MOISTURE REDISTRIBUTION AND TEXTURAL CHANGES IN STORED MODEL SANDWICHES

MOISTURE REDISTRIBUTION AND TEXTURAL CHANGES INSTORED MODEL SANDWICHES

ANN BARRETT1,4, ARMAND CARDELLO2, PAUL MAGUIRE1 andMICHA PELEG3

1Performance Enhancement and Food Safety TeamCombat Feeding Directorate

US Army Natick Soldier Systems CenterNatick, MA 01760-5018

2Product Optimization and Enhancement TeamUS Army Natick Soldier Systems Center

Natick, MA 01760-5018

3Department of Food ScienceUniversity of Massachusetts

Amherst, MA 01003

Received for Publication December 8, 2004Accepted for Publication August 26, 2005

ABSTRACT

Moisture migration and instrumental and sensory texture changes weredetermined in model cheese sandwich structures in which bread water activitywas varied by glycerol level. Sandwiches and components were evaluated bothafter assembly and 4-week ambient storage by uniaxial compression andmathematical description of stress-strain relationships, assessment of sensorycharacteristics by a trained panel, correspondence between predicted andactual stress-strain relationships, and mass-balance analysis of moisturemigration between cheese, bread crumb and bread crust. Textural changesoccurred as a result of both aging and moisture migration, and the accuracyof fitted “predicted” stress-strain relationships of the composites variedaccording to the extent of moisture migration. Sensory assessments of keytextural attributes of sandwich composites and components, before and afterstorage, were significantly correlated with measured mechanical parameters.

4 Corresponding author. TEL: 508-233-4516; FAX: 508-233-5191; EMAIL: [email protected]

Journal of Texture Studies 36 (2005) 569–588. All Rights Reserved.© Copyright 2005, Blackwell Publishing 569

KEY WORDS

Moisture migration, multilayer food, sandwiches, sensory-instrumentcorrelation, texture, water activity

INTRODUCTION

Interest in both shelf-stable foods and ready-to-eat convenience foods hasspawned efforts to stabilize complex, multilayer sandwich products. The U.S.military has, e.g., developed a line of water activity (Aw)-controlled sand-wiches specified to be stable for 18 months (Military Service RequirementADMN 02-11). Such products, however, are subject to the migration of mois-ture from higher to lower Aw regions if the composite is not initially matched.In such cases, equilibration to an eventual common Aw will yield a finalmoisture distribution different from what was originally formulated andintended (Labuza and Hyman 1998; Guillard et al. 2003).

A potential problem resulting from moisture redistribution is, of course,reduced microbial stability if the moisture-receiving component becomes highenough in Aw (and pH) to support growth. Even if the product remains micro-bially stable, moisture redistribution will inevitably alter texture. Classically, ahigher-moisture, higher-Aw filling is surrounded by a lower Aw bread or crust.In many products, the crust portion is expected to have a crisp texture, andincreases in moisture content will degrade this textural attribute and reducequality.

Shelf-stable sandwich-type foods are ideally formulated so that moistureredistribution is minimized, an objective only accomplished by equating Aw ineach layer. Formulating layers with precisely matched water activities,however, may not be feasible in every case. Freshly baked rolls, e.g., containinherent differences between the Aw of the crust and crumb; consequently,sandwiches constructed using these items will undergo at least a certain degreeof equilibration. For each product, an understanding of the effects of moistureredistribution on texture – and of formulation on the extent or rate of moistureredistribution – is needed.

A complicating factor concerning the stability of stored sandwiches isthat shelf-stable bread firms appreciably during storage, affecting both instru-mental texture (Barrett et al. 2000, 2002, 2005) and sensory attributes (Barrettet al. 2000, 2002) of the products. The firming of bread in sandwiches, giventhe simultaneous occurrence of moisture delivered from the filling to thebread, has not been addressed.

A method for predicting the texture of spongy composites fromthe mechanical parameters of individual components was described by

570 A. BARRETT ET AL.

Swyngedau and Peleg (1992). This technique is based on the fact that the levelof developed stress throughout a deformed assembly of multiple layers isconstant, whereas the levels of strain in each layer can differ depending on thedeformability characteristics of the individual components. Strain levels forthe components at discrete levels of stress are additive for the structure. Amathematical procedure for predicting strain, using the roots of pertinentfunctions describing the mechanical properties of individual layers, was dem-onstrated by Peleg (1993). A predicted stress-strain function for the compositecan be constructed from summed strain calculations at specified stressintervals.

Correspondence between the predicted and actual stress-strain relation-ships for multilayer foods can thus be used to assess the relative extent ofmoisture migration that occurred in the assemblies, in that less-than-predictedfirming of the bread component indicates plasticization due to moisture sorp-tion. The degree of the discrepancy between predicted and measured functionsis reflective of the degree of moisture transfer, which in turn influences sensoryperception of texture. Our objectives were to study the effects of moistureredistribution on the textural and sensory attributes of model sandwich struc-tures that were manipulated in original Aw difference through bread glycerolcontent.

MATERIALS AND METHODS

Bread and Sandwich Preparation

Standard, white military specification bread was prepared according tothe formulations shown in Table 1. Dry ingredients were premixed at lowspeed using a Hobart H-600 blender (Hobart Manufacturing Co., Troy, OH).Shortening and then water, or water and glycerol, were added. Batches weremixed at medium speed to develop the dough (~10 min) then formed into~75-g round rolls using an Fortuna A4-9670 dough divider (Adamatic,Eatontown, NJ). The rolls were proofed at 95% relative humidity and 32Cfor 40 min and baked at 175C for 25 min in a rotary oven (proofer and ovenby Hobart, Troy, NY).

Sandwiches were constructed by slicing the rolls in half lengthwise andby inserting a 0.7-cm thick by (approximately) 6 ¥ 6 cm slab of processedAmerican-type cheese (product # 8670 from Gamay Flavors, New Berlin, WI)between the halves. The long dimensions of the cheese slices were progres-sively trimmed and the slices weighed until 20-g specimens were obtained.The cheese was interfaced while the rolls were still slightly warm (~40C) sothat the cheese would better adhere to the crumb.

571MOISTURE AND TEXTURE CHANGES IN SANDWICHES

Rolls, sandwiches and slices of cheese were sealed in (polyethylene-foil-polypropylene) trilaminate pouches and blast frozen (at -20C). Thesamples were held frozen for no more than 1 week, pending analysis or storageschedule.

Sample Storage

Half of the rolls and sandwiches were put into storage at 22C andwithdrawn after 4 weeks – for instrumental, sensory and moisture-balanceanalysis. Moisture-balance analysis was additionally carried out on sand-wiches that had been stored for 8 weeks at 22C.

Physical Property and Moisture Analysis

Six freshly thawed rolls from each batch were analyzed for crust : crumbproportion and for the Aw and moisture content of the crust and crumb fractions(one replicate from each sample). Crust and crumb proportion was determinedby carefully separating the crust from the crumb (determination of crust wasbased on brown versus white color) with a razor blade and weighing thecomponents. The Aw of bread crust, bread crumb and cheese was measuredusing an AquaLab Aw meter (Decagon Devices, Pullman, WA). The moisturecontents of the cheese and bread components were determined by grinding thesamples and vacuum drying (�30 mmHg) for 12 h at 70C.

TABLE 1.DOUGH FORMULATIONS (% WEIGHT)

Ingredient (supplier) Formulation

Bread 1%

Bread 2%

Flour (ConAgra, Inc.) 54.2 51.1Water 30.6 28.6Shortening (ACH Food Company, Inc.) 9.0 8.5Glycerol (KIC Chemicals, Inc.) 0 6.0Yeast (Saf-instant) 2.4 2.2Salt (Morton, Inc.) 1.4 1.3Sucrose ester (Montello, Inc.) 1.1 1.0Gum arabic (Gum Technology, Inc.) 0.5 0.5Calcium sulfate (Archer Daniels Midland Arkady, Inc.) 0.3 0.3Xanthan gum (Kelco, Inc.) 0.3 0.3Encapsulated potassium sorbate (Balchem, Inc.) 0.1 0.1Cream flavor (David Michael & Co.) 0.1 0.1

100 100


Mechanical Analysis

Sandwiches stored for 4 weeks and freshly thawed sandwiches werecored into 20-mm diameter cylinders and trimmed so that 10 mm of breadremained on the top and bottom of the cheese. The trilayer cylinders werecompressed at 0.2 mm/s to 50% deformation using a TXT2 Texture Analyzer(Texture Technologies, Scarsdale, NY) interfaced with a Gateway E 4200computer (Gateway Inc., San Diego, CA), and force-deformation data wasacquired at 10 points per second.

Bread (20-mm diameter ¥ 20-mm high cylinders) and cheese (20-mmdiameter ¥ 7-mm high cylinders) stored for 4 weeks and freshly thawed wereindividually also compressed to 50% deformation according to the procedurepreviously mentioned to obtain mechanical parameters for the sandwich com-ponents that could subsequently be used to predict stress-strain relationshipsfor the composites.

Six replicates of the sandwich, bread and cheese lots were compressed.

Data Analysis: Mechanical Attributes

All data were converted to stress versus Hencky strain values.

Bread. The stress (s)-strain (e) relationships for the four bread speci-mens were fitted to the 3-parameter function,

se

e e=

¥+ ¥( )( ) ¥ -( )

C

C C1

2 31(1)

(in which C1 indicates relative compressive resistance or “firmness”; C2 is ashape parameter; and C3 is densification strain – strain at which the sample canno longer be reduced in volume, or 1 for very porous materials), whichdescribes the deformation behavior of foam-like, or “spongy” materials(Nussinovitch et al. 1991; Swyngedau et al. 1991; Barrett et al. 2002). Fittingof data was carried out using Sigma Plot (Jandel Scientific Co., San Rafael,CA). Fits of compression data to this function all had r 2 � 0.98.

Cheese. Cheese, unlike bread, is an incompressible material; i.e., cheesesamples maintain their volume during compression instead of collapsing intopore space. Therefore, horizontal deformation is compensated for by lateralexpansion of the specimens, i.e., “barreling out.” By contrast, deformation ofbread samples due to their porous structure does not produce an increase incross-sectional area. A schematic illustration of deformation of a representa-tive bread-cheese composite is shown in Fig. 1.


The stress-strain relationships for the stored and unstored cheese sampleswere thus first corrected for this increase in cross-sectional area (barreling outrelative to the bread), because it was assumed that the portion of the cheeseextending beyond the diameter of the bread did not contribute to stress. Thiscorrection factor corresponded to

A

A t

d

d t0 0

2

( )=

( )ÊË

ˆ¯ (2)

where A0 and d0 = initial cross-sectional area and diameter, respectively, andA(t) and d(t) = area and diameter at time, t. Because

dV

h02

0

4=p

(3)

and

hB,0 B

C

B

B

C

B

Compression

Force

hB(t)

d(t)

hC(t)

hB(t)

d0 d0

hC,0

hB,0

FIG. 1. SCHEMATIC REPRESENTATION OF COMPRESSION OFA BREAD-CHEESE COMPOSITE

The bread component, unlike the cheese component, does not increase in lateral area duringcompression, because the structure collapses into pore space.

B, bread; C, cheese; h, height; t, time; d, diameter.


d tV

h t( ) =

( )2 4

p(4)

where V = specimen volume, and h0 and h(t) are, respectively, initial heightand height at time, t, then

d

d t

VhV

h t

h t

h0

20

0

4

4( )ÊË

ˆ¯ =

( )

=( )p

p

(5)

Therefore, every stress value was multiplied by h(t)/h0.The corrected stress-strain functions were then fitted to

s e= C C4

5 (6)

in which C4 and C5 are parameters indicating relative dependence of stress onstrain and exponential character, respectively.

Sandwiches. The stress-stain relationships of the four sandwiches eachhad two distinct linear-appearing regions – one at low and another at highdeformation after a transition region. Firmness parameters were obtained bycalculating the apparent slopes of these regions,

C6 =DDse

(7)

using the first 20% (between 0 and 10% deformation, yielding C6a) of thestress-strain relationships and the final 20% (between 40 and 50% deforma-tion, yielding C6b) of the stress-strain relationships. The magnitudes of thestress-strain functions at maximum (50%) deformation were also recorded.

Representative stress-strain relationships for bread, cheese and the inter-faced composite are shown in Fig. 2.

Calculation of C4–C6 was accomplished using Minitab 7 (Minitab Inc.,State College, PA) statistical software.

Data Analysis: Prediction of Composite Stress-strain Relationships

The stress-strain profiles of the composites were estimated by the methodof Peleg (1993), using Mathcad Plus 6.0 software (Mathsoft, Cambridge, MA).By this procedure, the roots of the two functions for the bread and cheese


(calculated values using predetermined C1–C5 coefficients) are used to esti-mate the strain level of the composite sandwiches at specific stress levels, thusconstructing stress-strain functions from discrete values. First, the replicatedstress-strain relationships of the sandwiches were averaged into a representa-tive (mean) function for the sample. Then, 20 evenly spaced stress values wereextracted from the averaged relationships, using the entire (zero to maximum)stress range. Each stress value obtained from this procedure was then used inthe following procedure:

ee

e ee1

1 1

2 1 3 111

= =+( ) -( )

ÊËÁ

ˆ¯̃

root SC

C C, (8)

e e e2 4 2 25= =( )root S C C , (9)

e e eTOTAL hth h= +( )1

1 1 2 2 (10)

where S = stress, e1 = strain in the bread portion of the sandwich, e2 = strain inthe cheese portion of the sandwich, h1 = bread height, h2 = cheese height,ht = h1 + h2, and C1, C2, C3, C4 and C5 are parameters described in Eqs. (1) and(6). For each calculation, the particular stress level and specimen characteris-tics (i.e., dimensions and C1–C5) were entered, and e1, e2 and eTOTAL weredetermined.

Estimated stress-strain functions were compared to the actual, averagedstress-strain relationships by determining how much estimated strain deviated

Bread

Force

Deformation

Cheese Composite

FIG. 2. SCHEMATIC CURVES SHOWING THE TYPICAL SHAPESOF FORCE-DEFORMATION RELATIONSHIPS FOR BREAD, CHEESE AND

THE INTERFACED COMPOSITES


from (e.g., was less than) observed strain for each sandwich system. Thisprocedure was accomplished through regression analysis (Minitab Release 7),which fitted the constants for

Estimated Strain Observed Strain= + ( )a b (11)

where the coefficient, b, is a proportionality factor. By this method, a factor (b,or the slope of the equation) of one indicates perfect overlap, i.e., that thecomposite behaved as was estimated on the basis of analysis of the individualcomponents. A slope of �1 indicates that the specimens underwent higherlevels of strain than were predicted at the examined levels of stress, becauseof greater deformability of the material due to plasticization from moisturesorption.

Moisture Balance Analysis

At 4- and 8-week samplings, the bread and cheese (of sandwiches storedspecifically for moisture migration analysis) were separated, and the crustcarefully cut from the bread crumb (using brown versus white to distinguishcrust from crumb). Bread crumb and crust, and cheese were weighed, andmoisture content and Aw were measured by the aforementioned procedures. Amass balance that illustrated movement of water from cheese to bread crumband from bread crumb to bread crust was calculated at each sampling. Sixreplicates of each lot were analyzed, and results were averaged.

Sensory Texture Analysis

The sensory attributes of the sandwich assemblies and separated sand-wich components, before and after 4 weeks storage, were judged by a traineddescriptive panel of 11 individuals. Attribute intensities were judged using themethod of modulus-free magnitude estimation (Stevens 1953; Moskowitz1977). The panelists were given instruction on the use of the General Foodstexture profile method (Brandt et al. 1963) and on the magnitude estimationprocedure. All panelists had participated as members of an in-house laboratoryand descriptive texture profile panel for time periods ranging from 3 to15 years. All had participated in descriptive analysis of a wide range of breadand snack products, and all had prior experience on the use of magnitudeestimation scaling.

The panelists were presented with approximately 2.5 ¥ 2.5 cm squarespecimens of the sandwich assemblies with the crusts trimmed off, and alsocheese and bread crumb specimens that had been separated from the assem-blies. Evaluation consisted of partial compression of the samples withthe molar teeth and mastication up through swallowing. The panelists used


the psychophysical method of modulus-free magnitude estimation to judgethe perceived magnitude of the attributes listed in Table 2, the definitions ofwhich were developed during pretest evaluations of the products. The magni-tude estimation procedure involved assigning an arbitrary number to the firstsample to represent the perceived magnitude of the selected attribute inthe sample. Subsequent judgments of attribute intensities were made in a ratiorelative to the first sample. For example, if a sample was perceived to be twiceas firm as the first sample, it would be given a number twice as large for thatattribute; if a sample was perceived to be one-third as dense, it would be givena number one-third as large for that attribute.

Sensory data were normalized using modulus equalization (Moskowitz1977) to accommodate differences in the range of magnitude estimates used bythe panelists. Normalization involved calculating geometric means across allsamples and subjects for each attribute and session. Magnitude estimates foreach subject were multiplied by the ratio of the grand mean to the panelistmean for each attribute and session, thus producing a common scale for all thedata. Changes in magnitude estimates due to storage were furthermoreexplained in moisture redistribution.

Sensory data were also correlated with parameters of the stress-strainfunctions of the sandwiches (i.e., initial and final slopes, stress at 50% strain)by regression analysis.

RESULTS AND DISCUSSION

Initial Physical Properties of the Sandwich Components

Table 3 shows the initial moisture contents, water activities and specimenweights of the sandwich components. The addition of glycerol to the rolls

TABLE 2.SENSORY ATTRIBUTES AND DEFINITIONS*

Attribute Definition

First biteFirmness: The perceived force required to compress the sample between the molar teeth

before compactingDenseness: The perceived amount of material per unit volume during a single compression

with the molar teethMastication

Moistness: The perceived degree of moisture in the sample during chewingChewiness: The perceived total effort required to repeatedly compress the sample through

five chews

* Predetermined by the sensory panelists during initial training.


effectively reduced the Aw (and moisture content) of both crumb and crust.Glycerol also produced a relatively greater crust proportion in the rolls. Therewere large initial differences between the Aw of the crumb and crust in speci-mens formed from either formulation, creating a driving force for equilibrationwithin the rolls. The Aw of the cheese was substantially higher than that of thefresh 6% glycerol-containing rolls (crumb and, particularly, crust); the Aw ofthe cheese was only slightly higher than the 0% glycerol crumb but wellexceeded the Aw of the 0% glycerol crust. Thus, thermodynamic instability thatwould promote moisture redistribution existed in both systems, although to agreater extent in the 6% glycerol sandwich.

Mechanical Attributes: Changes through Storage

Parameters from Eqs. (1), (6) and (7), respectively, for bread, cheese andcomposites, determined from compression experiments (all before and afterstorage) are listed in Table 4. Shelf-stable bread crumb is known to firmappreciably during storage because of several contributing factors. Differentmechanisms that promote firming have been cited, including loss of plasticiz-ing moisture from the crumb to crust (He and Hoseney 1990; Baik andChinachoti 2003; Barrett et al. 2005); development of interactions betweengluten and starch (Martin et al. 1991); changes in the gluten fraction, partlydue to moisture redistribution from gluten to starch (Willhoft 1973; Kim-Shinet al. 1991); and starch retrogradation (Hug-Iten et al. 2001). In our experi-ments, C1 increased by 300–400%. The sandwich structures also became

TABLE 3.INITIAL PHYSICAL PROPERTIES OF SANDWICH COMPONENTS

Attribute Sandwich 1(0% Glycerol bread)

Sandwich 2(6% Glycerol bread)

Crumb mass (g) 47.6 (1.1)* 45.0 (3.3)Crumb proportion of roll (%) 72.2 (1.8) 69.0 (1.6)Crumb moisture content (wb %) 33.2 (1.4) 29.8 (0.90)Crumb Aw 0.92 (0.002) 0.87 (0.001)Crust mass (g) 18.4 (1.7) 20.2 (0.98)Crust proportion of roll (%) 27.8 (1.6) 31.0 (2.1)Crust moisture content (wb %) 21.4 (1.5) 20.8 (0.98)Crust Aw 0.87 (0.001) 0.82 (0.002)Cheese weight (g) 20 (~0)† 20 (~0)†Cheese moisture (wb %) 42.7 (1.5) 42.7 (1.5)Cheese Aw 0.93 (0.002) 0.93 (0.002)

* Numbers in parentheses are SDs.† Cheese specimens were trimmed to target weight.wb, wet basis.


somewhat firmer during storage, as evidenced by approximately a doubling inthe slopes of the stress-strain relationships. (The magnitudes of the six repli-cated stress-strain functions for the composites varied only modestly, and thecoefficients of variation for the highest stress levels in the functions were12.0 ± 0.32%.)

Predicted versus Actual Stress-Strain Functions

The sandwiches – primarily those constructed using the glycerol-containing rolls – did not firm to the degree expected based on the behavior of

TABLE 4.MECHANICAL PARAMETERS* OF BREAD, CHEESE AND COMPOSITES

Constituent Parameters fromEq. (1)

Parameters fromEq. (6)

Parameters fromEq. (7)

C1 (kPa) C2 C4 (kPa) C5 C6a C6b

Bread,0% glycerolTime 0

5.87(1.5)†

1.33(0.70)

– – – –

Bread,6% glycerolTime 0

6.25(2.3)

4.62(2.0)

– – – –

Bread,0% glycerol4 weeks

20.9(5.4)

2.49(1.1)

– – – –

Bread,6% glycerol4 weeks

25.7(11)

1.63(0.10)

– – – –

CheeseTime 0

– – 30.0(4.8)

0.990(0.090)

– –

Cheese4 weeks

– – 17.1(2.3)

0.750(0.060)

– –

Sandwich, 0%glycerol breadTime 0

– – – – 3.60(0.49)

6.62(0.62)

Sandwich, 6%glycerol breadTime 0

– – – – 2.68(0.86)

2.84(0.55)

Sandwich, 0%glycerol bread4 weeks

– – – – 6.08(1.2)

9.58(1.6)

Sandwich, 6%glycerol bread4 weeks

– – – – 4.77(0.83)

6.59(0.89)

* r2 for fits of individual stress-strain relationships to Eqs. (1), (6) and (7) were all �0.99.† Numbers in parentheses are SDs.


the noninterfaced constituents. Actual and root procedure-predicted stress-strain relationships for the sandwiches are shown in Fig. 3A–D. Correspon-dence between measured and predicted stress-strain functions was close forthe nonstored sandwich composites: slopes (b) from Eq. (11) (which indicatethe proportional magnitudes of the actual stress-strain relationships withrespect to the predicted stress-strain relationships) were 0.92 and 0.85 forsandwiches constructed using 0 and 6% glycerol bread, respectively. Corre-spondence between measured and predicted stress-strain functions was lowerfor stored sandwich systems: in these stored samples, slopes from Eq. (11)were 0.79 and 0.55 for composites containing 0 and 6% glycerol, respectively.

Higher-than-predicted strain in stored specimens was reflective of theplasticizing effect of moisture that had migrated from cheese to the bread,which caused the bread to firm less during storage than it would have hadmoisture content remained constant. Two countervailing processes were atwork: moisture migration, serving to reduce the mechanical parameters of thebread, and staling or aging, serving to increase the mechanical parameters ofthe bread. The substantially higher-than-predicted strain in the glycerol-

0% Glycerol sandwich, 0 timeA

C D

B

0

1

2

3

4

5

6

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Hencky strain

Str

ess

(kP

a)

Estimated

Observed

0% Glycerol sandwich, 4 weeks

0123456789

Hencky strain

Str

ess

(kP

a)

Estimated

Observed

6% Glycerol sandwich, 4 weeks

0

1

2

3

4

5

6

Hencky strain

Str

ess

(kP

a)Estimated

Observed

4

6% Glycerol sandwich, 0 time

Observed

Hencky strain

Estimated

Str

ess

(kP

a)

3.53

2.52

1.51

0.50

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

FIG. 3. CORRESPONDENCE BETWEEN PREDICTED (LINE) AND ACTUAL (TRIANGULARSYMBOLS) STRESS-STRAIN FUNCTIONS

(A) 0% glycerol bread after assembly (actual curve is 91% of predicted curve); (B) 6% glycerolbread after assembly (actual curve is 85% of predicted curve); (C) 0% glycerol bread after 4-week

storage (actual curve is 79% of predicted curve); (D) 6% glycerol bread after 4-week storage (actualcurve is 55% of predicted curve).


containing sandwich compared to the no-glycerol sandwich is consistent withthe relatively greater Aw difference between the cheese and bread crumb in thisspecimen.

Moisture Balance

Moisture migrated simultaneously from the cheese into the bread crumband from the bread crumb into the crust. Figures 4A,B illustrate this movementand show the moisture content of each component at each sampling. Thefigures also show that moisture migration was not complete at 4 weeks, butwas apparently close to complete after 8 weeks.

Table 5 shows the moisture mass balance within the sandwich systemsand confirms that relatively more moisture migration occurred in the glycerol-

Moisture loss/gain duringstorage: 0% Glycerol sandwich

20

25

30

35

40

45

0

Weeks

Mo

istu

re c

on

ten

t(w

b) Crust

Crumb

Cheese

Moisture loss/gain duringstorage: 6% Glycerol sandwich

20

25

30

35

40

45

0Weeks

Mo

istu

re c

on

ten

t(w

b) Crust

Crumb

Cheese

84

84

FIG. 4. MOISTURE BALANCE IN STORED SANDWICHES(A) 0% glycerol; (B) 6% glycerol. Moisture was drawn from the cheese into the bread crumb and

from the bread crumb into the crust. Relatively greater moisture migration occurred in theglycerol-containing sandwich.


TAB

LE

5.SA

ND

WIC

HM

ASS

BA

LA

NC

ET

HR

OU

GH

STO

RA

GE

Sand

wic

hty

peSt

orag

etim

e(w

eeks

)A

ttrib

ute

Com

pone

ntTo

tal

moi

stur

ein

sand

wic

hC

rust

Cru

mb

Che

ese

Con

stru

cted

from

brea

dco

ntai

ning

0%gl

ycer

ol0

Wei

ght

(ini

tial,

g)18

.4(1

.7)*

47.6

(1.1

)20

.0(~

0)M

oist

ure

cont

ent

(wb

%)

21.4

(1.5

)33

.2(1

.4)

42.7

(1.5

)W

eigh

tdr

ym

atte

r(g

)14

.531

.811

.5W

eigh

tw

ater

(g)

3.94

15.8

8.54

28.3

4M

oist

ure

cont

ent

(wb

%)

23.5

(0.1

4)34

.2(2

.5)

37.2

(0.1

5)W

eigh

tw

ater

(g)

4.44

16.5

6.79

27.8

8M

oist

ure

cont

ent

(wb

%)

24.6

(0.5

5)33

.7(0

.96)

34.8

(0.5

5)W

eigh

tw

ater

(g)

4.72

16.2

6.12

27.0

Con

stru

cted

from

brea

dco

ntai

ning

6%gl

ycer

ol0

Wei

ght

(ini

tial,

g)20

.2(1

.8)

45.0

(1.5

)20

.0(~

0)M

oist

ure

cont

ent

(wb

%)

20.8

(1.6

)29

.8(1

.5)

42.7

(1.5

)W

eigh

tdr

ym

atte

r(g

)16

.032

.011

.5W

eigh

tw

ater

(g)

4.20

13.0

8.54

25.7

4M

oist

ure

cont

ent

(wb

%)

25.7

(1.1

1)32

.5(0

.15)

30.8

(0.3

1)W

eigh

tw

ater

(g)

5.53

15.4

5.10

26.0

8M

oist

ure

cont

ent

(wb

%)

27.7

(0.3

8)32

.0(0

.70)

30.0

(0.5

5)W

eigh

tw

ater

(g)

6.13

15.1

4.46

25.7

*N

umbe

rsin

pare

nthe

ses

are

stan

dard

devi

atio

ns.

wb,

wet

basi

s.


containing sandwich: the amount of water in the bread crumb in these speci-mens increased 16% over 8 weeks compared to a 3% increase in theno-glycerol bread crumb. This substantially larger amount of moisture transferinto the glycerol-containing bread was driven by the relatively larger differ-ence in Aw between the cheese and bread crumb in these assemblies – and isconsistent with the relatively higher discrepancy between predicted and actualstress-strain relationships for this system.

Sensory Characteristics

The geometric mean magnitude estimates of perceived firmness, chewi-ness, denseness and moistness for the sandwich assemblies and for the disas-sembled sandwich components at zero time and after 4-week storage areshown in Fig. 5. SEs as a proportion of the geometric means of judgmentswere 4.8 ± 1.2, 4.8 ± 1.5, 4.5 ± 1.4 and 4.6 ± 1.3% for firmness, chewiness,denseness and moistness, respectively.

Magnitude estimates of firmness, chewiness, and denseness increasedsignificantly (P � 0.005) during storage in all cases for the bread, cheese andsandwiches. Increased perception of these attributes for bread or sandwiches is

First bite – denseness

First bite – firmness Mastication – chewiness

Mastication – moistness

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0% Glycerol6% GlycerolBread Cheese Sandwich

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Bread Cheese Sandwich

Bread Cheese Sandwich

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FIG. 5. CHANGE IN MAGNITUDE ESTIMATES (SENSORY DETERMINATIONS) OF (A)FIRMNESS, (B) CHEWINESS, (C) DENSENESS AND (D) MOISTNESS OF SANDWICHES

AND COMPONENTS FROM 0- TO 4-WEEKS STORAGEIncreased perception of resistance attributes and decreased perception of moistness after storage and

relatively more change in 0% glycerol sandwiches are shown.


reflective of the higher measured mechanical resistance (i.e., increased C1 forbread and increased slopes [C6a and C6b] for sandwiches) because of agingduring storage.

Concurrently, perceived moistness for the bread, cheese and sandwichesdecreased significantly (P � 0.001) during storage. Decreased perceivedmoistness for the sandwiches was not due to dehydration of the bread crumb,because crumb moisture content actually increased due to migration of waterfrom the cheese (Table 5) and was most likely associated with the aging, andconsequent increase in mechanical resistance, of the bread crumb.

Concerning the role of glycerol, analysis of variance conducted on thedata in Fig. 5 shows significantly higher (P � 0.005) perceptions of moistnessfor both the bread and sandwiches containing this plasticizer. Similarly, therewere significantly lower (P � 0.005) perceptions of firmness, chewiness anddenseness for these specimens. Although there were no significant interactioneffects between the presence/absence of glycerol in the samples and storagetime, the relative slopes of the plots in Fig. 5 suggest that there was less changeduring storage in perceptions of the glycerol-containing bread/sandwich speci-mens than in perceptions of the no-glycerol specimens (i.e., relatively lowerslopes between 0- and 4-week mean magnitude estimates for the glycerol-containing samples). This subtle effect is most likely due to the greater transferof moisture from the cheese to the bread within these assemblies (Table 5).Greater migration of moisture, and thus more pronounced plasticization of thecrumb of these samples, resulted in a smaller increase in perceptions ofresistance attributes.

Regarding the cheese, there was no main effect of glycerol on sensorycharacteristics, because the cheese did not contain glycerol. However, therewas a significant interaction effect between storage time and the presence/absence of glycerol in the sandwich (bread) for both cheese firmness(Ff = 5.24, P � 0.05) and cheese denseness (Fd = 4.47, P � 0.05). The plotsfor cheese in Fig. 5 reveal these interaction effects and similar, but moresubtle (i.e., did not reach statistical significance), interaction effects for per-ceived moistness and chewiness: in particular, the relatively greater loss ofmoisture in the cheese interfaced with the glycerol-containing bread(Table 5) was reflected in the comparatively more negative slope between 0and 4 weeks for perceived moistness for this sample. Cheese interfaced withglycerol-containing bread had relatively greater increases in perceived firm-ness, chewiness and denseness (i.e., comparatively higher slopes between 0and 4 weeks).

While the primary objective of judging sensory attributes was to quantifythe relative magnitudes of changes due to storage, “quality” or “acceptability”issues are presumably related to the degree samples differ from the fresh orunstored product.


Sensory attributes (perceived chewiness, firmness and denseness) for thesandwiches were also strongly correlated with mechanical attributes (Table 6).

CONCLUSIONS

Changes in the physical properties of sandwich structures during storageare due to both aging of components and moisture redistribution. Specifically,the migration of moisture from a higher-Aw sandwich filling into a lower-Aw

bread can plasticize the bread and lessen the storage-induced firming of thiscomponent. The relative extent of moisture redistribution in a stored compositeis reflected in the relative discrepancy between its measured stress-strainrelationship and the stress-strain relationship estimated from its noninterfacedcomponents. A correspondence between this discrepancy and the actualamount of migrated moisture was demonstrated.

Redistribution of moisture within sandwich structures also affects per-ceptions of textural characteristics. Judgments of the sandwich texture andindividual sandwich components were influenced by the relative extent ofmoisture migration. Control of moisture redistribution in complex foodsthrough formulation parameters that produce fairly matched Aw is necessary toensure stability and to minimize textural changes during storage.

TABLE 6.CORRELATION BETWEEN SENSORY AND INSTRUMENTAL

MEASUREMENTS FOR SANDWICH ASSEMBLIES, FROMREGRESSION ANALYSIS

Relationship t-ratio r 2

Chewiness-C6a† 6.4 0.90*Chewiness-C6b‡ 26 0.99*Chewiness-force at 50% strain 13 0.98*Firmness-C6a 5.4 0.93*Firmness-C6b 4.6 0.87*Firmness-force at 50% strain 5.8 0.92*Denseness-C6a 4.9 0.89*Denseness-C6b 2.9 0.71*Denseness-force at 50% strain 3.0 0.72*

* Significant at P � 0.05.† Apparent slope (initial) of the stress-strain relationship, a measure

of stiffness.‡ Apparent slope (final) of the stress-strain relationship, a measure

of stiffness.


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MOISTURE REDISTRIBUTION AND TEXTURAL CHANGES IN STORED MODEL SANDWICHES

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