A Fresh Perspective on Staling: The Significance of
Starch Recrystallization on the Firming of Bread
L.M. HALLBERG AND P. CHINACHOTI
ABSTRACT: Storage stability of standard white bread (SWB) and Meal, Ready-to-Eat (MRE) breads were studied
in terms of texture firming, amylopectin recrystallization, and water relations. SWB showed a more rapid increase in firmness during storage mainly due to the loss of moisture to the crust and surrounding environment.
The MRE, a long shelf-life military bread, firmed much slower due to the moisture loss inhibition (hermetic
pouch) and plasticization (by formulation). This work confirmed previous findings that in some cases, firming
of a bread can be strongly influenced by factors other than amylopectin crystallization. This is possible through
controlling changes in the amorphous domains earlier described from thermomechanical studies.
Keywords: bread, staling, water, military, crystallization
Introduction
Food Engineering and Physical Properties
S
TARCH RETROGRADATION (RECRYSTALLIZATION) HAS BEEN LONG
proposed to be the predominant factor in staling. Now, after
a century of research, starch retrogradation is still considered to
be one of the key factors contributing to staling (Schoch and
French 1947; Maga 1975; Krog and others 1988). Other factors
include changes in gluten functionality (Maga 1975; Kulp and
others 1981), moisture migration/redistribution (Leung 1983),
and, recently, the glassy-rubbery state of bread polymers
(Slade and Levine 1991). Most recent reviews of the composite
nature of bread with respect to phase change, water mobility,
and component interactions can be found elsewhere (Chinachoti and Vodovotz 2001).
Mobilization or plasticization of starch is the function of the
water of hydration (Biliaderis 1992; Slade and Levine 1991;
Zeleznak and Hoseney 1987; Chinachoti 1996a and b). The minimum relative amount of water to allow this total mobility to crystallize has been reported to be at approximately 40 to 50% water
(Biliaderis 1992).
Most researchers believe amylopectin crystallization is the
major factor in staling and is determined by correlation between measured crystallization and textural firmness (Krog
and others 1988). However, starch crystallization is not necessarily equivalent to staling as defined by firming; that is, the
recrystallization process of starch may or may not lead to texture firming. Other factors are known to be involved such as
gluten emulsifiers, fat, cellular structures, cell wall properties
(the relative roles of the continuous and discontinuous
phase), and so on (Rao and others 1992, Kou and Chinachoti
1990; Hallberg and Chinachoti 1992; Nussinovitch and others
1992). Rao and others (1992) suggested that crystallized
starch is in a discontinuous domain embedded in a continuous domain. Therefore, questions have been raised to what
extent or how significant the role of starch crystals are as compared to the continuous, amorphous domain (Schiraldi and
Fessas 2001; Chinachoti 1996a). This also suggests that, if
amylopectin is not the only factor, firming of bread could be
retarded also by physio-chemical modification of the noncrystalline phases.
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Hallberg and Chinachoti (1992) investigated the role of the
amorphous domains by characterizing the glass transition temperature ( Tg) using Differential Scanning Calorimetry (DSC)
and Dynamic Mechanical Analysis (DMA). It has been suggested that Tg of bread may increase during staling (Slade and Levine 1991) as a result of annealing and networking of amorphous chains. If stored long enough, the amorphous network
was described to mature leading to an increase in Tg. Additionally, if this is correct, changing a system Tg and storage temperature could lead to a modified staling rate ( Taub and others
1994; Slade and Levine 1991). This has been observed in a standard white bread (SWB) when stored in a highly permeable
polyethylene bag (Chinachoti and others 1992; Vodovotz and
others 1995; Chinachoti 1996b). In an MRE (Meal, Ready-toEat) bread, storing it in a hermetically sealed pouch at an ambient temperature for up to 3 y did not lead to detectable increase
in Tg as observed by DMA (Hallberg and Chinachoti 1992).
However, other changes can still occur during this long shelflife. This may also suggest that by keeping the amorphous
phase plasticized, hardening of bread could be minimized.
However, this leads to a question whether amylopectin recrystallization would still have a significant influence on texture
during storage. The MRE bread is a military pouch bread (Anon
1989) and contains glycerol, sucrose esters, lipids, and sorbic
acid to prolong its shelf-life. The average equilibrium pH and
a w of this bread is 5.0 and 0.86, respectively. These 2 types of
bread represent the actual breads on the consumer and military market shelves and also represent 2 different systems, one
fairly simple in composition (SWB) and the other (MRE) is formulated to prolong shelf-life up to 3 y.
The freshness of shelf-stable “meal ready-to-eat” (MRE)
bread is preserved by controlling water activity aw, pH, oxygen
content, and initial microbial load (Hallberg and others 1990;
Powers and Berkowitz 1990). Although the bread is microbiologically safe, physical and chemical changes can still occur during a
shelf-life of up to 3 y.
This work explored staling in MRE and SWB based on amylopectin crystallization and mechanical firming over an extended
storage time.
5/3/2002, 1:36 PM
Starch crystallization and bread staling . . .
Bread preparation
The formulas for the SWB and MRE breads are presented in
Table 1 and 2, respectively. Both breads were processed using
the straight dough method (modified procedures from Anon
1989, for MRE; and from Nussinovitch and others 1991, for SWB).
Four 5 lb batches of dough were prepared for each type of bread.
Each 5 lb batch made 10 loaves of bread. The dough content was
dry mixed, hydrated, developed, and fermented (85 F at 80%
RH) for approximately 2.5 h. It was then divided into 1.8 oz. (51 g)
pieces, molded, proofed, (95 F or 32 C, 90% RH) until rising level
with the top of the lidded pan (approximately 1 ¼ “ in height).
The baking was done at 375 F (190 C) for 20 min. Both SWB and
MRE breads were prepared in a similar fashion with a few exceptions. For the case of MRE bread, the dough needed less time (1
hr) than the SWB to ferment, since it did not have undissociated
potassium sorbate that inhibits yeast growth. Additionally, the
baking was done for 20 min at 330 F (166 C). The baked weights
were 48 g and 45 g for MRE and SWB, respectively. The volume of
the baked breads were approximately 2 ¼ 4 ½ 1 ¼ in3 as restricted by the pan lid. (The lid is to assure the fit of the breads in
the pouch as required in the military ration package). The specific volume of the baked MRE and SWB breads were 0.0043 and
0.0046 m3/Kg, respectively. The breads were cooled (on racks in a
clean, enclosed, proof box to limit the airflow and reduce the possibility of contamination) to less than 120 F, (but not less than
80 F) and then sealed intact in their respective packaging. MRE
breads were hermetically sealed in trilaminated pouches (polyester, aluminum foil, and polyolefin; Cadillac Products Inc., Ill.,
U.S.A.) with a Multivac sealer (Koch Supplies, Inc., Kansas City,
Mo., U.S.A.). SWB was sealed in 5 mil polyethylene bags, with a
Cryovac film sealer (Koch x 200, Koch Supplies, Inc., Kansas City,
Mo., U.S.A.). Both breads were sealed at ambient pressure, with a
50 g packet of oxygen scavenger (Fresh Pax, Multisorb Technologies, Buffalo, N.Y., U.S.A.) inserted to extend shelf-life. It should
be noted that the SWB was additionally preserved by 0.1% potassium sorbate in the formula to prevent molding, since it had a
higher moisture content.
Four sets of 10 breads from each 5 lb batch were made for this
study. The MRE bread samples were stored at 25 C and withdrawn at 0, 3, 6, 12, 17, 50, and 87 mo. SWB was withdrawn at 0, 3,
6, 7, and 17 mo, at which time mold became evident. Duplicate
samples were withdrawn for DSC, Instron, and moisture measurements. Duplicate measurements were made and averaged.
Samples for testing were taken from the geometric center of the
crumb (to assure the most uniform moisture content).
Thermal transitions by DSC
Approximately 15 mg of bread crumbs were placed in a stainless steel, hermetic sample pan (Perking Elmer, Somerset, N.J.,
U.S.A.). The sample, along with an empty reference pan, was
placed into the Differential Scanning Calorimeter (DSC) chamber and chilled with liquid nitrogen to –100 C. A gas purge of
pure nitrogen gas flowed (30 ml / min) through the furnace
chamber to eliminate any moisture in the head space (as found
in preliminary results, not shown). The DSC (Seiko Instruments
Int., model DSC 100, Torrence, Calif., U.S.A.) was calibrated using indium and programmed to heat the sample and empty reference pans from –100 C to 130 C at a rate of 10 C / min. Heat
flow recorded in mW as plotted against temperature.
Amylopectin crystallization was measured from an endother-
jfsv67n3p1092-1096ms20010199-SR.p65
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Table 1—Standard white bread formula
Ingredients
%
Flour
Water
Shortening
Sugar
Salt
Yeast
Calcium propionate
Potassium sorbate
TOTAL
57.5
34.5
3.0
2.7
1.0
1.0
0.2
0.1
100.00
mic melting enthalpy corresponding to a peak at 60 to 80 C
range. It was calculated in terms of melting calories per gram of
starch. This was done using the following conversion factors:
cal
1 g sample
65.5 g solids
SWB –———— ————— –————— g sample s
g solids
57.5 g flour
100 g flour
–––––———–
73.3 g starch
cal
1 g sample
71.7 g solids
MRE –———— ————— –————— g sample s
g solids
50.3 g flour
100 g flour
–––––———–
73.3 g starch
Where s is g solids / g bread. This calculation was done using a
starch content of 73.3% flour basis (Baik and Chinachoti 2000);
both SWB and MRE breads contained the same type of flour.
Texture measurement
Compression testing was performed using an Instron Universal Machine (Canton, Mass., U.S.A.) according to Peleg and others (1989) and Nussinovitch and others (1992). Each sample was
taken from the geometric center of each loaf in order to avoid the
crust; samples with larger air voids were eliminated. The samples
were cut into 50 mm dia discs with a thickness of 25 mm using a
sharp cutter. Uniaxial compression was done at the crosshead
speed of 10 mm / min and the containment cell of 50 mm diameter was used. Measurement was done immediately after samples
were removed from the package to prevent excessive moisture
loss before testing. Standard error was 3 Kpa.
Moisture content determination
Moisture content measurement was performed on each 10 g
sample using vacuum oven methods (48 hours at 21 C, 30 in Hg
vacuum, AOAC Method 1973).
Results and Discussion
Rheological characterization
Sigmoidal compressive stress-strain behavior has been found
in various types of bread (Peleg and others 1989; Swyngedau
and others 1991; Nussinovitch and others 1991, 1992; Kou and
Chinachoti 1990). The 3 parts to the typical curve- the shoulder,
the plateau, and the vertical ascent- represent the matrix deformation, cell wall collapse, and the cell wall material densification,
respectively. Nussinovitch and others (1992) and Kou and Chinachoti (1990) both characterized freshly made and staled white
bread.
Figure 1a and 1b shows the stress-strain relationship for MRE
and SWB, respectively. Over storage time, both breads showed a
5/3/2002, 1:36 PM
Food Engineering and Physical Properties
Materials and Methods
Starch crystallization and bread staling . . .
Table 2—MRE bread formula
Ingredients
%
Flour
Water
Shortening
Glycerol
Yeast
Salt
Sucrose ester
Gum arabic
Xanthan gum
Calcium sulfate
Sorbic acid
TOTAL
50.3
28.9
8.5
6.3
2.3
1.3
1.0
0.5
0.5
0.3
0.1
100.00
Food Engineering and Physical Properties
significant increase in firmness (stress) at a given strain. Note
the difference in time periods between the 2 breads. After 82 mo,
inspection revealed the package was intact. The sample was noticeably very firm to the touch even though moisture content was
measured to be relatively unchanged (23%, total basis) and the
package integrity was maintained.
The stress-strain curve from MRE bread showed no prominent shoulder (Figure 1a). This was due to smaller air cells inherent to this type of bread, which resulted in a higher density.
The SWB with full volume and large cells, after aging for 2 mo,
however, had a distinct shoulder (Steffens 1992; Peleg and others 1989).
Stress values at 30% deformation (Table 3) increased with
storage. For MRE bread, the stress value increased to 18 kPa with
the 1st 3 mo and remained relatively stable ( > 30 kPa) for 54 mo
(beyond the expected shelf-life of 3 y). SWB deformation showed
a relatively higher firmness than MRE bread staled. SWB stored
for 3 mo was very firm and unacceptable while MRE bread at 3 to
15 mo of storage was still soft and more acceptable (Steffens
1992). Only after an extremely long period (47 and 82 mo or 4 to 7
y) did the MRE bread become “staled” or noticeably firm.
The difference in firmness developed during aging of SWB
and MRE bread could be caused by a number of factors. One of
the obvious factors was the difference in the amount of moisture
loss through the package. While MRE bread retained all moisture
within the hermetically sealed pouch, a very significant amount
of moisture was lost through SWB samples due to the highly permeable polyethylene bag. As shown in Table 3, SWB moisture
content decreased from 43.8% to 32.0% moisture in 8 mo and to
17.8% moisture after 17 mo of storage. MRE bread, however, did
not change significantly in moisture content (Table 3). Another
factor could be the differences in composition and processing
conditions between the 2 breads. MRE bread, for instance, contained a significant amount of glycerol (in addition to other ingredients) which is an effective plasticizer. Although the 2 breads
were very different in composition, processing, and storage conditions, this exercise shows that firming of bread could be overcome during storage by modification of various parameters as
employed in the case of MRE bread.
80 C. Subsequent runs at 1 to 50 mo produced similar-sized endotherms (that is, enthalpies of 0.38 0.13 to 0.46 0.08 cal / g
sample, respectively). DSC data for this peak are shown in Table
3. Similar results were found for SWB.
MRE bread showed an increased amylopectin crystallization
within the 1st 3 mo of storage, and remained relatively constant
thereafter. The temperature range of the transition (Table 3) was
considerably higher than those values earlier reported for amylopectin in starch (Yuryev and others 1995) which could probably
be due to the solutes present. Factors, such as moisture content,
heating rate, sample size, and composition, might also contribute to the discrepancies.
The initial and peak temperatures ( To and Tp, respectively)
did not significantly change over time for MRE bread (Table 3).
This was also the case for SWB except for the 17 mo sample
when To and Tp increased due to the extensive moisture loss.
Enthalpy of amylopectin crystal melting (0.71 0.14 cal / g
starch) appeared to peak within the 1st 6 mo of storage for both
cases (Table 3). Then, the values decreased slightly and leveled
off for the remaining time of the study. SWB showed less amylopectin crystallinity. When compared the enthalpies for the 2
breads directly on a calorie per gram starch basis, the difference was more obvious (Table 3). This could be due to composition difference, an extensive moisture loss in SWB that retarded
the recrystallization process. Moisture has been found to have a
positive effect on crystallization of amylopectin (Chinachoti
Amylopectin recrystallization
DSC thermograms were obtained at up to 50 mo of storage.
Fresh MRE bread did not display a starch melting endotherm
(over 50 to 60 C range for amylopectin melting transition). After
6 mo of storage at 25 C, a small endotherm started to appear,
(0.31 0.06, cal / g sample) at a temperature range of 60 C to
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Figure 1a&1b—Stress-strain curves of stored MRE and SWB
5/3/2002, 1:36 PM
Starch crystallization and bread staling . . .
Table 3—Amylopectin melting peak from DSC.
Moisture Content
g / 100 g
sample
MRE
0
3
6
12
17
50
SWB
0
3
6
7
17
Stress (Kpa)
at 30%
deformation
Enthalpy
cal/g sample
cal/g starch
23.0
25.1
23.4
22.8
24.6
25.5
1.0
1.0
1.0
1.0
1.0
0.2
0.5
11.3
8.2
7.7
6.1
13.0
0.07
0.22
0.31
0.19
0.22
0.22
0.02
0.04
0.06
0.05
0.09
0.08
43.8
37.9
33.4
32.8
17.8
0.1
0.2
0.2
0.3
0.3
0.5
0.7
27.5
N/A
N/A
0.00
0.08 0.03
0.15 0.04
0.19 0.07
0.12 0.03
Table 4—Unfrozen water (1% UFW) measured and calculated in fresh MRE and SWB
Age
(mo)
MRE
0
4
6
12
17
50
SWB
0
3
7
8
17
Moisture
g / 100 g
sample
UFW%
total water
basis
UFW
g / g sample
DSC ice melting
Onset To (8C) Peak Tp (8C)
23.90.1
23.10.3
23.40.4
22.80.1
24.60.3
25.50.2
75.80.3
80.20.5
88.00.2
79.70.6
77.61.8
76.80.1
0.170.01
0.190.01
0.200.01
0.180.02
0.170.01
0.180.02
-23.00.3
-28.01.8
-25.01.5
-22.71.4
-24.70.3
-28.10.8
-12.20.6
-17.60.4
-17.20.8
-13.52.1
-15.90.3
-16.12.0
43.80.1 62.70.08 0.200.01
37.90.2 74.71.5 0.290.01
32.80.3 76.12.9 0.270.01
32.00.6 78.40.1 0.270.02
17.80.3 95.70.2 0.180.03
-10.10.4
-12.70.9
-12.70.0
-12.71.2
-1.64.4
-1.80.2
-5.80.4
-5.50.2
-5.90.9
-12.10.1
and Steinberg 1986). Additionally, the presence of glycerol in
MRE would have an additional plasticizing effect promoting
more crystallization of the starch.
Freezable and unfreezable water
Table 4 shows the endothermic ice-melting enthalpy expressed as the amount of unfreezable water (UFW) obtained under the experimental condition. In SWB the unfrozen water was
found to remain relatively constant over 8 mo of storage even
though there was a significant moisture loss from the sample.
Over this period the sample lost some of its freezable fraction of
water. At 19 mo, the amount of unfrozen water was much lower
and contributed to almost all of the water present (0.170 g / g
bread out of a total moisture of 0.178 g / g bread, Table 4). Therefore, most of the freezable water was lost from the sample and
some unfrozen water was also removed.
This was found to coincide with a significant change in DMA
(Dynamic Mechanical Analysis) thermograms reported earlier
by Chinachoti and others (1992) and Vodovotz and others
(1995). In this previous work, it was found that very old white
bread (19 mo) drastically increased in thermomechanical transition midpoint temperature and the temperature range and
this would be contributed by the extensive loss of moisture
(Chinachoti and others 1992). However, when compared with a
fresh bread subjected to drying to similar moisture, 10%, the
jfsv67n3p1092-1096ms20010199-SR.p65
1095
0.18
0.57
0.79
0.48
0.56
0.57
0.05
0.10
0.15
0.12
0.23
0.21
0.00
0.20 0.08
0.35 0.09
0.44 0.16
0.23 0.06
Melting temperature
Onset To
Peak Tp
( 8 C)
( 8 C)
60.5
59.1
59.0
58.8
57.0
60.3
2.5
3.1
2.8
2.5
3.5
2.1
N/A
60.5 2.4
61.7 1.9
66.2 2.2
77.1 1.5
70.4
69.0
75.3
77.2
70.9
75.2
0.5
1.2
2.0
1.7
0.9
3.1
N/A
66.0 0.8
68.5 2.2
70.8 2.0
83.3 2.8
DMA thermogram was somewhat different (narrower in range,
more or less unimodal) from that of aged bread (much broader
and individual tan peaks were bimodal). Thus, it was concluded that the extensive loss in moisture to the point when all
freezable water was depleted at least partly led to an increase
in glassy rubbery transition temperature range ( Vodovotz and
others 1995). It must be noted, however, that a regular bread
stales much sooner than this (within 2 wk after baking) and in
this range a significant change in DMA (Tg) transition has been
reported (Chinachoti 1996a and b).
The loss of freezable water (or the gain in UFW fraction, Table
4) in SWB agreed with the loss in total moisture content during
storage. On the other hand, MRE bread retained all of its water
(due to a far better packaging material used). The precise role of
freezable and unfreezable water on staling of bread is still unclear. It has been proposed by a number of investigators that the
loss in freezable water during storage of a bread is a result of the
migration of freezable water from the amorphous matrix to the
crystalline hydrate of retrograded starch (Leung and others1983;
Slade and Levine 1991; Wynne-Jones and Blanshard 1996). However, the molecular dynamic investigation of water mobility using oxygen-17 Nuclear Magnetic Resonance (NMR) of staling
bread has shown that the decrease in spin-spin relaxation time
(T2) was due to a redistribution in water within the more mobile
amorphous domains (Kim-Shin and others 1991; Chinachoti and
others 1992). From a solid state deuterium and oxygen-17 NMR
study of staled bread, the observed increase or change in crystalline starch was not accompanied by an observable change in
NMR results (Chinachoti 1996b; Kim-Shin and others 1991).
Unfreezable water has been reported to be highly mobile (Li
and others 1998). The water may distribute in various regions of
breads (Ruan and others 1996; Chen and others 1997). Even
within the starch granules it exhibits a distribution in molecular
motions with respect to its location in various regions, such as
growth ring and channels, and semi-crystalline lamellae and the
crystalline domains. More work is clearly needed to investigate
water mobility distribution in various regions as well as its influence on the texture of bread
Mechanisms of firming
There are a number of possible reasons for firming of bread.
The most obvious is the extensive moisture loss. As reported recently (Baik and Chinachoti 2000), moisture loss (freezable water
loss) from crumb to crust could lead to a significant firming. They
5/3/2002, 1:36 PM
Food Engineering and Physical Properties
Age
(mo)
Starch crystallization and bread staling . . .
Food Engineering and Physical Properties
also have found that when glycerol is present, firming during
storage was not mainly due to amylopectin recrystallization but
rather related to changes in the non-crystalline region (Baik
2001; Baik and Chinachoti 2001). Tang and others (2000) investigated water distribution in various starch amorphous and crystalline domains using nuclear magnetic resonance to determine
water distributions undergoing upon freezing and heating. Application of such technique to determine the actual amount and
mobility of water neighboring various starch and gluten domains
would in the future provide useful information on the molecular
role of water in staling.
Amylopectin recrystallization of SWB and MRE breads increased over storage as observed from the enthalpy ( Table 3),
but more extensive recrystallization was found for MRE bread.
This could have an effect on firming.
Efforts to retain moisture in a hermetically sealed package
and to plasticize some domains by adding humectants helped
keep the softer texture of the MRE bread over an extensive storage despite the rather extensive amylopectin recrystallization.
Since bread is a composite material, its rheological properties are
more likely to be governed by changes that occur in a continuous
phase rather than the embedded starch crystals in a discontinuous phase. The contribution of crystalline starch on bread rheological properties is expected to become more significant when
their domain (crystal) size becomes large enough.
Other physical factors might have also been involved in the
stress-strain data and consequently the firmness measurement.
These are air cell geometry and size, the density of the bread, and
the cell wall thickness (Peleg and others 1989). Additionally, different formulation and composition (and inter- or intra-molecular interaction) could also contribute to the difference in texture.
Conclusion
T
HE DATA PRESENTED HERE CONFIRMS THAT CHANGES IN THE AMOR-
phous and crystalline domains could influence the texture of
bread. For the case of MRE bread, a greater contribution from
amorphous components is suggested.
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MS 20010199 Submitted 4/23/01, Accepted 2/1/02, Received 2/11/02
The authors would like to acknowledge support from the U.S. Army (Natick) for Dr. Hallberg
and from Massachusetts Agricultural Experiment Station, MAS811. Technical advice from
Dr. Irwin A. Taub, Senior Scientist, U.S. Army (Natick) is also acknowledged.
Author Hallberg is with the U.S. Army and Biological Chemical Command, Natick Soldier Center, Combat Feeding Program, Natick, MA 01760.
Author Chinachoti is with the Dept. of Food Science, Univ. of Massachusetts, Amherst, MA 01003. Direct inquiries to author Chinachoti (Email:
pavinee@foodsci.umass.edu).
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