Chaska, MN) 2 Akara data obtained from Plahar (2004) 3 Values are on a dry weight basis

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Table 5.4: Texture profile analysis results of moin-moin samples at room temperature as affected by starting material, water:cowpea solids ratio, and cooking time.

Firmness (N)

Energy (mJ)

Cohesiveness (A2/A1)

Gumminess (N)

Springiness (mm)

Adhesiveness (mJ)

Chewiness (N*mm)

Starting Material Cowpea Flour 27.93a 69.59a 0.28a 7.39a 2.47a 0.65c 19.26a

Imbibed Cowpea 18.31b 50.96b 0.12b 2.17b 0.88c 1.21b 1.92b

Cowpea Meal 13.69c 47.78b 0.14b 1.99b 1.23b 1.60a 2.50b

Water:Cowpea 3.25 24.90a 72.21a 0.19b 4.87b 1.71b 1.15a 10.18b

3.75 23.80a 64.24b 0.23a 5.95a 2.10a 1.00a 15.73a

4.25 12.08b 34.29c 0.20b 2.58c 1.53c 1.23a 4.68c

Cooking Time 20 24.99a 64.45a 0.19b 5.03a 1.56b 0.96b 9.52a

40 16.87b 52.64b 0.22a 4.04b 1.96a 1.27a 10.77a a,b,c Values in a column for each treatment variable not followed by the same letters are significantly different (α=0.05).

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Table 5.5: Averaged texture profile analysis values as affected by treatment levels of cowpea material, cooking time, and water:cowpea solids ratio.

Material Cooking Time Water:Solids Firmness

(N) Energy

(mJ) Cohesiveness

(A2/A1) Gumminess

(N) Springiness

(mm) Adhesiveness

(mJ) Chewiness

(N*mm)

3.25 47.56 114.44 0.20 9.35 1.91 0.35 17.96 3.75 45.32 108.77 0.24 10.82 2.39 0.24 25.87 20

4.25 13.99 33.67 0.23 3.23 1.49 1.20 4.92 3.25 23.57 70.50 0.30 7.14 3.14 0.80 23.26 3.75 21.63 52.19 0.42 9.08 3.55 0.44 32.27

Flour

40

4.25 14.70 36.01 0.31 4.51 2.36 0.94 10.81 3.25 16.87 51.73 0.19 3.16 1.37 1.53 4.32 3.75 15.20 44.36 0.14 2.14 1.08 1.17 2.36 20

4.25 11.01 33.74 0.14 1.50 1.10 1.28 1.68 3.25 17.43 71.46 0.13 2.31 1.28 1.96 2.97 3.75 13.04 51.63 0.14 1.77 1.36 2.14 2.42

Meal

40

4.25 8.61 33.76 0.13 1.09 1.16 1.50 1.29 Imbibed 40 3.25 18.31 50.96 0.12 2.17 0.88 1.21 1.92

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Table 5.6: Objective measurements of color, viscosity, and puncture tests for moin-moin samples equilibrated to room temperature. Color measurements are reported in the Hunter vector system of luminosity (L*), hue angle, and color intensity (Chroma). Viscosity is reported in cPoise. Puncture tests results are reported as peak force (N), maximum extension at peak force (mm) and energy of force curve (mJ). Color Viscosity1 Puncture

L* Hue Chroma

cPoise

Peak Force

(N)

Extension (mm)

Energy (mJ)

Starting Material Cowpea Flour 71.98a 1.42b 24.25a 404.78c 2.26a 7.26a 8.37a

Imbibed Cowpea 74.02a 1.43a 23.68b 733.33a 0.82c 5.97b 2.66c

Cowpea Meal 72.80a 1.41b 21.83c 522.00b 1.67b 6.07b 6.44b Water:Cowpea

3.25 72.80a 1.42b 23.80a 631.33a 2.21a 6.53ab 7.98a 3.75 73.20a 1.43a 21.82c 427.83b 2.06b 6.92a 8.30a 4.25 71.48a 1.40c 23.46b 356.50c 1.28c 6.40b 4.62b

Cooking Time 20 71.95a 1.43a 23.79a - 2.21a 6.50a 7.42a 40 73.00a 1.40b 22.48b - 1.67b 6.70a 6.72b

a,b,c Values followed by different letters in the same column within the same treatment level are significant (α=0.05).

1 Viscosity was measured on moin-moin batter at room temperature before cooking.

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Table 5.7: Sensory hedonic and intensity ratings from an expert sensory panel (4 panelists) comprised of panelists who were familiar with cowpea/legume products either on a cultural or research basis.

Color Surface Smoothness Texture Chewiness Springiness Cohesiveness General

Handling

Starting Material Cowpea Flour 58.56a 98.16a 5.27ab 10.18a 83.82a 92.08a 5.52ab

Imbibed Cowpea 52.00a 28.75b 3.75b 16.00a 47.37b 66.63b 5.90a Cowpea Meal 57.47a 79.79a 5.56a 10.94a 58.13ab 70.30ab 4.25b

Water: Solids 3.25 55.58a 81.50b 5.39a 12.08a 69.56a 79.69a 5.83a 3.75 55.30a 99.17a 5.28a 9.31a 74.66a 83.89a 5.63a 4.25 62.72a 79.66b 5.38a 10.78a 65.94a 78.36a 5.47a

Cooking Time 20 50.47b 93.39a 5.02a 9.85a 71.54a 82.51a 5.27b 40 64.52a 80.27b 5.65a 11.64a 68.64a 78.86a 6.00a

a,b Values followed by different letters in the same column in the same treatment level are significant (α=0.05)

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Chapter 6

SUMMARY AND CONCLUSIONS

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Milling cowpea flour in a cyclone assisted attrition mill demonstrated a

reasonable degree of control for end product uniformity, with respect to geometric

particle mean diameter (dgw) and yield. It was shown that positioning of the airflow

restriction valve played a significant role in the final dgw of cowpea flour. Turbine

speed, airflow restriction, and preconditioning cowpea seed moisture content exerted

influential forces on the dgw and yield of cowpea flour. Preconditioning of cowpea

seeds at the selected temperatures showed no affect on the dgw or yield of cowpea

flour. Milling of cowpea flour under minimal sample treatment conditions (i.e. 4%

moisture at 20oC) was comparable to cowpea flour produced under extensive

sample treatment conditions (i.e. 0.5% moisture at -80oC).

Trypsin inhibitor (TI) content of thermally treated soy flake at 30, 35, and 40%

moisture content showed minimal to no reduction of TI when compared to the

respective untreated samples. Initial TI content of ADM soy flake showed an

average of 30% residual TI for all samples analyzed when compared to common raw

soybean as reported by Liu (1997).

Cyclone assisted attrition milling of soy flake using a Super Wingmill DM-200

produced soy flour with an average dgw of 18 μm, which is consistent with the dgw for

similar operational parameters used for production of cowpea flour. Suspensions

created using soy flour that had been subjected to thermal treatment (40% moisture,

110oC, 5 min) and attrition milling had an overall lower viscosity than those from

other treatments. With regard to the temperature of the water used for

homogenizing the suspensions, relatively high temperatures created unstable

suspensions whereas lower temperatures produced stable suspensions. The levels

of anti-caking agent used in this study showed no effect on the rate of suspension

separation.

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Moin-moin prepared from cowpea flour was shown to produce an overall

firmer product when compared to that made from imbibed cowpea seed. The ratio of

water to cowpea solids was shown to affect the end product texture indicating that

cowpea starch was the dominate component in moin-moin structure attributes.

Cooking moin-moin for longer time periods produced a sticker and less firm product

than a shorter cooking time. Moin-moin prepared from cowpea flour with dgw of

17μm showed significantly lower adhesiveness values (TPA) than that prepared from

either the imbibed cowpea seed or from cowpea meal suggesting that amylose-lipid

complexes were easily formed due to the smaller particles sizes (increased surface

area). Sensory analysis of moin-moin identified a preference based upon various

treatment conditions; however, conclusive results would require a larger sensory

panel.

Overall, it was shown that a significant reduction in particle size of a legume

alters its functional properties when used in the respective food systems employed in

this study. These alterations in legume flour functional properties can be either a

positive or negative influence on consumer products depending on the consumer’s

preference. The objective measurements selected in this experiment can be used as

an indicator of consumer preference once a preference has been identified.

Effect of Cyclone Assisted Milling on Legume Flour

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