CEREAL SCIENCE TECHNOLOGY

CEREAL SCIENCE

&

TECHNOLOGY

Prof. Dr. rer. agr. B.D. Rohitha Prasantha

Dept. of Food Science & Technology

Faculty of Agriculture

University of Peradeniya

Objectives

• Starchs of cereals

Starch structure

Starch gelatinization

Modified starches

Cellulose structure

Cellulose ingredients

• Proteins of cereals

• Other constituents of cereals

• Glass transition

Starch Starch is the cheapest and most abundant food

biopolymer in the world

Starch in cereals 60 –75%

Starch effects on the physical properties of foods

Ex: Gelling,

Thickening,

H2O retention,

Adhesion,

Stabilizing etc.

Starch Sources

Common source: Corn, Wheat, Rice, Cassava, Potato, Sweet potato, Arrowroot, Sorghum, Sago etc.

Industrial source:

Corn starch Root/Tuber starch Other Cereals

Common corn Potato Wheat

Waxy corn Cassava Rice

(amylose 0%) Sweet potato Barley

High-amylose

(55% -70%)

Other mutants

• Sago is a starch extracted from the pith

of sago palm stems, Metroxylon sagu

• It is a major a staple food for New

Guinen

• Arrowroot

Starch Sources

Starch Applications

• Non-Food Applications of Starches Adhesives Construction Textile

Explosives Mining Paper

Dry cells Cosmetic Pharmaceutical

Bioderadable plastic

• Food Applications of Starches Canning

Filling: viscosity, suspension and opacity agent

Body or texture agent: soups, sauces, puddings

and gravies

Beverages: coffee, teas or chocolate

Starch Applications

Cereals and Snacks Extruded snacks

Chips etc.

Fried foods

Ready-to-eat cereals

Bakery Pies

Fillings, glazes

Custards and icings

Cakes, donuts,

danish

Icing sugar

• Food Applications of Starches

Batters and Breadings Coated fried foods

Frozen battered:

vegetables, fish and meat

Dry mix coatings

Cooked Meat Binder Binder for formed meat

Smoked meats

Pet foods

Starch Applications

Dressings, Soups and

Sauces

Pourable salad dressings

(high shear)

Instant dry salad dressing

Low-fat dressing

Canned gravies and sauces

Frozen gravies and sauces

Frozen Foods

Fruit fillings

Oriental foods

Soups, sauces

Cream-based

products


Starch Applications

Flavours and Beverage

Encapsulation of flavors, fats, vitamins, spices etc. Spray dried flavors for dry beverage mixes Beverage emulsions Liquid and powdered non-dairy creamers

Confectionery

Dusting powder Panned candies

Jelly gums Confectioners sugar

Hard gums


Functional Properties of Starches in

Food

Specific viscosity (hot and cold forms)

Thin boiling (faster canning heat transfer)

Viscosity resistance acid or mechanical sheer

Freeze-thaw stability (natural / modified)

Gel texture, body at various temperatures

Clarity, opacity

Tolerance to processing conditions


Food

Oil retention, high or low

Resistance to setback (gel formation)

Flow properties

Emulsion stabilizing capacity

Mouthfeel, lubricity, palate-coating

Suspension characteristics

Adhesiveness


Food

Crystallinity

Long shelf-life stability

Hygroscopicity

Color

Anti-caking

Cold-water swelling or dispersibility

Swelling character and resistance to swelling

Film-forming properties

Starch Composition

Amylose (20-30%)

Amylopectin (70-80%)

Both are polymers of

α-D-glucose units

Starch Composition

• Amylose:

Linear a- 1,4 glucose chain. (some branch a-1,6

approximately every 200 glucose units). MW< 0.5

million. Strong films, firm gel formation, blue with I2

• Amylopectin:

Linear a-1,4 chain with an a-1,6 branch

approximately every 20 glucose units.

MW 50 – 500 million. Weak films, Non-gel to soft

gel formation, Reddish brown with I2

Amylose & Amylopectin Structure

Amylose

Amylpectin

Starch Composition

• Amylose: amylopectin ~ 1:3

Waxy starches all amylopectin

High amylose mutants up to 70%

amylose

Starch Type Amylose (%) Amylopectin (%)

Corn (waxy) 25 (< 1) 75 (> 99)

Wheat 25 75

Rice (waxy) 18 (< 1) 82 (99)

Potato 20 80

Tapioca 17 83

Amylose Waxy amylose: 0 - 5%

Very low amylose: 5 - 12%

Low amylose : 12 - 20%

Intermediate amylose: 20 - 25%

High amylose: 25 - 33%

Commercially

Low amylose <20 %

Medium amylose: 21 - 25%

High amylose: 26 - 33%

Amylose can be separated from Amylopectin by selective precipitation

Forms insoluble complexes with butanol and other organic substances with hydrophilic groups

In water

Amylose molecules tendency to orient themselves in parallel

Amylose & Amylopectin Structure

Amylose & Amylopectin Structure In diluted solution: Starch precipitation

Concentrated solution:

Steric hindrance leads to formation of gel call “retrogradation”

Steric effects cause Steric hindrance - nonbonding

interactions influence the shape

Amyloses

Affinity for I2, Fatty acids and long chain alcohols

Formed helical complex Gelling: ppt. or crystallization fraction

Amylose & Amylopectin

Amylopectin:

Large size and presence of branches reduces the mobility of molecule Tendency to form sufficient amount of hydrogen bonds

Amylopectin: Gelling resistant and Stringy (tough or rubbery) Cooked rice with

low amylose is soft and sticky

high amylose is firm and fluffy


Characteristics Amylose Amylopectin

Shape essential linear branched

Linkage α – 1,4 (some α –

1,6)

α – 1,4 and α – 1,6

Molecular weight < 0,5 million 50 – 500 million

Film forming strong weak

Gelling firm soft

Color with iodine blue reddish brown

Three chain types: A,

B and C

A chains are not

branched

B chains are branched

C chain has the

reducing end R

One reducing end per

amylopectin

molecules

Different Types of Chains for Amylopectin


Amylose: when cooled, forms a gel due to

hydrogen bonding between the linear chains

Amylopectin: when cooled, thickens but

does not form a gel or very slow formation,

slow retrogradation due to branched structure

Starch Granules SG 1st in a plant cell as minute points

Grow rapidly in the cell

SG has specific size and shape

It is a characteristic of plant spp.

Starch granules vary in diameter

Tapioca SG: 5-35 µm Maize SG: 5-25 µm

Potato SG: 15-100 µm Rice SG: 3-8 µm

SGs are essentially insoluble in cold water

Starch Granules Structure

Starch granules: discrete plastids,

semicrystallin aggregates structure

Shape: spherical, polygonal, round, irregular,

lenticular; ø 1μm - 100 μm

Shape and size: fingerprints for starch origin

Growth of SG

• Starch Grows by apposition

The new layer deposit outside of the

granule

varying thickness

amount of available CHO

time

• Layers become apparent after treatment

with dilute acid or enzyme

Starch Granules Size Distribution

• Important for specific applications

• Basic physical characteristic

• Can be value-added

e.g.: Small granule size of rice starch

fine fabrics

skin cosmetics


Techniques

Light Microscopy

Confocal Microscopy

X-ray Diffraction

Atomic force Microscopy (AFM)

Transmission Electron Microscopy

(TEM)

Scan Electron Microscopy (SEM)


• Amylose and Amylopectin molecules:

organized in granules as alternating semi-

crystalline and amorphous layers form as

growth rings

• The semi-crystalline layer: ordered

regions composed of amylopectin double

helices

Formed by short amylopectin branches

Crystalline structures known as the

crystalline lamellae


• The amorphous regions:

semi-crystalline layers and the amorphous

layers are composed of

Amylose and non-ordered

Amylopectin branches

Amylose A crystals

Amylose and the branching

point of amylopectin contribute

to the amorphous region and

the outer chains of amylopectin

contribute to the crystalline

region


Crystallinity

3 types of X-ray patterns: A,B and C

Pattern A: most cereals starches

Pattern B: Potato, root starches and

retrograded starch

Pattern C: Pea and bean starches

(C = mixture of A+B)

Heat + H2O

B potato starch A starch

Pattern V: Not found in naturally but after

gelatinization lipid related starch

• Amylopectin double-helical chains can either form the more

Type B: more open hydrated hexagonal crystallites Type A: denser crystallites


Amylose and Glycemic Index

AUC = Area under blood glucose curve

GI = AUC of food/ AUC of given glucose x 100

High amylose low GI (but

depends on resistant starch content)

High amylose low α-amylose

hydrolysis due to long linear

amylose molecule

Starches RDS – Rapidly digestive starch

Amorphous and Dispersed starch

Digest to glucose within 20 min.

SDS – Slow digestive Starch

Low amorphous starch and raw starch

Type A and C SG structure

Digest to glucose within 100 min.

RS – Resistant starch (no digestion)

Type of RS Description Food sources Resistance

minimized

RS1 Physically

protected

Whole- or partly milled grains

and seeds, legumes

Milling, chewing

RS2 Un-gelatinized

resistant granules,

slowly hydrolyzed

by a-amylase

Raw potatoes, green

bananas, legumes, high

amylose corn

Food processing

and cooking

RS3 Retrograded

starch

Cooked and cooled potatoes,

bread, cornflakes, food

products with repeated moist

heat treatment

Processing

conditions

RS4 Chemically

modified starches

due to cross-

linking with

chemical reagents

Foods in which modified

starches have been used (for

example, breads, cakes)

Less susceptible

to digestibility in

vitro

Gelatinization, pasting and

retrogradation

Native Starch insoluble in cold water

Different mouth-feel due to native SG

Increases in T: irreversible changes SG

Gelatinaization and pasting

Change food texture, viscosity and WHC

After heating:

Reassociation of the starch molecules

Starch “retrogradation”

Starch Gelatinization When granular birefringence is lost

Begin when viscosity increases

Birefringence

Birefringence of SG can be observed

under polarized light

SG has higher degree of molecular order

Birefringence

Polarized-light micrographs of SG

(a) SG prior to heating in

excess H2O

(b) SG after heating to a

temp. that melts the B-

type crystallites

Starch characteristics

Gel Formation Sol Fluid Starch Paste Gel

Semisolid Paste Forms after cooling <100º C Requires Sufficient Amylose

SG hydrate upon heating, time and

pH


Gel Formation

Amylose

Corn starch

↑ Amylose Opaque Gel

Potato, Tapioca

↓ Amylose Clearer Gel

Waxy Hybrids - Corn & Cross Sorghum

↓↓ Amylose Do Not Gel

Dextrinization

Breakdown of starch molecules

Presence of dry heat

Smaller sweeter-tasting

Dextrin molecules

Thickening power

diminished


Starch Gelatinization

Starch heating in H2O: Granules take up H2O and swell

Lose crystalline structure and change is irreversible

Starch molecules leaches out of the swollen granule

Mixture becomes viscous, thickened, forms a sol

Starch gelatinization

Breaks down the intermolecular bonds of

starch due to water and heat

• High amt. H2O enter into the SG: 1st

absorbed into the amorphous area of

double helical structures of amylopectin

• Heating: swelling while and H2O enter

into the crystalline regions (CR): melt and

break the CR

H2O is a Plasticizer: Reduce the # and

size CR

Heat diffuse into the CR: chains

separate into an amorphous form

Gelatinization temperature depends:

Crop, amount of water, pH, salt, sugar, fat and protein


• Cooled to an ambient temp.

Gelatinized starches form either a

firm gel or a flowing paste

(due to stronger H bonds)

•Gel ages OR Frozen and thawed

formed strong interact within chains

Starch molecules start to realign Form strong hydrogen bonds Squeeze out or release H2O called

“syneresis”

Syneresis can be reduced by chemically modifying the starch

Due to longer storage:

crystallization of starch chains in the gel called “retrogradation”



Gelatinization: also addition of an alkaline hydroxide or hypochloride

The alkaline reagents initiate oxidisation of the starch molecules

disrupting the composition of SG

enable SG to absorb some water

Oxidized

potato

starch over

10 g of ZnO


When SG place in water

Water freely into SG

Increase: 30% dry wt. & 5% vol.

Hydration is reversible

Structural damage is irreversible

Measured by visco-amylo graph

Measure the relative viscosity at constant

rate of heating 1.5 oC/ min

Carboxymethyl cellulose buffer (phosphate

buffer pH 6.8) - baseline viscosity


granule starts to expand rapidly called

“gelatinization”

at gelatinization: bundles of starch separate

due to absorption of H2O

viscosity of the slurry increases to a peak

Produce amorphous starch mass

on cooling the starch start to realign

2nd rise in viscosity called “set-back”

higher the amylose: greater the setback

Visco-Amylo Graph

Starch-water or flour-water suspension is heated in a

rotating bowl and then cooled down to room temp., both

under controlled conditions

Visco-Amylo Graph Profile

Time

Vis

co

sit

y 65oC

90oC 30oC

heating constant temp.

50oC 95oC 95oC 50oC Hold

60o C 70o C 80o C 95o C

30o C

Cooling

Pasting

Shear thinning

Setback

http://www.public.iastate.edu/~pkeeling/functn3.htm




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VAG Data

Alternative to VAG Profile

0

15

30

45

60

75

90

105

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0 10 20 30 40 50 60 70 80 90 100

Tem

per

atu

re (

oC

)

Vis

cosi

ty (

mP

a s

)

Time (min.) Mean viscosity of Bg 360 Mean viscosity of At 405

Temperature

Property Bg 360 flour

(Mean ± SD)

At 405 flour

(Mean ± SD)

Peak viscosity (mPa s) 6660±275a* 7400±329b

Through viscosity (mPa s) 1116.7±126a 2083.3±29b

Final viscosity (mPa s) 8850±600a 7233.3±208b

Breakdown viscosity (mPa s) 5543.3 5316.7

Setback viscosity (mPa s) 2190 -166

Pasting temperature (°C) 67.1±0.3a 63.4±0.4b

Pasting time (min) 30±1a 25±2b

Breakdown viscosity (mPa s) = peak viscosity - through

viscosity

Setback viscosity (mPa s) = final viscosity - peak viscosity

Consistency (mPa s) = final viscosity - through viscosity


Starch Gelatinization Temp. (°C)

Waxy maize 63 - 72

Wheat 52 - 63

Tapioca 59 - 70

Potato 56 - 66

High amylose maize 110 - 120

• Viscosity profile extremely helpful to determining

starch behavior under different condition

comparing the relative differences between starch

effect of starch modifying reagents gelatinization

pasting of modifying starch

Starch

Gelatinization

Characteristics of starch solutions

Crop Viscosity Transparency Stirring

Proof

Retrogradation

Potato very high clear low-

medium

medium

Maize medium opaque medium high

Wheat Low-

medium

turbid medium high

Waxy

maize

Medium-

high

clear Low very low

Heating Starch in Limited Water

Difficult to study character starch in limited

water system

Differential Scanning calorimeter (DSC) is

beneficial

DSC measures heat flow as function of

temperature

DSC measures heat flow as function of temp.

DSC Output: endothermic peak (ET peak)

Water: Starch, 2:1 => Sharp ET peak

Endothermic: A transition which absorbs energy

Exothermic: A transition which releases energy

Factors affecting gelatinization

Acid hydrolyses glucosidic bonds

short chain polymers

(oligosaccharides)

Acid added to pies or gravies (vinegar,

tomatoes) for thinning the starch

mixtures

Agitation destroys swollen granules

Enzymes are used to hydrolyze starch • a-amylase breaks long starch

polymers short chain

polymers (oligosaccharides)

• b-amylase removes maltose

from the end of starch

polymers

• Glucoamylase removes

glucose from the end of starch

polymers


a-amylase digested SG form barley seed

Acid/Dextrinization Occur due to acid and enzymes

Lemon pudding or pie => Lemon

juice is added early in the

gelatinization process

Amylopectin get less "thick"

Amylose get shorter chain length

Decreased starch paste viscosity

and gel strength


Sugar affects the gelatinization of

starch Starch pudding

High-moisture cake

• Sugar interacts with the amorphous

areas of SG

• Sugar inhibits swelling SG by competing

with H2O (reducing aw)

• Starch disperse decrease paste

viscosity and gel strength

• increased starch gelatinization Temp.


Starch pudding less viscose or less firm gel Cake collapse => starch structure is delayed

or inhibited The type of sugar influences the degree of

gelatinization Graph

• Sugar conc. High Onset of gelatinization

temp.

• Delaying the onset of gelatinization high in

Fructose > Glucose> Maltose > Sucrose

Sugar + dry starch mix before adding water: help

prevent clumping

Cont.

Types of sugar impact on the degree,

temperature and/or speed of gelatinization of

Starch

Swelling Power (SP)

of Starch

Swelling

Centrifugation W1 W2

SP = W1/W2

Swelling of gelatinized starch granules

SP is hydration capacity of Starch

Swelling power depend on

Type of Starch

Physical Characteristics

Chemical modification

Rice

flour

Physico-chemical properties (±S.D)

Amylose

content

(%)

Damage

starch (%)

Swelling

power

(g/g)

Water

absorption

capacity

(%)

Bg 360 31.1±0.1 2.6±0.1 6.3±0.1 196.8±2

At 405 14.4±0.1 4.2±0.6 6.8±0.1 248.5±4

Retrogradation Retrogradation is associated with • When the gelatinized product is cooled, the

amylose fraction retrogrades immediately while the

amylopectin remains in an amorphous state

• Ageing of cooked rice

• Syneresis of starch past after freeze–thaw cycles

• Opaqueness development of

Starch past

Starch gel

Starch dispersion

• Starch Stabilization => Starch substitution

Retrogradation

Retrogradation is associated with

• Production of resistance starch

• Slow digesting starch

• Film formation by amylose

Intensifying starch retrogradation for

value-added products

Retrogradation Manipulation

Structural Modification

Chemical modification , substitution

Enzymatic modification

Genetic modification

Adding sugars, lipids etc..

Problems of native starches

The lack of free flowing properties

or water repellency of starch

granules

Insolubility or failure of granules to

swell and became viscous in cold

water

Excess or uncontrolled viscosity

upon cooking

Cohesive or rubbery texture of

cooked starches

Breakdown during extended cooking

when expose to shear or low pH

Lack of clarity and the tendency of

starch sols to become opaque

Problems of native starches

Characteristics of some native starch

Starch Amylose

(%)

Clarity Gel

structure

Texture

Potato 20 Clear Non-gel Salve

Common

corn

27 Slight

opacity

Firm-gel Gel

Waxy corn 0 Clear Non-gel Paste

Cassava 22 Clear Soft-gel Gel

High-

amylose

corn

55 Opaque Rigid-gel Gel

Modified

Starches

Depolymerized

Acid Hydrolysis: 2 types

Acid thinning or thin-boiling starch

• Hydrolyzed starch slurry using HCl (other acids)

• Radom cleavage of molecule chain

• Starch remains in SG : dry flour can be obtained

• Low viscosity than native starch

• Hydration at low temperature

• Retain gel structure

• High adhesiveness • Use for confectionery

(high sweetener-solid)

Modified Starches

Dextrinization

Dry roasting (non slurry) of starch with

acid

Producing dextrin

Cold water soluble

Develop light tan to yellow color in

foods

Very low viscosity and high solubility May or may not gel

Snakes, bakery products, desserts, flavors

and colors

Modified Starches

Oxidation • Bleaching starch with NaHOCl

• Limited food functionality compare to

other MFS

• Low viscosity compare to NS

• Low heat hydration

• Low gelling properties

• High dry powder flow and do not caking

• Dusting for marshmallows and chewing-

gum

• Use for pharmaceutical industry for tablet

Modified Starches

Instant starches (IS)

Two types

Substantially degraded into fragments during manufacturing

Good stability, appearance, clarity and smooth texture

Use of IS: dry-mix pudding, gravies and sauces and extruded products

MODIFIED STARCH

Modified Starches

pH stability, viscosity stability, process tolerance, good textural properties, shelf-stable, emulsification stability, better surface appearance.

spaghetti sauce Water 62.37%

Crosslinked /substituted corn starch 3%

Tomato paste (32%) 30%

Salt 1.8%

Sugar 2.0%

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