EFFECT OF BREAD CRUMB STRUCTURE AND COMPOSITION ON RATE OF MOISTURE SORPTION IN SANDWICH COMPOSITES

EFFECT OF BREAD CRUMB STRUCTURE ANDCOMPOSITION ON RATE OF MOISTURE SORPTION IN

SANDWICH COMPOSITES

ANN BARRETT1,3, UMRAZ SAJJAD1 and GONUL KALETUNC2

1Performance Enhancement and Food Safety TeamCombat Feeding Directorate

US Army Natick Soldier RD&E CenterNatick, MA 01760-5018

2Department of Food, Agricultural and Biological EngineeringThe Ohio State University590 Woody Hayes Drive

Columbus, OH 43210-1057

Accepted for Publication September 10, 2008

ABSTRACT

Shelf-stable sandwiches will undergo thermodynamically driven migra-tion of moisture from relatively high to relatively low aw regions, therebypotentially compromising stability and acceptance. While differences in aw

determine the final moisture-content distribution, rates of migration can beaffected by physicochemical properties of the moisture-receiving phase.Lowered aw bread crumb that varied in: bulk density/cell size, oil content,resistant starch content and gluten content was produced and interfaced withhigher aw cheese and stored for 4 weeks. The bread was sliced into 5 mmsections parallel to the interface and added moisture per gram calculated foreach section weekly, and distributions of g added moisture/g dry weight versusdistance were determined. Treatment varied distribution parameters, andsorption was reduced in higher density, oil content and resistant starch contentbread crumb. Adjustment of bread physicochemical properties can improve theshelf life of stored sandwiches.

PRACTICAL APPLICATIONS

Migration of moisture from higher to lower water activity regions instored sandwich composites can be slowed – and thus, product quality

3 Corresponding author. TEL: +508-233-4516; FAX: +508-233-5181; EMAIL: [email protected]

Journal of Food Processing and Preservation 34 (2010) 460–475.460 DOI: 10.1111/j.1745-4549.2008.00351.x

This article is a US Government work and is in the public domain in the USA.Journal compilation © 2009 Wiley Periodicals, Inc.

enhanced – by adjusting the physicochemical properties of the bread phase.Specifically, increasing the bulk density, oil content and resistant starch contentof the bread/crust component will slow sorption from higher aw fillings.

INTRODUCTION

Shelf-stable foods are convenient and are becoming increasingly popular.The military, due to mass-distribution constraints, has even more stringentshelf life requirements than industry, necessitating at least an 18-month shelflife at 27C. However, “fresh-seeming” foods, particularly baked products, arerelatively new and popular items on the army feeding menu. Shelf-stablesandwiches, produced using specially developed “Meal, ready-to-eat” bread,are one such ration item, in which controlled water activity/controlled pHbread is combined with an equivalently shelf-stable filling. Water activity inmilitary specification bread and sandwiches is usually reduced through partialreplacement of moisture with glycerol, both in the bread crumb and filling.However, despite close matching of water activities during development, somemoisture equilibration in the sandwich is inevitable. During storage, moisturemigrates from the filling to the crumb and eventually to the bread crust – thelowest aw domain in the sandwich – until equilibrium is established. Moisturemigration from filling into crumb has been described by Guillard et al. (2003)as a combination of interface monolayer saturation, vapor and liquid diffusion,and capillary flow. Moisture migration may potentially produce undesirableeffects in products, such as reduced microbial stability, deleterious texturalchanges and reduced sensory quality (Labuza and Hyman 1998).

While thermodynamic instability determines the ultimate moisture profilein a composite, the rate of moisture redistribution can vary; furthermore,adjusting the kinetics of moisture migration can potentially increase thestorage period within which a product remains highly acceptable. Forexample, increased structural tortuosity potentially can provide a physicalhindrance to moisture migration; also, reduced chemical affinity (e.g., reduced“wetting” of the receptor surfaces) can render the matrix relatively less recep-tive to moisture sorption. Several physicochemical factors have been reportedto slow diffusion or sorption in model food systems. A reduced rate of diffu-sion in relatively low porosity samples was reported by Marousis et al. (1991)for drying of granular starch slabs and by Fu et al. (2003) for heating/drying ofdough. Analogously, the rate of rehydration of dried carrots was found toincrease with increasing porosity by Marabi and Saguy (2004). Ohtsuka et al.(1994), using nuclear magnetic resonance (NMR) analysis of starch gels,reported lowered apparent diffusion coefficients with either retrogradation orincreasing concentration, theoretically due to the thickness and number of

461BREAD CRUMB STRUCTURE AND SANDWICH COMPOSITES

polymeric structures/barriers in the network. Similarly, studies on wheyprotein gels showed reduced sorption into and expressible moisture from moreconcentrated and interconnected sample structures (Barrett et al. 1998).

Both decreased porosity and increased lipid content were found to reducethe effective moisture diffusivity in model sponge cakes (Roca et al. 2006). Fatcoating of a flour-based crunchy product was similarly reported to increasehydration time (Weglarz et al. 2007). Another potential chemically basedinhibitor of sorption into flour-based systems is aging, which presumablyincreases molecular associations through amylopectin crystallization (Baikand Chinachoti 2000; Baik et al. 2003) and likely cross-linking of starch andgluten (Martin and Hoseney 1991; Martin et al. 1991; Barrett et al. 2000).Aging of the polymeric matrix in flour-based baked foods has been foundthrough NMR analysis (Ruan et al. 1996) to contribute to a population of lessbound water, theoretically due to exclusion of water from the network.

Our objectives were to systematically alter the structure and chemicalcomposition of the bread used in interfaced composites while maintaining thethermodynamic driving force for diffusion (e.g., aw difference with a moremoist layer), and to quantify the effects of these changes on moisture sorptionrates. Crumb structure was adjusted through a combination of yeast contentand proofing time. Oil content was adjusted in order to vary the hydrophobicityof the crumb. Resistant starch was included in order to increase the crystal-linity of the crumb. Gluten was added in order to increase intermolecularinteraction in the crumb. Standard bilayer composites were constructed frominterfaced blocks of bread crumb and commercial cheese, and stored for 4weeks. Bread samples were sectioned weekly for moisture distribution analy-sis. Added-moisture versus distance profiles at each analysis time was fitted toan empirical equation in order to determine the effect of treatment on moisturesorption characteristics of the bread crumb. Calculated moisture sorptionparameters for each bread type were compared statistically in order to deter-mine significance of structural/chemical effects.

MATERIALS AND METHODS

Formulations and Baking

The four conditions tested separately in standard “Meal, ready-to-eat”(MRE) white pan bread (Table 1) were adjustment of crumb bulk density,addition of (liquid) oil, addition of resistant starch and addition of gluten.Bread production was varied according to the formulation and process changeslisted in Table 2. Proof times were varied in order to obtain – except in the caseof deliberately varied structure – as comparable raise volumes as possible.Loaf center temperatures were monitored every 5 min after 40 min, and baking

462 A. BARRETT ET AL.

times adjusted accordingly to ensure a constant end temperature (97C). Afterbaking and cooling, the crust from each loaf was cut off and the crumb waspackaged in 50 ¥ 30 cm trilaminate pouches. The sealed pouches were brieflymaintained at -10C until physical property analysis or construction of sand-wich assemblies.

TABLE 1.BASE BREAD FORMULATION*

Ingredient Percent

Flour (ConAgra, Omaha, NE) 47.7Water 27.0Shortening (ACH Food Co, Memphis, TN) 8.0Glycerol (KIC Chemicals, New Paltz, NY) 8.0Yeast (Saf-instant, Lesaffre Yeast Corp., Milwaukee, WI) 6.0Salt (Morton, Chicago, IL) 1.0Sucrose ester (Montello, Tulsa, OK) 1.0Gum arabic (Gum Technology, Tucson, AZ) 0.5Calcium sulfate (ADM Arkady, Decatur, IL) 0.3Xanthan gum (Kelco, Atlanta, GA) 0.3Encapsulated potassium sorbate (Balchem, Hampton, NY) 0.3

* 2500 g batches mixed.

TABLE 2.VARIATIONS TO BASE FORMULATION AND BAKING PROCEDURE*

Variable Concentrationsused (%)

Adjustments to baseformulation

Prooftimes (h)

Bakingtimes (min)

Yeast content 136

Yeast reduced; otheringredient ratios retained

1-1/211

504545

Oil content 05

1015

Shortening eliminated; otheringredient ratios retained

1111-1/2

45455050

Resistant starch content 05

10

Starch replaced equal weightof flour

111-1/2

455050

Gluten content 05

10

Other ingredient ratios retained 111-1/2

454550

* Baking procedure: in Hobart H-600 mixer, blend dry ingredients, then shortening, then glycerol andmix on intermediate speed for 6 min, until development; let rest for 10 min; divide into three loaves(~830 g each) and place in 26 ¥ 11 cm aluminum pans; proof at 95% RH/32C and bake at 177C forspecified times. Proofer and convection/rotary oven by Hobart.


Physical Property Analysis

Bread crumb for each variable was analyzed for: water activity (AqualabSeries 3TF aw meter, in triplicate); dry-basis moisture content, by vacuumdrying (HotPack/SP Industries 207380 vacuum oven, for 16 h at 70C, intriplicate); bulk density, by weighing 2-cm diameter by 2-cm high (cored)cylinders (in triplicate); and mean area cell size, using image analysis(Olympus Cue2 image analyzer) according to the sample preparation andprocession procedures described by Barrett and Peleg (1992), in which a cutsurface was subjected to inking in order to delineate cell walls, and the visuallycircumscribed cell interiors were counted and measured. Mean cell area sizeswere obtained by acquiring cumulative populations of ~100 cells.

Sandwich Construction and Storage

Approximately 5 ¥ 5 ¥ 3 cm blocks of bread crumb were cut from theloaves using a sharp knife, interfaced with equivalently sized blocks of pro-cessed cheese (Kraft Velveeta, with aw of 0.92 � 0.003) along the larger facesand secured together with plastic wrap. The cheese was sufficiently adhesiveand the wrapping sufficiently secure to ensure complete contact between thebread and cheese. The assemblies were then sealed in 15 ¥ 25 cm trilaminatepouches, maintained at 4C, and withdrawn for analysis after 1, 2, 3 and 4weeks. Three sandwich replicates for each variable and each analysis timewere prepared.

Retrograded Bread Specimens

A few loaves of the high and low density variables were packaged(without crust) and stored for 2 weeks at 22C before sandwich assembly inorder to assess the relative effects of retrogradation on moisture sorptionkinetics.

Analysis

Absorbed Moisture Profile Determination. At each analysis time, thebread was separated from the cheese and sliced, parallel to the contact face,into four 5-mm thick sections using a razor blade, and the sections immedi-ately weighed. Care was taken to minimize air exposure – i.e., the specimenswere kept in a plastic bag at all times except for slicing and prompt weighing.The sections were vacuum-dried (HotPack 207380 oven) for 16 h at 70C, thenreweighed, and dry basis moisture content (mcdb), were calculated.

mcInitial sample weight dried sample weight

dried sampldb = −ee weight

(1)


Four sections of bread – between 0–5 mm, 5–10 mm, 10–15 mm and15–20 mm from the cheese–bread interface – for each of the specimen repli-cates were evaluated.

The moisture adsorbed by the bread per gram of dry crumb (mad) in eachsection was determined by taking the difference between the dry basis mois-ture content at each time point and the initial dry basis moisture content. Theadsorbed moisture as a function of distance from the cheese–bread interfacewas fitted to an empirical equation of the following form:

m C bxad = ( )exp (2)

in which C is the fitted adsorbed moisture (per gram of dry bread) at thecheese–bread interface, b indicates the relative steepness of the moisturecontent gradient in the sample, and x is distance from the interface. Midpointsof each section (i.e., x = 2.5, 7.5, 12.5 and 17.5 mm) were used in Eq. (2). Totalper gram of dry bread moisture absorbed by the specimens into the 20-mmwide measurement blocks was also determined at each time point.

Statistical Analysis of Moisture Sorption Effects. Significance ofeffects of structural attributes (bulk density) and formulation variables (percentoil content, percent resistant starch content and percent added gluten content)on parameters b and C from Eq. (2) and on total moisture absorbed weredetermined through multiple regression analysis. Factors with Student t val-ues � 2.0 were considered significant. Statistical analyses were performedusing Minitab Release 10 software (Minitab, Inc., State College, PA).

RESULTS AND DISCUSSION

Bread Physical Properties

Crumb water activity (0.88 for most samples) was roughly constant andvaried at most by only �0.01. The bulk density and mean cell size of eachspecimen are shown in Fig. 1a,b, respectively. Bulk density decreased withincreased yeast content, as was anticipated, and was accompanied by a corre-sponding increase in mean cell size – indicating generally more expansion ofthe structure during proofing. Added oil had a similar but less pronouncedeffect on structure, also progressively reducing density and increasing meancell size. Added resistant starch or gluten had effects on structure opposite tothose of yeast or oil; both generally increased density and reduced mean cellsize.


Sorption Behavior: Effects of Spatial Distance and Time

Spatial plots of added moisture per gram of dry bread crumb versusdistance from the interface for 1, 2 and 4 week storage/equilibration times areshown in Fig. 2a–d. In each plot, added moisture versus distance relationshipstrend downward (i.e., relatively more moisture sorption nearer the cheeseinterface), with relationships showing progressively increased overall sorption(i.e., generally higher curves) and becoming less negative in slope (i.e., indi-cating a more uniform added moisture profile) with storage time. In each plot,there is progressively more “spread” between the curves at larger distancesfrom the interface, indicating fairly fast saturation of the interface region andthen slower diffusion into the interior of the bread. Table 3 shows that storagetime, or an interaction effect of storage time, in most cases significantlyincreased both “b” from Eq. (2) (indicating a decreased difference betweeninterface and interior moisture) and total moisture absorbed.

0.4

0.5

0.6

0.7

0.8

0.9

0 5 10 15

Percent Addition

Yeast

Oil

Starch

Gluten

Den

sity

y (g

/cc)

0.3

0.6

0.9

1.2

0 5 10 15Percent Addition

Mea

n C

ell s

ize

(sq

. cm

.)

Yeast

Oil

Starch

Gluten

a

b

FIG. 1. EFFECT OF FORMULATION/TREATMENT VARIABLES ON (A) CRUMB DENSITYAND (B) MEAN CELL SIZE


High Density Bread

0

0.05

0.1

0.15

0.2

0.25

0 5 10 15 20

Distance From Interface (mm)

Gra

ms

Ad

ded

H2O

/G

ram

s D

ry B

read

1 Week

2 Weeks

4 Weeks

Medium Density Bread

0

0.05

0.1

0.15

0.2

0.25

0 5 10 15 20

Distance From Interface (mm)G

ram

s A

dd

ed

H2O

/ G

ram

s D

ry B

read

1 Week

2 Weeks

4 Weeks

Low Density Bread

0

0.05

0.1

0.15

0.2

0.25

0 5 10 15 20Distance From Interface (mm)

Gra

ms

Ad

ded

H2O

/G

ram

s D

ry B

read

1 Week

2 Weeks

4 Weeks

a

b

FIG. 2. EFFECTS OF VARYING BREAD VARIABLE ON SORPTIONVERSUS DISTANCE RELATIONSHIPS

(A) Density on sorption, (B) oil content on sorption, (C) resistant starch content on sorption and (D)added gluten content on sorption


0% Resistant Starch Bread

0

0.05

0.1

0.15

0.2

0 5 10 15 20


Gra

ms

Ad

ded

H2O

/ G

ram

s D

ry B

read

1 Week

2 Weeks

4 Weeks 10% Resistant Starch Bread

0

0.05

0.1

0.15

0.2

0 5 10 15 20


Gra

ms

Ad

ded

H2O

/G

ram

s D

ry B

read

1 Week

2 Weeks

4 Weeks20% Resistant Starch Bread

0

0.05

0.1

0.15

0.2

0 5 10 15 20


Gra

ms

Ad

ded

H2O

/G

ram

s D

ry B

read

1 Week

2 Weeks

4 Weeks

c

0% Added Gluten Bread

0

0.06

0.12

0.18

0 5 10 15 20


Gra

ms

Ad

ded

H2O

/G

ram

s D

ry B

read

1 Week

2 Weeks

4 Weeks 5% Added Gluten Bread

0

0.06

0.12

0.18

0 5 10 15 20


Gra

ms

Ad

ded

H2O

/G

ram

s D

ry B

read

1 Week

2 Weeks

4 Weeks

10% Added Gluten Bread

0

0.06

0.12

0.18

0 5 10 15 20


Gra

ms

Ad

ded

H2O

/G

ram

s D

ry B

read

1 Week

2 Weeks

4 Weeks

d

FIG. 2. CONTINUED


Average coefficients of variation for added moisture/g replicates were:11% for the varying density batch, 18% for the varying oil content batch, 14%for the varying resistant starch content batch and 9% for the varying (added)gluten content batch. Fits for regression-determined C and b, in which allreplicates were combined, consistently had P < 0.05.

Sorption Behavior: Effects of Structure and Formulation

Density Effects. Increasing the bread crumb bulk density, with formu-lation (other than yeast content) constant among the samples, reduced mois-ture sorption, as is evidenced by the progressively lowered (added mad versusdistance from the interface) curves (Fig. 2a). Increasing density significantlydecreased interface, added moisture (C) and reduced total moisture absorbedby the specimen (Fig. 3a and Table 3). It is likely that less expanded breadstructures have relatively less capillary draw (i.e., fewer and smaller channels)and generally more tortuous diffusion paths compared to more expanded breadstructures, effectively slowing the rate of moisture migration.

TABLE 3.SIGNIFICANT EFFECTS* OF PROCESS/FORMULATION ON

MOISTURE MIGRATION PARAMETERS

Bread variable Moisture migrationparameter

Significantlyaffected by

t Ratio

Crumb bulk density Intercept* Crumb bulk density -2.3Time -3.9Bulk density ¥ time -3.9

Slope† Time 6.3Bulk density ¥ time 3.3

Total mass absorbed Crumb bulk density -5.4

Oil content Intercept Oil content -2.7Oil content ¥ time -3.9

Slope Time 5.6Total mass absorbed Oil content -7.0

Time 4.8

Resistant starch content Intercept Starch content -2.8Total mass absorbed Starch content -5.6

Starch content ¥ time -2.9

Added gluten content Intercept Added gluten content -2.1Slope Time 3.5

Gluten content ¥ time 2.0Total mass absorbed Time 4.1

* C from Eq. (2) (fitted added moisture per gram at interface).† b from Eq. (2) (fitted added moisture per gram gradient).


Oil Effects. Increasing oil content in the crumb had effects on sorptionanalogous to and more pronounced than those due to increasing density,significantly reducing both C and total moisture absorbed (Table 3). As canbe seen in Figs. 2b and 3b, the amount of added moisture per gram for

0

0.1

0.2

0.3

0.4

0.5

High, 1 wk

High, 2 wks

High, 3 wks

High, 4 wks

Med, 1 wk

Med, 2 wks

Med, 3 wks

Med, 4 wks

Low, 1wk

Low, 2wks

Low, 3wks

Low, 4wks

a

b

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0%, 1 wk

0%, 2 wks

0%, 3 wks

0%, 4 wks

5%, 1 wk

5%, 2 wks

5%, 3 wks

5%, 4 wks

10%, 1 wk

10%, 2 wks

10%, 3 wks

10%, 4 wks

15%, 1 wk

15%, 2 wks

15%, 3 wks

15%, 4 wks

FIG. 3. EFFECTS OF VARYING BREAD VARIABLE ON TOTAL SORPTION(g WATER/g DRY BREAD)

(A) Density, (B) oil content, (C) resistant starch content and (D) added gluten content


15% oil bread is approximately half that for 0% oil bread; and sorption isreduced progressively with oil content. Oil rendered the bread network gen-erally more hydrophobic and may also have produced a lipid surface“coating” on pores.

0

0.1

0.2

0.3

0.4

0.5

0%, 1 wk

0%, 2 wks

0%, 3 wks

0%, 4 wks

10%, 1 wk

10%, 2 wks

10%, 3 wks

10%, 4 wks

20%, 1 wk

20%, 2 wks

20%, 3 wks

20%, 4 wks

c

d

0

0.1

0.2

0.3

0.4

0.5

0.6

0%, 1 wk

0%, 2 wks

0%, 3 wks

0%, 4 wks

5%, 1 wk

5%, 2 wks

5%, 3 wks

5%, 4 wks

10%, 1 wk

10%, 2 wks

10%, 3 wks

10%, 4 wks

FIG. 3. CONTINUED


Resistant Starch Effects. Resistant starch affected moisture migrationprimarily at the highest concentration, 20%, at which level sorption wasreduced by roughly half compared to that for the no-added-starch sample(Figs. 2c and 3c). Added-starch level significantly reduced both C and totalsorption (Table 3). The ordered, crystalline nature of this starch most likely hasless tendency to draw and hold water than the amorphous gelatinized starch inthe flour that it replaced. While the bread containing the highest level ofresistant starch did have a higher density and smaller cells than the no-added-starch sample (attributes which themselves could have served to slow diffu-sion), comparison with results for the high-density (adjusted through yeastcontent) specimen shows relatively markedly less migration into the starch-containing sample – indicating a definite effect of this ingredient itself ininhibiting sorption.

Gluten Effects. The primary effect of added gluten was that it magnifiedthe effect of time on sorption. As can be seen in Fig. 2d, the spread between the1, 2 and 4 week curves becomes greater with gluten content, also indicated bya significant gluten content ¥ time interaction effect (Table 3), i.e., glutenpromoted greater moisture penetration into the specimens with time. However,no significant effect of gluten on total moisture was determined, and corre-spondingly, Fig. 3d shows no discernable trend in total moisture sorption withgluten content. It may be that added gluten, by reducing the overall proportionof starch, also over time reduced the proportion of crystallizing starch in thesamples which, as was demonstrated by the resistant starch-containing speci-mens, can inhibit sorption.

Aging Effects. The inhibiting effects of crumb aging on moisture sorp-tion are demonstrated by comparison between Fig. 3a and Fig. 4, whichrespectively show cumulative (total) moisture sorption into fresh and stored-before-composite-construction bread (high density and low density variables).Crumb aged without crust for 2 weeks (in which no moisture was lost)absorbed substantially less water (~40%) than did interfaced fresh crumb. Thisreduction in sorption is also reflected in the generally lower interface moisturecontents (C) and steeper negative slopes in the added moisture/g versus dis-tance relationships (b) for the aged-before-interfacing bread, compared totheir corresponding fresh-bread composites (Table 4). Presumably, the storedcrumb would have relatively greater retrograded, crystalline starch (Baik andChinachoti [2000] demonstrated storage-induced amylopectin crystallinityeven in bread crumb that had been separated from crust to prohibit moistureloss), which would have relatively less affinity for water than the more amor-phous fresh crumb. These results are therefore consistent with those showingthe diffusion-inhibiting benefit of added resistant starch.


CONCLUSIONS

The stability of multicomponent, multiple-water activity foods can beincreased by adjusting the properties of the lower aw phase. Moisture migrationcan be slowed through kinetic approaches that render the receptor component

0

0.1

0.2

0.3

0.4

FIG. 4. TOTAL SORPTION (g WATER/g DRY BREAD) INTO AGED (2 WEEKS) BREAD(HIGH AND LOW DENSITY SAMPLES)

TABLE 4.FITTED PARAMETERS FOR ADDED-MOISTURE CURVES,

COMPARISON OF AGED VERSUS FRESH CRUMB

Sample Week Fitted addedmoisture/gat interface (C)†

Fitted addedmoisture/ggradient (b)†

Significance*

Low density bread,6% yeast

1 0.32 -0.14 **1, aged bread 0.13 -0.18 ***2 0.16 -0.057 ***2, aged bread 0.11 -0.11 ***3 0.15 -0.043 ***3, aged bread 0.091 -0.091 ***

High density bread1% yeast

1 0.15 -0.11 **1, aged bread 0.12 -0.18 ***2 0.13 -0.072 **2, aged bread 0.14 -0.113 0.13 -0.048 **3, aged bread 0.14 -0.15 ***

* P < 0.05; ** P < 0.01; *** P < 0.001.† From Eq. (2).


less hydrophilic or more structurally hindered. Three such approaches –increasing bread density, oil content and crystallinity – have been demon-strated to significantly reduce measured moisture sorption parameters. Theseapproaches are practical (nonthermodynamic) processing and formulationtechniques that can potentially improve shelf life.

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EFFECT OF BREAD CRUMB STRUCTURE AND COMPOSITION ON RATE OF MOISTURE SORPTION IN SANDWICH COMPOSITES

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