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Effect of freezing and frozen storage on the staling of part-baked bread
Food Research International 36 (2003) 863–869 www.elsevier.com/locate/foodres
Effect of freezing and frozen storage on the staling of part-baked bread Marı´a Eugenia Ba´rcenasa, Mo´nica Harosb, Carmen Beneditob, Cristina M. Rosellb,* a Universidad de las Ame´ricas, Puebla, Mexico Laboratorio de Cereales, Instituto de Agroquı´mica y Tecnologı´a de Alimentos (CSIC), P.O. Box 73, 46100 Burjassot, Valencia, Spain
b
Received 31 March 2003; accepted 10 June 2003
Abstract The effect of part-baking, freezing, frozen storage, thawing, rebaking on the aging behaviour of bread was evaluated. The amylopectin modification during the process was assessed by differential scanning calorimetry (DSC), while changes in bread quality were followed by crumb hardness measurements. During frozen storage no retrogradation of amylopectin was detected in the part-baked dough. When analysing the aging of the rebaked samples, it was observed that the time of frozen storage produced a progressive increase of the retrogradation temperature range of the amylopectin, and also great energy was required for amylopectin melting at longer storage period, indicating that structural changes of amylopectin were produced during frozen storage. Regarding the quality of the fresh bread resulted after rebaking, crumb hardness increase with the time of frozen storage, and also the hardening rate during aging was dependent on that time. Crumb hardness results of the fresh bread and also DSC studies indicate that some changes are produced during the frozen storage. # 2003 Elsevier Ltd. All rights reserved. Keywords: Part-baking; DSC; Amylopectin; Freezing; Gelatinization; Retrogradation; Aging; Part-baked bread; Hardness
1. Introduction Bakery products have a very short shelf life; their quality is highly dependent on the period of time between baking and consumption. During storage, a loss of bread freshness parallel to an increase in crumb hardness produces a loss of consumer acceptance, that process is usually referred as bread staling (Hebeda, Bowles, & Teague, 1990). This is a complex process that involves several physico-chemical changes, although the exact mechanism still remains unclear. However, since starch is the major constituent in the bread crumb, the physical changes accompanying the retrogradation of starch have been suggested as the main cause of bread staling (Biliaderis, 1992; Seow & Teo, 1996; Zobel & Kulp, 1996). Differential scanning calorimetry (DSC) has been extensively applied to study starch retrogradation. DSC can measure the enthalpy associated to amylopectin recrystallization, and provides a way of * Corresponding author. Tel.: +34-96-390-00-22; fax: +34-96-36363-01. E-mail address: [email protected] (C.M. Rosell). 0963-9969/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0963-9969(03)00093-0
monitoring the progressive magnitude of staling endotherm (Biliaderis, 1992). Many efforts have been focussed on the development of different additives and enzymes for extending the shelf life of the bread products by retarding the staling process in the stored bread. Different emulsifiers like sodium/calcium stearoil lactylate, mono/diglicerides (Stampfli & Nerste, 1995; Twillman & White, 1988) and hydrocolloids like carboxymethylcellulose, guar gum, alginate, and xanthan (Armero & Collar, 1996; Davidou, Le Meste, Debever, & Bekaer, 1996; Rosell, Rojas, & Benedito, 2001) have been successfully used as antistaling agents in wheat breads. In addition, different a-amylases, hemicellulases and lipases (Haros, Rosell, & Benedito, 2002; Leo´n, Duran, & Benedito, 2002; Martı´nez-Anaya, Devesa, Andreu, Escriva, & Collar, 1999; Rosell, Haros, Escriva, & Benedito, 2001) are widely used for retarding the bread staling. A different approach for increasing the shelf life of bread quality is by applying freezing temperatures to the breadmaking process. Freezing temperatures can be applied to bread dough interrupting the breadmaking before proofing, nevertheless frozen bread dough led to
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bread of reduced volume (Inoue & Bushuk, 1991, 1992; Veron, 2000), mainly due to the physical damage caused to the protein network structure (Varriano-Marston, Hsu, & Mahdi, 1980) and the deterioration of yeast during frozen storage. Despite those problems can be minimized by using strong wheat flour and freeze-tolerant yeasts, the breadmaking process of frozen bread dough still has several requirements related to wheat flour quality, freezing and thawing conditions and thawed dough handling (Bhattacharya, Langstaff, & Berzonsky, 2003). In addition, freezing temperatures can be also applied to the partially baked bread, called part-baked bread or pre-baked bread. Freezing the pre-baked bread is an easy way to prolong the shelf life of the bread keeping its freshness. The interrupted baking method (pre-baking) used for bread was initially developed for improving the bread quality (Labutina, Puchkova, Gubiev, Ilyasov, & Kats, 1981; Morgenstern, 1985; Stephan, 1977). Several studies have been conducted focussed in determining the optima time and temperature for part-baking (Ferreira, Watanabe, & Benassi, 1999; Fik & Surowka, 2002; Stephan, 1977), the microbial quality of the part-baked bread (Doulia, Katsinis, & Mougin, 2000; Leuschner, O’Callaghan, & Arendt, 1999), and the quality of fresh bread after the finish baking (Ferreira et al., 1999; Fik & Surowka, 2002), concluding that bread obtained by the part-baking process has sensory and textural properties close to those of the bread obtained by a conventional method. Despite the popularity of the process in France and UK, there is scarce information about the staling of the frozen part-baked bread concerning starch changes, namely amylopectin retrogradation, and crumb hardening during post-baking storage. The objective of the present work was to analyse the effect of freezing and frozen storage on the staling of partially baked wheat bread. For analysing the bread staling process two different approaches have been performed. Firstly, the amylopectin behaviour was followed by differential scanning calorimetry for simulating the baking process and quantifying the retrogradation during storage. Secondly, hardness increase during storage was evaluated in the wheat bread crumb from pre-baked bread after different frozen storage times.
Ingredients were mixed in a 50-g bowl Brabender farinograph (Brabender, Germany), after 4 min mixing, the resulting bread dough was rested for 10 min, moulded and then proofed in a fermentation cabinet at 28 C and 80% relative humidity up to three times the initial dough volume (approximately 90 min). This proofed bread dough was further used for simulating the baking process in the calorimeter. 2.2. Differential scanning calorimetry (DSC) A differential scanning calorimeter (Perkin-Elmer DSC-7) was used as an oven to simulate the baking process (Leo´n, Dura´n, & Benedito, 1997). In this study a modified procedure was followed in order to imitate the interrupted breadmaking process. Thermal properties of starch were analysed through part-baking, frozen storage, final baking process and refrigeration storage. A complete scheme of the temperature transitions is shown in Fig. 1. 2.2.1. Part-baking stage To simulate the partial baking, bread dough samples (18–20 mg) were weighed in stainless steel pans (PE 0319-0218). An empty capsule was used as a reference. After sealing, the capsules were heated from 25 to 90 C, that was the temperature reached in the centre of the crumb during part-baking measured by thermocouples (results not shown). Then capsules were cooled to 40 C at 10 C/min. Twenty-seven replicates for each sample were run. Thermal transitions of starch samples were defined as To (onset), Tp (peak of gelatinization) and Tc (conclusion). The enthalpy associated with starch gelatinization (Hg) was calculated as the area enclosed by the straight line and endotherm curve, and it was expressed in Joules per grams of dry sample.
2. Materials and methods 2.1. Dough samples Commercial Spanish flour, obtained from the local market was used in this study. A basic bread recipe consisted in wheat flour (50 g), compressed yeast (1.0 g), salt (1.0 g) and water (27.2 g, up to optimum absorption) was used for analysing starch thermal properties.
Fig. 1. Temperature profile reached in the centre of the crumb during an interrupted baking process including part-baking stage, frozen storage stage and finish baking stage or rebaking.
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2.2.2. Frozen storage Capsules were immediately placed in a freezer at 35 C for 15 min and then stored at 18 C for 7, 15 and 30 days.
3. Results and discussion
2.2.3. Finish-baking stage or rebaking At different frozen storage times, capsules were thawed at 25 C for 15 min and heated again in the calorimeter from 25 to 110 C at 10 C/min to complete the baking process. Nine replicates for each sample were run.
The DSC technique was chosen for studying the behaviour of wheat starch during a two-step baking process, because the baking process can be simulated removing the possible interference promoted by the water loss, since the calorimetry analysis used hermetic capsules. Thermal properties of the starch were followed with the DSC (Fig. 2). Two endotherms appeared in the thermograms when the fermented dough was baked in the DSC by increasing the temperature from 25 to 90 C, temperature reached in the centre of the bread dough in the pre-baking process (results not shown). This behaviour is typical for starch when heated in the presence of a limited amount of water (Biliaderis, Maurice, & Vose, 1980; Califano & An˜on, 1990, Jovanovich, Zamponi, Lupano, & An˜on, 1992). The first peak of the thermogram corresponded to the gelatinization process of the amorphous phase of the starch, which appeared between 67.2 C and 83.4 C and had an enthalpy of 1.43 J/g (d.b.) (Table 1). The peak of the second endotherm appeared at 88.1 C and had 0.39 J/g (d.b.) of enthalpy. The same behaviour was previously reported by Ferrero, Martino, and Zaritzky (1993), Leo´n et al. (1997) and Andreu, Collar, and Martinez-Anaya (1999), who assigned the first endothermic peak to the gelatinization peak and the second one to the melting of the more stable crystalline structure of starch. Regarding the later, its enthalpy value is greatly dependent on the amount of water. In fact, the enthalpy value in this study was slightly lower than the 0.76 J/g reported by
2.2.4. Aging or refrigeration storage After the complete baking (B) capsules were stored at 4 C for analysing the amylopectin retrogradation during bread staling. Samples after 2, 4 and 7 days of storage were scanned in the DSC from 25 to 110 C at 10 C/min. The retrogradation enthalpy (Hr) of amylopectin was calculated as the area enclosed by the straight line and endotherm curve, and it was expressed in joules per gram of dry sample. The retrogradation index (RI) was defined as (Hr/Hg) (Duran, Leo´n, Barber, & Benedito, 2001). Three replicates for each sample were run. 2.3. Breadmaking procedure A straight dough process was performed for preparing the bread samples. The bread dough recipe, based on flour weight, consisted of: 6500 g of wheat flour (14% mb), 2% compressed yeast, 2% salt and water up to optimum consistency. Ingredients were optimally mixed, rested for 10 min, divided (150 g), kneaded, rested again for 10 min, mechanically sheeted and rolled and finally proofed at 28 C and 80% relative humidity (up to three times the initial dough volume). The partbaking was conducted at 165 C for 7 min, then loaves were cooled to 40 C in the crumb centre and next frozen to 18 C at a temperature of 35 C for 105 min. Frozen loaves were wrapped in polyethylene films and stored for different intervals at 18 C. Samples for analysis were taken at 7, 14, 28, 42 days and thawed at room temperature. After unwrapping, part-baked breads were baked at 195 C for 14 min and cooled at room temperature for 1 h. Full-baked bread volume was determined by seed displacement, and crumb hardness was measured in a Texture Analyser TA-XT2i (Stable Micro Systems, Surrey, UK). Since this type of bread is for daily consumption, the hardness increase at 25 C was followed during the first 24 h.
3.1. Thermal properties of wheat starch during heating/ frozen/heating cycles
2.4. Statistical analysis Multiple sample comparison was statistically analysed with the Statgraphics Plus 5.0. Fisher’s least significance difference (LSD) test was used to describe means at the 5% significance level.
Fig. 2. Differential scanning calorimetry thermograms during the process; a, part-baking procedure (Peak 1: gelatinization, Peak 2: melting or fusion endotherm); b, finish-baking process; c, thermogram of amylopectin retrogradation after storage at 4 C (Peak 3: amylopectin retrogradation).
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Table 1 Comparison of thermal parameters for wheat starch through the partbaking and in the retrogradation after 30 days frozen storage, rebaking and 7 days of aging at 4 C
To ( C) Tp ( C) Tc ( C) H (J/g) RI
Gelatinizationa
Retrogradationb
67.20.9 75.30.4 83.40.4 1.430.29 –
38.40.6 58.40.6 78.60.8 9.220.47 7.40.7
T1To, onset temperature; Tp, peak temperature; Tc, conclusion temperature; RI, retrogradation index. a Meanstandard deviation (n526). b Meanstandard deviation (n53).
Leo´n et al. (1997), besides Andreu et al. (1999) showed enthalpies ranging from 0.03 to 0.13 J/g (d.b.). These differences can be explained due to the different materials, recipes and processing conditions. At different intervals of the frozen storage, the partbaked samples were reheated in the DSC simulating the finish-baking process. In the second scan (Fig. 2), corresponding to the finish-baking appeared the second endotherm, the melting or fusion endotherm, which indicates that during the frozen storage of the partbaked samples no detectable retrogradation of amylopectin is produced. Ferrero et al. (1993) described similar findings when analysing frozen pastes of wheat flour, they did not detect the retrogradation peak for any freezing rate used after 12 h at 20 C. Ferrero and Zaritzky (2000) also reported that retrogradation of amylopectin was not detected by DSC in corn starch frozen samples stored for 91 days at 80 C. Rapid freezing prevented crystallization of both amylose and amylopectin, producing a homogeneous structure upon thawing and the absence of amylopectin retrogradation peaks (Ferrero et al., 1993).
fast crystallization of amylose and slow recrystallization of amylopectin (Biliaderis, 1992). A progressive increase of the retrogradation endotherm was observed with the aging time (Fig. 3), obtaining a very well defined peak after 7 days storage at 4 C. 3.3. Effect of part-baking, frozen storage and rebaking of bread dough on the aging during storage When bread samples from different frozen storage times were aged, they showed a significant decrease (P < 0.05) in the onset temperature of the retrogradation endotherm with the time of frozen storage, while the peak and conclusion temperature were barely affected (Table 2). As a consequence the retrogradation temperature range significantly increased with the frozen storage time. Ribotta et al. (2003) obtained similar results when studying the effect of dough frozen storage on the retrogradation temperature transition, although in this case only after long storage, 150 and 230 days. The lowering of To might be due to the modification of the amount of water available, since the water content controls the amylopectin melting during retrogradation (Leo´n et al., 1997; Ribotta et al., 2003; Zeleznak & Hoseney, 1986). Lu and Grant (1999) and Bhattacharya et al. (2003) clearly showed that the amount of freezable water increases during dough frozen storage. Bhattacharya et al. (2003) attributed that effect to the deterioration of gluten network during frozen resulting in a migration of water and an increase in the freezable water. However in the part-baked bread the protein network has already been denatured in the part-baking process before freezing, therefore the explanation of a redistribution of the water present in the system seems more plausible in this type of process (Berglund, Shelton, & Freeman, 1991). In addition, it should be also considered that in baked bread a more complex
3.2. Thermal properties of amylopectin during aging Samples submitted to the second heating were stored at 4 C for analysing the aging behaviour of the amylopectin when a part-baking was applied and the possible effect of the frozen storage. Aging was performed at 4 C for accelerating the retrogradation of the amylopectin, because Ribotta, Leo´n, and An˜on (2003) described higher retrogradation rate at 4 C than at 20 C. After different storage times (2, 4, 7 days), samples were scanned again and the thermogram showed a well defined peak corresponding to the amylopectin retrogradation (Fig. 2), which showed lower values of transition temperatures (To, Tp and Tc) than the gelatinization peak (Table 1). The enthalpy of the retrogradation endotherm (Hr) was higher than the gelatinization enthalpy at all the storage times tested. Retrogradation involves
Fig. 3. DSC thermograms corresponding to aged baked-dough at 4 C for 2, 4 and 7 days. Baked dough was obtained after part-baking, frozen storage and rebaking.
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Table 2 Effect of 2 days aging at 4 C on the thermal properties of frozen part-baked breada Frozen time (days)
To ( C)
Tp ( C)
Tc ( C)
Tr ( C)
RI
7 15 30
46.7 0.9a 42.4 0.4b 38.9 1.0c
62.20.9a 60.60.8a 59.92.6a
77.81.4a 78.01.9a 78.12.1a
31.1 1.5a 35.6 2.1b 39.2 1.2c
5.41.3a 5.01.9a 5.10.6a
T2To, onset temperature; Tp, peak temperature; Tc, conclusion temperature; Tr, retrogradation temperature range; RI, retrogradation index. a Meanstandard deviation, n=3, means within columns followed by the same letter are not significantly different (P<0.05).
interactions are involved like between inner and outer crumb, starch and gluten and spatial populations of water with different mobility (Czuchajowska & Pomeranz, 1989). Regarding the retrogradation index (RI), no significant differences were found after 2 days aging, but at longer refrigeration storage a progressive increase of the retrogradation enthalpy was observed. Fig. 4 presents the effect of frozen storage on the retrogradation enthalpy of the amylopectin. Despite no differences were observed at 2 days of storage, different trends were observed with prolonged storage, reaching higher retrogradation enthalpies with higher time of storage in frozen conditions. Nevertheless, after 4 days of storage, retrogradation enthalpy achieved an asymptotic behaviour. Frozen storage did not introduce differences in the rate of amylopectin retrogradation, but affected the total enthalpy of amylopectin retrogradation, obtaining higher enthalpies at longer times of frozen storage. As previously has been discussed, even at freezing temperatures a redistribution of water and different interactions between proteins and starch could yield a more ordered amylopectin crystallites which require higher energy for melting. In fact, Ribotta et al. (2003) described changes in both the structure and arrangement of amylose and amylopectin during the frozen storage of bread doughs, which were reflected in the amylopectin retrogradation during aging.
3.4. Specific volume and hardness of the bread from interrupted baking process
Fig. 4. Effect of time of frozen storage at 18 C of the part-baked samples on the amylopectin retrogradation during aging at 4 C. The different series correspond to the time of frozen storage.
Fig. 5. Effect of part-baking, freezing, frozen storage at different times, thawing and rebaking on the crumb hardness during aging at 25 C. The different series correspond to the time of frozen storage.
No significant effect was observed in the specific volume of the fresh bread, neither in the shape index (Table 3), however when the crumb hardness was evaluated a progressive significant increase of the crumb hardness was observed with the time of frozen storage, and that increase was more evident beyond 14 days of storage. Fik and Surowka (2002) found no significant correlation between the overall sensory evaluation and the storage time in frozen conditions, they did not found either correlation with the texture parameters, although a wide variation between samples was found. 3.5. Hardness increase during aging of part-baked bread after the finish-baking In Fig. 5 can be observed a significant increase (P < 0.05) in the crumb hardness of the bread with the frozen storage time, therefore some changes occurred during frozen storage of the part-baked bread. Fik and Surowka (2002) found no statistically significant differences in the crumb hardness of bread after different time of frozen storage, only the crumb adhesiveness was correlated with the storage time, although described a high variation in the parameters analysed due to the heterogeneity of the material.
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Table 3 Effect of part-baking, freezing, frozen storage and rebaking on the quality of the fresh breada Frozen time (days)
Specific volume (ml/g)
Shape index
Hardness (g)
Hardening rate (g/day)
0 7 14 28 42
4.000.15a 3.850.11a,b 4.000.09a 3.700.13b 3.850.11a,b
1.590.06a 1.600.05a 1.600.04a 1.600.05a 1.590.03a
313.98.3a 327.720.7a,b 346.020.8b 373.432.1c 422.321.4d
11.50a 12.52b 14.46c 14.53d 14.53d
T3Shape index: width/height ratio. a Meanstandard deviation, n=3, means within columns followed by the same letter are not significantly different (P<0.05).
As was expected, a progressive increase of the hardness was observed during the aging, and when analysed the rate of hardening (slope of the hardening curve) it was obtained faster hardening at longer period of frozen storage, reaching an asymptotic trend beyond 14 days of frozen storage. Fik and Surowka (2002) described that the freezing process itself produces the greatest changes on sensory and textural properties of fresh bread from part-baked bread, but they did not follow the possible effect of the frozen storage on the aging behaviour of the resulting bread. Results obtained in this work show that the freezing process itself did not produce effect on the bread aging, in opposition the time of frozen storage influenced the rate of hardening during aging, moreover when prolonged frozen storage are performed. Crumb hardness results of the fresh bread and also the hardening rate during aging indicate that some changes are produced during the frozen storage, which corroborates the previous results obtained when analysing the amylopectin retrogradation by DSC.
4. Conclusions The frozen storage of part-baked dough does not eliminate the processes occurring after baking, such as interactions among the diverse components, which become evident during the aging or storage of the resulting bread. The DSC analysis of the amylopectin behaviour during the part-baking, freezing, frozen storage, thawing, rebaking and aging revealed that the time of storage produces an increase in the retrogradation temperature range and the total enthalpy for amylopectin melting. Regarding the crumb hardness, results indicate that the time of frozen storage determine the hardening rate during aging.
Acknowledgements This work was financially supported by Ministerio de Ciencia y Tecnologia Projects (MCYT, AGL2001-1273
and AGL2002-04093-C03-02) and Consejo Superior de Investigaciones Cientı´ficas (CSIC), Spain. M.E. Ba´rcenas would like to thank her grant from Universidad de las Americas, Puebla, Mexico.
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Effect of freezing and frozen storage on the staling of part-baked bread Marı´a Eugenia Ba´rcenasa, Mo´nica Harosb, Carmen Beneditob, Cristina M. Rosellb,* a Universidad de las Ame´ricas, Puebla, Mexico Laboratorio de Cereales, Instituto de Agroquı´mica y Tecnologı´a de Alimentos (CSIC), P.O. Box 73, 46100 Burjassot, Valencia, Spain
b
Received 31 March 2003; accepted 10 June 2003
Abstract The effect of part-baking, freezing, frozen storage, thawing, rebaking on the aging behaviour of bread was evaluated. The amylopectin modification during the process was assessed by differential scanning calorimetry (DSC), while changes in bread quality were followed by crumb hardness measurements. During frozen storage no retrogradation of amylopectin was detected in the part-baked dough. When analysing the aging of the rebaked samples, it was observed that the time of frozen storage produced a progressive increase of the retrogradation temperature range of the amylopectin, and also great energy was required for amylopectin melting at longer storage period, indicating that structural changes of amylopectin were produced during frozen storage. Regarding the quality of the fresh bread resulted after rebaking, crumb hardness increase with the time of frozen storage, and also the hardening rate during aging was dependent on that time. Crumb hardness results of the fresh bread and also DSC studies indicate that some changes are produced during the frozen storage. # 2003 Elsevier Ltd. All rights reserved. Keywords: Part-baking; DSC; Amylopectin; Freezing; Gelatinization; Retrogradation; Aging; Part-baked bread; Hardness
1. Introduction Bakery products have a very short shelf life; their quality is highly dependent on the period of time between baking and consumption. During storage, a loss of bread freshness parallel to an increase in crumb hardness produces a loss of consumer acceptance, that process is usually referred as bread staling (Hebeda, Bowles, & Teague, 1990). This is a complex process that involves several physico-chemical changes, although the exact mechanism still remains unclear. However, since starch is the major constituent in the bread crumb, the physical changes accompanying the retrogradation of starch have been suggested as the main cause of bread staling (Biliaderis, 1992; Seow & Teo, 1996; Zobel & Kulp, 1996). Differential scanning calorimetry (DSC) has been extensively applied to study starch retrogradation. DSC can measure the enthalpy associated to amylopectin recrystallization, and provides a way of * Corresponding author. Tel.: +34-96-390-00-22; fax: +34-96-36363-01. E-mail address: [email protected] (C.M. Rosell). 0963-9969/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0963-9969(03)00093-0
monitoring the progressive magnitude of staling endotherm (Biliaderis, 1992). Many efforts have been focussed on the development of different additives and enzymes for extending the shelf life of the bread products by retarding the staling process in the stored bread. Different emulsifiers like sodium/calcium stearoil lactylate, mono/diglicerides (Stampfli & Nerste, 1995; Twillman & White, 1988) and hydrocolloids like carboxymethylcellulose, guar gum, alginate, and xanthan (Armero & Collar, 1996; Davidou, Le Meste, Debever, & Bekaer, 1996; Rosell, Rojas, & Benedito, 2001) have been successfully used as antistaling agents in wheat breads. In addition, different a-amylases, hemicellulases and lipases (Haros, Rosell, & Benedito, 2002; Leo´n, Duran, & Benedito, 2002; Martı´nez-Anaya, Devesa, Andreu, Escriva, & Collar, 1999; Rosell, Haros, Escriva, & Benedito, 2001) are widely used for retarding the bread staling. A different approach for increasing the shelf life of bread quality is by applying freezing temperatures to the breadmaking process. Freezing temperatures can be applied to bread dough interrupting the breadmaking before proofing, nevertheless frozen bread dough led to
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bread of reduced volume (Inoue & Bushuk, 1991, 1992; Veron, 2000), mainly due to the physical damage caused to the protein network structure (Varriano-Marston, Hsu, & Mahdi, 1980) and the deterioration of yeast during frozen storage. Despite those problems can be minimized by using strong wheat flour and freeze-tolerant yeasts, the breadmaking process of frozen bread dough still has several requirements related to wheat flour quality, freezing and thawing conditions and thawed dough handling (Bhattacharya, Langstaff, & Berzonsky, 2003). In addition, freezing temperatures can be also applied to the partially baked bread, called part-baked bread or pre-baked bread. Freezing the pre-baked bread is an easy way to prolong the shelf life of the bread keeping its freshness. The interrupted baking method (pre-baking) used for bread was initially developed for improving the bread quality (Labutina, Puchkova, Gubiev, Ilyasov, & Kats, 1981; Morgenstern, 1985; Stephan, 1977). Several studies have been conducted focussed in determining the optima time and temperature for part-baking (Ferreira, Watanabe, & Benassi, 1999; Fik & Surowka, 2002; Stephan, 1977), the microbial quality of the part-baked bread (Doulia, Katsinis, & Mougin, 2000; Leuschner, O’Callaghan, & Arendt, 1999), and the quality of fresh bread after the finish baking (Ferreira et al., 1999; Fik & Surowka, 2002), concluding that bread obtained by the part-baking process has sensory and textural properties close to those of the bread obtained by a conventional method. Despite the popularity of the process in France and UK, there is scarce information about the staling of the frozen part-baked bread concerning starch changes, namely amylopectin retrogradation, and crumb hardening during post-baking storage. The objective of the present work was to analyse the effect of freezing and frozen storage on the staling of partially baked wheat bread. For analysing the bread staling process two different approaches have been performed. Firstly, the amylopectin behaviour was followed by differential scanning calorimetry for simulating the baking process and quantifying the retrogradation during storage. Secondly, hardness increase during storage was evaluated in the wheat bread crumb from pre-baked bread after different frozen storage times.
Ingredients were mixed in a 50-g bowl Brabender farinograph (Brabender, Germany), after 4 min mixing, the resulting bread dough was rested for 10 min, moulded and then proofed in a fermentation cabinet at 28 C and 80% relative humidity up to three times the initial dough volume (approximately 90 min). This proofed bread dough was further used for simulating the baking process in the calorimeter. 2.2. Differential scanning calorimetry (DSC) A differential scanning calorimeter (Perkin-Elmer DSC-7) was used as an oven to simulate the baking process (Leo´n, Dura´n, & Benedito, 1997). In this study a modified procedure was followed in order to imitate the interrupted breadmaking process. Thermal properties of starch were analysed through part-baking, frozen storage, final baking process and refrigeration storage. A complete scheme of the temperature transitions is shown in Fig. 1. 2.2.1. Part-baking stage To simulate the partial baking, bread dough samples (18–20 mg) were weighed in stainless steel pans (PE 0319-0218). An empty capsule was used as a reference. After sealing, the capsules were heated from 25 to 90 C, that was the temperature reached in the centre of the crumb during part-baking measured by thermocouples (results not shown). Then capsules were cooled to 40 C at 10 C/min. Twenty-seven replicates for each sample were run. Thermal transitions of starch samples were defined as To (onset), Tp (peak of gelatinization) and Tc (conclusion). The enthalpy associated with starch gelatinization (Hg) was calculated as the area enclosed by the straight line and endotherm curve, and it was expressed in Joules per grams of dry sample.
2. Materials and methods 2.1. Dough samples Commercial Spanish flour, obtained from the local market was used in this study. A basic bread recipe consisted in wheat flour (50 g), compressed yeast (1.0 g), salt (1.0 g) and water (27.2 g, up to optimum absorption) was used for analysing starch thermal properties.
Fig. 1. Temperature profile reached in the centre of the crumb during an interrupted baking process including part-baking stage, frozen storage stage and finish baking stage or rebaking.
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2.2.2. Frozen storage Capsules were immediately placed in a freezer at 35 C for 15 min and then stored at 18 C for 7, 15 and 30 days.
3. Results and discussion
2.2.3. Finish-baking stage or rebaking At different frozen storage times, capsules were thawed at 25 C for 15 min and heated again in the calorimeter from 25 to 110 C at 10 C/min to complete the baking process. Nine replicates for each sample were run.
The DSC technique was chosen for studying the behaviour of wheat starch during a two-step baking process, because the baking process can be simulated removing the possible interference promoted by the water loss, since the calorimetry analysis used hermetic capsules. Thermal properties of the starch were followed with the DSC (Fig. 2). Two endotherms appeared in the thermograms when the fermented dough was baked in the DSC by increasing the temperature from 25 to 90 C, temperature reached in the centre of the bread dough in the pre-baking process (results not shown). This behaviour is typical for starch when heated in the presence of a limited amount of water (Biliaderis, Maurice, & Vose, 1980; Califano & An˜on, 1990, Jovanovich, Zamponi, Lupano, & An˜on, 1992). The first peak of the thermogram corresponded to the gelatinization process of the amorphous phase of the starch, which appeared between 67.2 C and 83.4 C and had an enthalpy of 1.43 J/g (d.b.) (Table 1). The peak of the second endotherm appeared at 88.1 C and had 0.39 J/g (d.b.) of enthalpy. The same behaviour was previously reported by Ferrero, Martino, and Zaritzky (1993), Leo´n et al. (1997) and Andreu, Collar, and Martinez-Anaya (1999), who assigned the first endothermic peak to the gelatinization peak and the second one to the melting of the more stable crystalline structure of starch. Regarding the later, its enthalpy value is greatly dependent on the amount of water. In fact, the enthalpy value in this study was slightly lower than the 0.76 J/g reported by
2.2.4. Aging or refrigeration storage After the complete baking (B) capsules were stored at 4 C for analysing the amylopectin retrogradation during bread staling. Samples after 2, 4 and 7 days of storage were scanned in the DSC from 25 to 110 C at 10 C/min. The retrogradation enthalpy (Hr) of amylopectin was calculated as the area enclosed by the straight line and endotherm curve, and it was expressed in joules per gram of dry sample. The retrogradation index (RI) was defined as (Hr/Hg) (Duran, Leo´n, Barber, & Benedito, 2001). Three replicates for each sample were run. 2.3. Breadmaking procedure A straight dough process was performed for preparing the bread samples. The bread dough recipe, based on flour weight, consisted of: 6500 g of wheat flour (14% mb), 2% compressed yeast, 2% salt and water up to optimum consistency. Ingredients were optimally mixed, rested for 10 min, divided (150 g), kneaded, rested again for 10 min, mechanically sheeted and rolled and finally proofed at 28 C and 80% relative humidity (up to three times the initial dough volume). The partbaking was conducted at 165 C for 7 min, then loaves were cooled to 40 C in the crumb centre and next frozen to 18 C at a temperature of 35 C for 105 min. Frozen loaves were wrapped in polyethylene films and stored for different intervals at 18 C. Samples for analysis were taken at 7, 14, 28, 42 days and thawed at room temperature. After unwrapping, part-baked breads were baked at 195 C for 14 min and cooled at room temperature for 1 h. Full-baked bread volume was determined by seed displacement, and crumb hardness was measured in a Texture Analyser TA-XT2i (Stable Micro Systems, Surrey, UK). Since this type of bread is for daily consumption, the hardness increase at 25 C was followed during the first 24 h.
3.1. Thermal properties of wheat starch during heating/ frozen/heating cycles
2.4. Statistical analysis Multiple sample comparison was statistically analysed with the Statgraphics Plus 5.0. Fisher’s least significance difference (LSD) test was used to describe means at the 5% significance level.
Fig. 2. Differential scanning calorimetry thermograms during the process; a, part-baking procedure (Peak 1: gelatinization, Peak 2: melting or fusion endotherm); b, finish-baking process; c, thermogram of amylopectin retrogradation after storage at 4 C (Peak 3: amylopectin retrogradation).
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Table 1 Comparison of thermal parameters for wheat starch through the partbaking and in the retrogradation after 30 days frozen storage, rebaking and 7 days of aging at 4 C
To ( C) Tp ( C) Tc ( C) H (J/g) RI
Gelatinizationa
Retrogradationb
67.20.9 75.30.4 83.40.4 1.430.29 –
38.40.6 58.40.6 78.60.8 9.220.47 7.40.7
T1To, onset temperature; Tp, peak temperature; Tc, conclusion temperature; RI, retrogradation index. a Meanstandard deviation (n526). b Meanstandard deviation (n53).
Leo´n et al. (1997), besides Andreu et al. (1999) showed enthalpies ranging from 0.03 to 0.13 J/g (d.b.). These differences can be explained due to the different materials, recipes and processing conditions. At different intervals of the frozen storage, the partbaked samples were reheated in the DSC simulating the finish-baking process. In the second scan (Fig. 2), corresponding to the finish-baking appeared the second endotherm, the melting or fusion endotherm, which indicates that during the frozen storage of the partbaked samples no detectable retrogradation of amylopectin is produced. Ferrero et al. (1993) described similar findings when analysing frozen pastes of wheat flour, they did not detect the retrogradation peak for any freezing rate used after 12 h at 20 C. Ferrero and Zaritzky (2000) also reported that retrogradation of amylopectin was not detected by DSC in corn starch frozen samples stored for 91 days at 80 C. Rapid freezing prevented crystallization of both amylose and amylopectin, producing a homogeneous structure upon thawing and the absence of amylopectin retrogradation peaks (Ferrero et al., 1993).
fast crystallization of amylose and slow recrystallization of amylopectin (Biliaderis, 1992). A progressive increase of the retrogradation endotherm was observed with the aging time (Fig. 3), obtaining a very well defined peak after 7 days storage at 4 C. 3.3. Effect of part-baking, frozen storage and rebaking of bread dough on the aging during storage When bread samples from different frozen storage times were aged, they showed a significant decrease (P < 0.05) in the onset temperature of the retrogradation endotherm with the time of frozen storage, while the peak and conclusion temperature were barely affected (Table 2). As a consequence the retrogradation temperature range significantly increased with the frozen storage time. Ribotta et al. (2003) obtained similar results when studying the effect of dough frozen storage on the retrogradation temperature transition, although in this case only after long storage, 150 and 230 days. The lowering of To might be due to the modification of the amount of water available, since the water content controls the amylopectin melting during retrogradation (Leo´n et al., 1997; Ribotta et al., 2003; Zeleznak & Hoseney, 1986). Lu and Grant (1999) and Bhattacharya et al. (2003) clearly showed that the amount of freezable water increases during dough frozen storage. Bhattacharya et al. (2003) attributed that effect to the deterioration of gluten network during frozen resulting in a migration of water and an increase in the freezable water. However in the part-baked bread the protein network has already been denatured in the part-baking process before freezing, therefore the explanation of a redistribution of the water present in the system seems more plausible in this type of process (Berglund, Shelton, & Freeman, 1991). In addition, it should be also considered that in baked bread a more complex
3.2. Thermal properties of amylopectin during aging Samples submitted to the second heating were stored at 4 C for analysing the aging behaviour of the amylopectin when a part-baking was applied and the possible effect of the frozen storage. Aging was performed at 4 C for accelerating the retrogradation of the amylopectin, because Ribotta, Leo´n, and An˜on (2003) described higher retrogradation rate at 4 C than at 20 C. After different storage times (2, 4, 7 days), samples were scanned again and the thermogram showed a well defined peak corresponding to the amylopectin retrogradation (Fig. 2), which showed lower values of transition temperatures (To, Tp and Tc) than the gelatinization peak (Table 1). The enthalpy of the retrogradation endotherm (Hr) was higher than the gelatinization enthalpy at all the storage times tested. Retrogradation involves
Fig. 3. DSC thermograms corresponding to aged baked-dough at 4 C for 2, 4 and 7 days. Baked dough was obtained after part-baking, frozen storage and rebaking.
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Table 2 Effect of 2 days aging at 4 C on the thermal properties of frozen part-baked breada Frozen time (days)
To ( C)
Tp ( C)
Tc ( C)
Tr ( C)
RI
7 15 30
46.7 0.9a 42.4 0.4b 38.9 1.0c
62.20.9a 60.60.8a 59.92.6a
77.81.4a 78.01.9a 78.12.1a
31.1 1.5a 35.6 2.1b 39.2 1.2c
5.41.3a 5.01.9a 5.10.6a
T2To, onset temperature; Tp, peak temperature; Tc, conclusion temperature; Tr, retrogradation temperature range; RI, retrogradation index. a Meanstandard deviation, n=3, means within columns followed by the same letter are not significantly different (P<0.05).
interactions are involved like between inner and outer crumb, starch and gluten and spatial populations of water with different mobility (Czuchajowska & Pomeranz, 1989). Regarding the retrogradation index (RI), no significant differences were found after 2 days aging, but at longer refrigeration storage a progressive increase of the retrogradation enthalpy was observed. Fig. 4 presents the effect of frozen storage on the retrogradation enthalpy of the amylopectin. Despite no differences were observed at 2 days of storage, different trends were observed with prolonged storage, reaching higher retrogradation enthalpies with higher time of storage in frozen conditions. Nevertheless, after 4 days of storage, retrogradation enthalpy achieved an asymptotic behaviour. Frozen storage did not introduce differences in the rate of amylopectin retrogradation, but affected the total enthalpy of amylopectin retrogradation, obtaining higher enthalpies at longer times of frozen storage. As previously has been discussed, even at freezing temperatures a redistribution of water and different interactions between proteins and starch could yield a more ordered amylopectin crystallites which require higher energy for melting. In fact, Ribotta et al. (2003) described changes in both the structure and arrangement of amylose and amylopectin during the frozen storage of bread doughs, which were reflected in the amylopectin retrogradation during aging.
3.4. Specific volume and hardness of the bread from interrupted baking process
Fig. 4. Effect of time of frozen storage at 18 C of the part-baked samples on the amylopectin retrogradation during aging at 4 C. The different series correspond to the time of frozen storage.
Fig. 5. Effect of part-baking, freezing, frozen storage at different times, thawing and rebaking on the crumb hardness during aging at 25 C. The different series correspond to the time of frozen storage.
No significant effect was observed in the specific volume of the fresh bread, neither in the shape index (Table 3), however when the crumb hardness was evaluated a progressive significant increase of the crumb hardness was observed with the time of frozen storage, and that increase was more evident beyond 14 days of storage. Fik and Surowka (2002) found no significant correlation between the overall sensory evaluation and the storage time in frozen conditions, they did not found either correlation with the texture parameters, although a wide variation between samples was found. 3.5. Hardness increase during aging of part-baked bread after the finish-baking In Fig. 5 can be observed a significant increase (P < 0.05) in the crumb hardness of the bread with the frozen storage time, therefore some changes occurred during frozen storage of the part-baked bread. Fik and Surowka (2002) found no statistically significant differences in the crumb hardness of bread after different time of frozen storage, only the crumb adhesiveness was correlated with the storage time, although described a high variation in the parameters analysed due to the heterogeneity of the material.
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Table 3 Effect of part-baking, freezing, frozen storage and rebaking on the quality of the fresh breada Frozen time (days)
Specific volume (ml/g)
Shape index
Hardness (g)
Hardening rate (g/day)
0 7 14 28 42
4.000.15a 3.850.11a,b 4.000.09a 3.700.13b 3.850.11a,b
1.590.06a 1.600.05a 1.600.04a 1.600.05a 1.590.03a
313.98.3a 327.720.7a,b 346.020.8b 373.432.1c 422.321.4d
11.50a 12.52b 14.46c 14.53d 14.53d
T3Shape index: width/height ratio. a Meanstandard deviation, n=3, means within columns followed by the same letter are not significantly different (P<0.05).
As was expected, a progressive increase of the hardness was observed during the aging, and when analysed the rate of hardening (slope of the hardening curve) it was obtained faster hardening at longer period of frozen storage, reaching an asymptotic trend beyond 14 days of frozen storage. Fik and Surowka (2002) described that the freezing process itself produces the greatest changes on sensory and textural properties of fresh bread from part-baked bread, but they did not follow the possible effect of the frozen storage on the aging behaviour of the resulting bread. Results obtained in this work show that the freezing process itself did not produce effect on the bread aging, in opposition the time of frozen storage influenced the rate of hardening during aging, moreover when prolonged frozen storage are performed. Crumb hardness results of the fresh bread and also the hardening rate during aging indicate that some changes are produced during the frozen storage, which corroborates the previous results obtained when analysing the amylopectin retrogradation by DSC.
4. Conclusions The frozen storage of part-baked dough does not eliminate the processes occurring after baking, such as interactions among the diverse components, which become evident during the aging or storage of the resulting bread. The DSC analysis of the amylopectin behaviour during the part-baking, freezing, frozen storage, thawing, rebaking and aging revealed that the time of storage produces an increase in the retrogradation temperature range and the total enthalpy for amylopectin melting. Regarding the crumb hardness, results indicate that the time of frozen storage determine the hardening rate during aging.
Acknowledgements This work was financially supported by Ministerio de Ciencia y Tecnologia Projects (MCYT, AGL2001-1273
and AGL2002-04093-C03-02) and Consejo Superior de Investigaciones Cientı´ficas (CSIC), Spain. M.E. Ba´rcenas would like to thank her grant from Universidad de las Americas, Puebla, Mexico.
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