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Different approaches for improving the quality and extending the shelf life of the partially baked bread: low temperatures and HPMC addition
Journal of Food Engineering 72 (2006) 92–99 www.elsevier.com/locate/jfoodeng
Different approaches for improving the quality and extending the shelf life of the partially baked bread: low temperatures and HPMC addition Marı´a Eugenia Ba´rcenas a, Cristina M. Rosell b
b,*
a Universidad de las Ame´ricas, Sta Catarina Martir, Cholula, Puebla-72820, Me´xico Cereal Laboratory, Institute of Agrochemistry and Food Technology (IATA-CSIC), P.O. Box 73, 46100 Burjassot, Valencia, Spain
Received 27 July 2004; accepted 11 November 2004 Available online 23 December 2004
Abstract The use of refrigeration or positive temperature storage as an alternative to frozen storage for extending the shelf life of partially baked bread is described. In addition, the effect of hydroxypropylmethylcellulose (HPMC) on the baking performance of those products is detailed. With this purpose, the quality and shelf life of the bread from partially baked bread stored at frozen temperatures ( 25 C) for 42 days was compared with those of that from positive temperature stored partially baked bread (2 C) stored for 10 days. In both cases, batches in the presence and absence of HPMC were performed. The characteristics evaluated were the microstructure by cryo-scanning electron microscopy (cryo-SEM), the sensory quality, the technological properties (specific volume, moisture content, width/height ratio, crumb hardness), the crumb grain and the staling rate. The microstructure of the bread crumb from the positive temperature stored sample was almost intact compared with that of the bread from its frozen counterpart. The positive temperature storage led to bread with better specific volume, low crumb hardness and slower hardening rate during staling than the bread stored at frozen temperatures. Regarding the sensory quality, loaves from positive temperature stored par-baked bread did not differ in appearance and aroma, but had lower scores in taste and texture than the samples from frozen par-baked bread. The presence of HPMC improved the technological parameters and diminished the staling rate without affecting the sensory attributes. The beneficial effect of the HPMC was more evident on the samples from frozen par-baked bread, showing a microstructure without any damage due to ice crystals growth and similar to that observed in the samples from positive temperature stored parbaked bread. 2004 Elsevier Ltd. All rights reserved. Keywords: Par-baked bread; HMPC; Frozen storage; Positive temperature storage; Microstructure
1. Introduction The important economic losses resulting from bread staling have encouraged the search of methods for controlling or at least delaying the phenomenon. Among the proposed techniques, the addition of enzymes, emulsifiers and hydrocolloids have been extensively studied (Ba´rcenas, Haros, & Rosell, 2003; Jime´nez & *
Corresponding author. Tel.: +34 963 900022; fax: +34 963 636301. E-mail address: [email protected] (C.M. Rosell).
0260-8774/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.11.027
Martı´nez-Anaya, 2001; Martı´nez, Andreu, & Collar, 1999; Mettler & Seibel, 1993; Rosell, Haros, Escriva´, & Benedito de Barber, 2001; Twillman & White, 1988). A different approach for extending the shelf life of the bread consists in baking the bread until the crumb is formed, and to stop the baking before the Maillard reactions starts on the crust. This partially baked bread can be kept frozen or at room temperature. Final baking is done before consumption. In this way, it is possible to adjust the bread baking to the rate of selling. In addition, the partial baking and the further frozen
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storage allow the consumers to have fresh bread at any time of the day, widen the number of possible retailers, and also facilitate the work of the retailers (Fik & Surowka, 2002; Leuschner, OÕCallaghan, & Arendt, 1999; Vulicevic, Abdel-Aal, Mittal, & Lu, 2004). Frozen storage of the partially baked bread is an expensive process due to the high costs involved in the cold chain for frozen products. The use of positive temperature storage would reduce the refrigeration load of the process, including freezing and storage of the partially baked bread. Moreover, keeping the product at a positive temperature would suppress the damage caused by ice crystals during freezing. Nevertheless, keeping the bread at positive temperatures exposes the bread to an increased risk of deterioration during storage such as microbial and fungi growth or enzymatic reactions. Bread is a product of daily consumption, highly demanded, so sometimes long-storage of partially baked bread is not necessary and positive temperature storage could be an acceptable alternative to freezing. Amylopectin retrogradation is known to be one of key phenomena involved in bread staling. Although staling is accelerated at low temperatures, retrogradation should not represent a major problem since the retrograded amylopectin is melted during the second baking (Leuschner, OÕCallaghan, & Arendt, 1997). The addition of hydrocolloids to the bread formulation leads to better quality and extended shelf life (Ba´rcenas, Benedito, & Rosell, 2004; Davidou, Le Meste, Debever, & Bekaert, 1996; Ribotta, Pe´rez, Leo´n, & An˜o´n, 2004; Rosell, Rojas, & Benedito de Barber, 2001; Sharadanant & Khan, 2003). This effect has been described with the derivatives of cellulose, such as carboxymethyl cellulose and hydroxypropylmethylcellulose, which have reliable properties due to their synthetic origin. Hydroxypropylmethylcellulose (HPMC) has been successfully used in the straight dough process (Armero & Collar, 1996; Guarda, Rosell, Benedito, & Galotto, 2004; Rosell, Rojas, et al., 2001), due to its ability to act as an emulsifier, and to its capacity in strengthening the cell walls and in enhancing water retention (Bell, 1990; Dziezak, 1991; Sarkar & Walker, 1995). Therefore the effect of the HPMC on the quality of partially baked bread could be of interest. The aim of this study was to compare the subzero (frozen) and positive temperature storage conditions of partially baked bread, and to evaluate the use of the HPMC as a bread improver and antistaling agent for the baked products obtained with these technologies.
2. Materials and methods Commercial wheat flour (14% moisture content, 12.5% protein) was purchased from local market. Alveographic properties of the wheat flour were 54 mm of tenacity, 144 mm of extensibility and 199 · 10 4 J of
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deformation energy. Compressed yeast was used as a starter. HPMC (Methocel K4M) was given by Dow Chemical (France). 2.1. Interrupted breadmaking process The bread recipe consisted in wheat flour (6.5 kg), compressed yeast (2%, flour basis), salt (2%, flour basis), and water (up to optimum consistency of 500 Brabender Units) was used in this study. HPMC when added was used at 0.5% concentration (flour basis). All the ingredients were mixed, rested for 10 min, divided in portions (150 g), kneaded and mechanically sheeted and rolled. Bread dough proofing was carried out in a proving cabinet at 28 C and 85% relative humidity up to the optimum dough volume increase. Proofed loaves were partially baked at 165 C for 7 min. Partially baked loaves (or par-baked loaves) were cooled at room temperature until the center of the crumb reached 40 C. 2.2. Storage of the partially baked loaves at subzero and positive temperatures In the case of frozen storage, partially baked bread was placed into a freezer at 35 C for a faster cooling and kept until the crumb center reached 6 C. The products were wrapped in polypropylene bags, and placed in a storage chamber at 25 C for 42 days. For the final baking, unwrapped frozen partially baked breads were thawed at ambient temperature for 30 min, baked at 195 C for 14 min, and finally cooled at ambient temperature for 60 min. In the case of positive temperature storage, partially baked bread was kept in a temperature controlled cabinet at 2 C, until that temperature was reached in the crumb core. Positive temperature stored partially baked breads were wrapped and kept at 2 C for 10 days. Final baking and cooling were performed as described before; with the exception that baking was only for 10 min, instead of the 14 min required in the case of frozen parbaked bread, due to the additional heat necessary to complete the thawing. For the staling studies, fully baked bread was packed again in polypropylene bags and stored at 25 C for 24 h. 2.3. Cryo scanning electron microscopy (cryo-SEM) A Jeol JSM-5410 scanning electron microscope equipped with a CT-1500 C cryo-unit (Oxford Instruments) was used. The sample was placed on the cryospecimen holder, and cryo-fixed in slush nitrogen (6 210 C), then transferred to the cryo-unit in the frozen state, where it was fractured, sublimated (15 min at 90 C) and sputter coated with gold (4 min, 2 mbar). Finally, the sample was transferred the microscope where it was observed at 15 kV and 130 C.
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2.4. Organoleptic properties evaluation Sensory analysis was carried out by an untrained panel and scored on a scale of 1 (least favourable) to 9 (most favourable). The attributes evaluated were visual appearance, aroma, taste and texture. For each one of attributes the average of the judges response was calculated. Bread was considered acceptable if their mean values were above 5 (neither like nor dislike). 2.5. Technological evaluation of the bread The technological parameters for determining bread quality included volume (rapeseed displacement), weight, specific volume and width/height ratio of the central slice. The water content was measured following the standard method (44-15A, AACC, 1995). The crumb hardness was carried out in a texture analyser TA-XT2i (Stable Microsystems, Surrey, UK). A 2 cm thick slice was compressed with a 25 mm probe up to 50% at 100 mm/min speed. 2.6. Image acquisition and analysis The crumb grain of the loaves was assessed using a digital image analysis system (Crowley, Grau, & Arendt, 2000). Images were taken from the centre of the bread slice and were captured using a Hewlett Packard flatbed scanner (HP ScanJet 4400c, Hewlett Packard, USA) supporting HP Precisionscan Pro 3.1 software (Hewlett Packard, USA). A single 40 mm · 40 mm square field of view was evaluated for each image. The clear colours were adjusted to 160 units and the dark ones to 120 units, while the middle colours to 2.2. Images were scanned fullscale in 256 grey levels at 150 dpi (dots per inch). Data were processed using SigmaScan Pro 5 software (Jandel Corporation, US). The crumb grain features chosen were number of cells, mean cell area and cell to total area ratio. 2.7. Statistical analysis In order to assess significant differences among samples, a multiple comparison analysis of samples was performed using the Statgraphics Plus 5.0 program. FisherÕs least significant differences (LSD) test was used to describe means with 95% confidence. 3. Results and discussion 3.1. Microstructure of the fully baked bread from partially baked bread stored at different temperatures and in the presence and absence of HPMC The micrographs of the cell walls of the fully baked bread from partially baked bread stored at frozen and
positive temperatures obtained by cryo scanning electron microscopy are displayed in Fig. 1. There was no difference between the bread from refrigerated parbaked bread stored for 0 and 10 days (Fig. 1a and b). In both micrographs of the cell walls well defined starch granules, filaments from the protein network that hold the bread structure, and numerous cavities that constitutes the gas phase of the crumb were observed. The cell walls of the bread from frozen par-baked bread without storage (Fig. 1c) had a similar picture to the positive temperature stored baked goods. The micrographs of those products were comparable to those obtained by Rojas, Rosell, Benedito de Barber, Pe´rez-Munuera, and Lluch (2000) in analysing the structure of the bread crumb obtained by conventional process. The microstructure of the bread from par-baked bread stored for 42 days at frozen temperatures (Fig. 1d) had instead a more compact appearance, showing less cavities of minor size. Moreover, it was not possible to distinguish the two characteristic phases of the solid crumb; those are the continuous or solid one formed by the protein network and the leached molecules of the starch polymers, and the discontinuous one constituted by the starch granules. In addition, in the cell walls of this sample it was possible to observe some places of dry and coarse appearance. In the presence of HPMC (Fig. 1e–h), the constituents of the crumb seemed to be intimately linked due to the presence of an agglutinating material that could be the HPMC. This was even more evident in the samples that were not stored which appear to be covered by a veil-like film. In all the samples containing HPMC, the starch granule structure was better defined. It should be stressed out that in the presence of HPMC the frozen samples without and with frozen storage (Fig. 1g and h) had similar microstructure than their counterparts from positive temperature stored par-baked bread (Fig. 1e and f). These results indicate that positive temperature storage and freezing did not affect crumb microstructure. However, it is evident that frozen storage promoted changes in crumb microstructure. It is possible that the growth of the ice crystals during the frozen storage damages the structure of the crumb constituents, as has been observed in bread dough (Berglund, Shelton, & Freeman, 1991; Naito et al., 2004; Ribotta et al., 2004; Varriano-Marston, Hsu, & Mahdi, 1980). On the other hand, the appearance of the samples containing HPMC indicates that this hydrocolloid was integrated in the structure of the crumb and interacts with its constituents exerting a shielding effect. The formation of physical entanglements or real links between the HPMC and the gluten proteins or between the HPMC and the starch granules has been already suggested (Armero & Collar, 1998; Collar, Armero, & Martı´nez, 1998).
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Fig. 1. Cryo-SEM micrographs showing the effect of storage temperature and HPMC (3500·). (a,e) wheat bread from partially baked bread after positive temperature storage and full baking; (b,f) wheat bread from partially baked bread after 10 days of positive temperature storage and full baking; (c,g) wheat bread from partially baked bread after freezing, thawing and full baking; (d,h) wheat bread from partially baked bread after freezing, 42 days of frozen storage, thawing and full baking. Control: a–d; HPMC: e–h. The arrows show starch granules (G), cavities (C), protein network (N), and damaged zones (DZ).
3.2. Quality attributes of the fully baked bread obtained from partially baked bread stored at different temperatures and in the presence and absence of HPMC The quality attributes of the fully baked loaves obtained from positive temperature stored/frozen par-
baked bread in the presence and absence of HPMC is summarised in Table 1. In general, the loaves from frozen par-baked bread had lower specific volume than those from positive temperature stored par-baked bread. This could be a consequence of the damage of the protein network that was promoted by the growth of the
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Table 1 Quality attributes of wheat bread from partially baked bread stored at both subzero (frozen) and positive temperatures after full baking Positive temperature stored Specific volume (cm3/g) Width/height ratio Moisture content (%) Hardness (g) Number of cells Mean cell area (mm2) Cell to total area ratio
Frozen
Control
HPMC
Control
HPMC
3.98 ± 0.07a,b 1.48 ± 0.01a 34.39 ± 0.02a 268.0 ± 23.4b 618 ± 30a 0.80 ± 0.04a 0.31 ± 0.02a
4.33 ± 0.07c 1.57 ± 0.03a 35.59 ± 0.03c 202.4 ± 24.2a 646 ± 32a 0.68 ± 0.03a 0.27 ± 0.01a
3.86 ± 0.14a 1.59 ± 0.03a 35.32 ± 0.03b 422.3 ± 23.4c 667 ± 33a 0.54 ± 0.02a 0.27 ± 0.02a
4.06 ± 0.06b 1.51 ± 0.03a 37.09 ± 0.03d 283.7 ± 13.8b 607 ± 30a 0.75 ± 0.04a 0.29 ± 0.02a
Positive temperature storage: 10 days; frozen storage: 42 days. Means within rows followed by the same letter were not significantly different (P < 0.05).
ice crystals during frozen storage. Damaged protein would not be strong enough to hold the crumb bringing about a reduction of the bread volume, as has been observed in the bread obtained from frozen dough (Bhattacharya, Langstaff, & Berzonsky, 2003; Havet, Mankai, & Le Bail, 2000; Ribotta et al., 2004; Seguchi, Nikaidoo, & Morimoto, 2003; Sharadanant & Khan, 2003). The presence of HPMC produced a significant increase in the specific volume of loaves from frozen and positive temperature stored par-baked bread. The improving effect of the HPMC on the volume of bread obtained by conventional process has already been reported (Armero & Collar, 1996; Guarda et al., 2004; Rosell, Rojas, et al., 2001). This result could be attributed to the ability of the hydrocolloid chains to release water molecules associated to them in a hydrated system such as a bread dough, and the subsequent interaction among them, leading to a polymeric network when are exposed to high temperatures (Bell, 1990; Dziezak, 1991; Haque, Richardson, Morris, Gidley, & Caswell, 1993; Sarkar & Walker, 1995). The HPMC network shields the bread dough from the gas release during baking, allowing a better development of the loaves and in consequence, great volume. The water content of the loaves from positive temperature stored par-baked bread (Table 1) was significantly (P < 0.05) lower than that of those from their frozen counterparts, likely due to differences in the duration of the final baking or to condensations during thawing of frozen samples. Concerning the effect of the HPMC on the moisture content, the baked products containing HPMC had significantly (P < 0.05) higher moisture content than those in absence of HPMC. The crumb hardness (Table 1) was significantly (P < 0.05) higher in the samples from frozen par-baked bread than in those from their positive temperature stored counterpart. Considering the amylopectin retrogradation as the main responsible of the crumb hardening (Gray & Bemiller, 2003) and that the optimum temperature for its retrogradation ranges from 3 to 5 C (Slade & Levine, 1987), it should be expected higher
crumb hardness in the sample from positive temperature stored par-baked bread. However, it should be taken into account that the par-baked bread requires a second baking to obtain bread. Melting of the already retrograded amylopectin is occurring during this second baking (Leuschner et al., 1997). Therefore, the amylopectin retrogradation that is expected during the storage of par-baked bread at positive temperatures is annulled during the second baking. The high crumb hardness observed on the loaves from frozen samples probably results from the mechanical damage of the crumb structure due to the mechanical damage provoked by crystals during freezing plus recoarsening during the frozen storage. This explanation is based on the Kou and Chinachoti (1991) findings that described a relationship between the mechanical damage of the crumb structure and its hardness. Those authors also described that the crumb structure undergoes an irreversible damage when compressed during a prolonged period, and there is a positive relationship between the degree of compression and the crumb hardness. Conversely, the growth of the ice crystals during the storage at frozen temperatures could promote the rupture of the starch granules, with the subsequent leaching of intragranular amylose, favouring in consequence the formation of complexes between intra and intergranular amylose that are responsible of the crumb hardness in the immediate hours after baking (Hug-Iten, Escher, & Conde-Petit, 2003). Other plausible explanation for the higher hardness of the samples from frozen par-baked bread could be their lower specific volume, since there is an inverse relationship between the hardness and volume (Armero & Collar, 1996). The presence of HPMC produced a significant reduction of the crumb hardness; this phenomenon was more obvious in the bread from frozen par-baked bread, which had similar hardness than the samples from positive temperature stored control. These results support the hypothesis described in the microstructure observations, in that the HPMC could protect the crumb constituents against the ice crystals damage. The hydrocolloids have been widely used in the ice cream
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3.3. Sensory quality of the fully baked bread obtained from partially baked bread stored at different temperatures and in the presence and absence of HPMC The sensory analysis of the fully baked breads from positive temperature stored and frozen par-baked bread in the absence and presence of HPMC is shown in Table 2. There were no significant (P < 0.05) differTable 2 Sensory quality of wheat bread from partially baked bread stored at both subzero (frozen) and positive temperatures after full baking Appearance
Taste
Texture
Positive temperature stored Control 7.1 ± 1.3a 6.7 ± 1.2a HPMC 6.8 ± 1.3a 6.6 ± 1.3a
5.8 ± 1.9b 6.0 ± 1.5b
5.6 ± 1.9b 6.2 ± 1.5b
Frozen Control HPMC
6.8 ± 1.2a 6.2 ± 1.6a,b
7.1 ± 1.3a 6.9 ± 1.3a
6.9 ± 1.2a 7.0 ± 1.5a
Aroma
7.0 ± 1.1a 6.4 ± 1.1a
Positive temperature storage: 10 days; frozen storage: 42 days. Means within columns followed by the same letter were not significantly different (P < 0.05).
ences regarding appearance and aroma. In general, the taste was better in the samples from frozen par-baked bread than that in those from positive temperature stored par-baked bread, but in the presence of HPMC this preference disappeared. Concerning the texture, loaves from frozen samples were better scored and no differences were detected in the presence of HPMC. The microstructure results revealed the damage of crumb constituents in the bread from frozen storage in absence of HPMC, so that a high score in texture was not expected. A possible explanation could be that damaged crumb was appreciated as a less elastic crumb, which could be pleasant to the judges whose perceived a wide variety of attributes associated to texture. In fact, likely the higher score of the bread from frozen samples concerning taste could be related to the better texture perceived by the panel. The presence of HPMC did not modify the sensory quality of the loaves, although it has been described that HPMC addition improved the sensory attributes of the typical Chilean bread (Guarda et al., 2004). 3.4. Staling of the fully baked bread obtained from partially baked bread stored at different temperatures and in the presence and absence of HPMC The staling of those loaves was followed by measuring the crumb hardness increase during the 24-h storage at 25 C (Fig. 2). The loaves from frozen par-baked bread showed higher hardness increase than those from positive temperature stored par-baked bread. This rapid staling could be also attributed to the damage produced by the growth of the ice crystals on the crumb structure. The possible rupture of both the starch granules and the protein network that hold the crumb, could promote the diverse phenomena responsible of the crumb hardening, such as the formation of linkages between gluten
Control c
450
Hardness increase (g)
production for delaying or reducing the growth of ice crystals during storage. Caldwell, Goff, and Stanley (1992) demonstrated using scanning electron microscopy that ice creams containing hydrocolloids had smaller ice crystals than the control, before and after the storage at fluctuating temperatures. It has been proposed that the hydrocolloids modify the kinetic properties of the non-frozen phase, because their concentration in the liquid phase surrounded the ice crystals limits the water diffusion from the small to the big crystals. In consequence, the hydrocolloids limit both the Ostwald ripening and the recrystallization phenomena (Goff, 1992), which could support that the HPMC also protects the crumb structure during freezing and frozen storage. In addition, the low crumb hardness of the products containing HPMC can be attributed to their high moisture content, since several studies reported the inverse relationship between the moisture content and the hardness (Fik & Surowka, 2002; He & Hoseney, 1990; Hug-Iten et al., 2003; Rogers, Zeleznak, Lai, & Hoseney, 1988). On the other hand, the microstructure analysis could support a possible interaction between HPMC and the bread constituents, namely amylose, which interferes with the hardening process during the first hours after baking. The lower hardness of the bread containing HPMC agrees with previous results obtained with conventional breadmaking process (Armero & Collar, 1996; Guarda et al., 2004; Rosell, Rojas, et al., 2001). Concerning, other quality attributes of the bread, such as width/height ratio and the grain structure of the crumb, there were no significant differences among the samples tested (Table 1).
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400 350 300 250
Control a,b
HPMC b HPMC a
200 150 100 50 0 Positive temperature storage
Subzero temperature storage
Fig. 2. Hardness increase during ageing of wheat bread from partially baked bread stored at subzero (frozen) or positive temperatures. Frozen storage: 42 days; positive temperature storage: 10 days. Ageing conditions: 24 h at 25 C.
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proteins and starch (Martin, Zeleznak, & Hoseney, 1991), the water redistribution between crumb and crust (Piazza & Masi, 1995) and among the polymers present in the crumb (Chen, Long, Ruan, & Labuza, 1997), and the amylopectin recrystallization (Schoch & French, 1947; Xie, Dowell, & Sun, 2004). The addition of HPMC reduced the crumb hardening of both samples (positive temperature stored and frozen) during staling. Different mechanisms could be responsible for this antistaling effect, such as the higher moisture content present in the samples containing the polymer, and the possible interaction of the HPMC with the polysaccharides, proteins and other bread constituents that hinder the formation of complexes between them. In the loaves from frozen par-baked bread, the HPMC effect could be also due to the previously described effect of the HPMC limiting the diffusion of water molecules and in consequence the growth of the ice crystals, which in turn decreases the damage on the crumb structure. The antistaling effect of the HPMC has been already reported in the bread obtained by conventional breadmaking process (Armero & Collar, 1998; Guarda et al., 2004). Finally, it should be stressed out that the bread is a very complex system, even when obtained from a basic formulation, for which the staling mechanisms are not completely understood. Therefore, the addition of positive temperature storage and frozen storage and the presence of bread improvers as HPMC increase the complexity of the problem.
4. Conclusions The addition of HPMC to the par-baked bread recipe resulted in baked products of better quality and with low hardening rate of the crumb, without affecting the sensory attributes. The HPMC had a protective effect against the damage promoted by freezing and frozen storage, which was supported with the scanning electron microscopy analysis of the crumb structure. The micrographs revealed an intimate interaction between the HPMC chains and the constituents of the bread crumb. The positive temperature storage seems to be a good alternative to the frozen storage, in fact in this case, parbaked bread led to better specific volume, softer crumbs and lower hardening rate in comparison with the bread from frozen par-baked. However, it would be necessary to perform some additional studies on the sensory quality and the microbiology safety in long storage periods.
Acknowledgments This work was financially supported by Ministerio de Educacio´n y Ciencia Projects (AGL2001-1273,
AGL2002-4093) and Consejo Superior de Investigaciones Cientı´ficas (CSIC), Spain. M.E. Ba´rcenas would like to thank her grant from Universidad de las Ame´ricas, Puebla, Me´xico. Authors also thank Dr. I. Perez-Munuera for her technical assistance in the microstructure analysis.
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M.E. Ba´rcenas, C.M. Rosell / Journal of Food Engineering 72 (2006) 92–99 Haque, A., Richardson, R. K., Morris, E. R., Gidley, M. J., & Caswell, D. C. (1993). Thermogelation of methylcellulose. Part II: Effect of hydroxypropyl substituents. Carbohydrate Polymers, 22, 175–186. Havet, M., Mankai, M., & Le Bail, A. (2000). Influence of the freezing condition on the baking performances of French frozen dough. Journal of Food Engineering, 45, 139–145. He, H., & Hoseney, R. C. (1990). Changes in bread firmness and moisture during long-term storage. Cereal Chemistry, 67, 603–605. Hug-Iten, S., Escher, F., & Conde-Petit, B. (2003). Staling of bread: Role of amylose and amylopectin and influence of starch-degrading enzymes. Cereal Chemistry, 80(6), 654–661. Jime´nez, T., & Martı´nez-Anaya, M. A. (2001). Amylases and hemicellulases in breadmaking, degradation by-products and potential relationship with functionality. Food Science and Technology International, 7(1), 5–14. Kou, Y., & Chinachoti, P. (1991). Structural damage in bread staling as detected by recoverable work and stress–strain model analysis. Cereal Foods World, 36(10), 888–892. Leuschner, R. G. K., OÕCallaghan, M. J. A., & Arendt, E. K. (1997). Optimization of baking parameters of part-baked and rebaked Irish brown soda bread by evaluation of some quality characteristics. International Journal of Food Science and Technology, 32, 487–493. Leuschner, R. G. K., OÕCallaghan, M. J. A., & Arendt, E. K. (1999). Moisture distribution and microbial quality of part baked breads as related to storage and rebaking conditions. Journal of Food Science, 64(3), 543–546. Martin, M., Zeleznak, K., & Hoseney, R. (1991). A mechanism of bread firming. I. Role of starch swelling. Cereal Chemistry, 68(1), 498–503. Martı´nez, J. C., Andreu, P., & Collar, C. (1999). Storage of wheat breads with hydrocolloids, enzymes and surfactants: Anti-staling effects. Leatherhead Food RA Food Industry Journal, 2, 133–149. Mettler, E., & Seibel, W. (1993). Effects of emulsifiers and hydrocolloids on whole wheat bread quality: A response surface methodology study. Cereal Chemistry, 70(4), 373–377. Naito, S., Fukami, S., Mizokami, Y., Ishida, N., Takano, H., Koizumi, M., et al. (2004). Effect of freeze-thaw cycles on the gluten fibrils and crumb grain structures of breads made from frozen doughs. Cereal Chemistry, 81(1), 80–86. Piazza, L., & Masi, P. (1995). Moisture redistribution throughout the bread loaf during staling and its effect on mechanical properties. Cereal Chemistry, 72(3), 320–325. Ribotta, P., Pe´rez, G. T., Leo´n, A., & An˜o´n, C. (2004). Effect of emulsifier and guar gum on microstructural, rheological and
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baking performance of frozen bread dough. Food Hydrocolloids, 18, 305–313. Rogers, D. E., Zeleznak, K. J., Lai, C. S., & Hoseney, R. C. (1988). Effect of native lipids, shortening, and bread moisture on bread firming. Cereal Chemistry, 65(5), 398–401. Rojas, J. A., Rosell, C. M., Benedito de Barber, C., Pe´rez-Munuera, I., & Lluch, M. A. (2000). The baking process of wheat rolls followed by cryo scanning electron microscopy. European Food Research and Technology, 212, 57–63. Rosell, C. M., Haros, M., Escriva´, C., & Benedito de Barber, C. (2001). Experimental approach to optimize the use of a-amylases in breadmaking. Journal of Agricultural and Food Chemistry, 49(6), 2973–2977. Rosell, C. M., Rojas, J. A., & Benedito de Barber, C. (2001). Influence of hydrocolloids on dough rheology and bread quality. Food Hydrocolloids, 15, 75–81. Sarkar, N., & Walker, L. C. (1995). Hydration–dehydration properties of methylcellulose and hydroxypropylmethylcellulose. Carbohydrate Polymers, 27, 177–185. Schoch, T. J., & French, D. (1947). Studies on bread staling. I. The role of starch. Cereal Chemistry, 24, 231–249. Seguchi, M., Nikaidoo, S., & Morimoto, N. (2003). Centrifuged liquid and breadmaking properties of frozen-and-thawed bread dough. Cereal Chemistry, 80(3), 264–268. Sharadanant, R., & Khan, K. (2003). Effect of hydrophilic gums on the quality of frozen dough: II. Bread characteristics. Cereal Chemistry, 80(6), 773–780. Slade, L., & Levine, H. (1987). Recent advances in starch retrogradation. In S. S. Stivala, V. Crescenzi, & I. C. M. Dea (Eds.), Industrial polysaccharides—the impact of biotechnology and advanced methodologies (pp. 387–430). New York: Gordon and Breach. Twillman, T. J., & White, P. J. (1988). Influence of monoglycerides on the textural shelf life and dough rheology of corn tortillas. Cereal Chemistry, 65(3), 253–257. Varriano-Marston, E. K., Hsu, K. H., & Mahdi, J. (1980). Rheological and structural changes in frozen dough. Bakers Digest, 54(1), 32–41. Vulicevic, I. R., Abdel-Aal, E-S. M., Mittal, G. S., & Lu, X. (2004). Quality and storage life of par-baked frozen breads. Lebensmittel Wissenchaft und Technologie, 37, 205–213. Xie, F., Dowell, F. E., & Sun, X. S. (2004). Using visible and nearinfrared reflectance spectroscopy and differential scanning calorimetry to study starch, protein, and temperature effects on bread staling. Cereal Chemistry, 81(2), 249–254.
Different approaches for improving the quality and extending the shelf life of the partially baked bread: low temperatures and HPMC addition Marı´a Eugenia Ba´rcenas a, Cristina M. Rosell b
b,*
a Universidad de las Ame´ricas, Sta Catarina Martir, Cholula, Puebla-72820, Me´xico Cereal Laboratory, Institute of Agrochemistry and Food Technology (IATA-CSIC), P.O. Box 73, 46100 Burjassot, Valencia, Spain
Received 27 July 2004; accepted 11 November 2004 Available online 23 December 2004
Abstract The use of refrigeration or positive temperature storage as an alternative to frozen storage for extending the shelf life of partially baked bread is described. In addition, the effect of hydroxypropylmethylcellulose (HPMC) on the baking performance of those products is detailed. With this purpose, the quality and shelf life of the bread from partially baked bread stored at frozen temperatures ( 25 C) for 42 days was compared with those of that from positive temperature stored partially baked bread (2 C) stored for 10 days. In both cases, batches in the presence and absence of HPMC were performed. The characteristics evaluated were the microstructure by cryo-scanning electron microscopy (cryo-SEM), the sensory quality, the technological properties (specific volume, moisture content, width/height ratio, crumb hardness), the crumb grain and the staling rate. The microstructure of the bread crumb from the positive temperature stored sample was almost intact compared with that of the bread from its frozen counterpart. The positive temperature storage led to bread with better specific volume, low crumb hardness and slower hardening rate during staling than the bread stored at frozen temperatures. Regarding the sensory quality, loaves from positive temperature stored par-baked bread did not differ in appearance and aroma, but had lower scores in taste and texture than the samples from frozen par-baked bread. The presence of HPMC improved the technological parameters and diminished the staling rate without affecting the sensory attributes. The beneficial effect of the HPMC was more evident on the samples from frozen par-baked bread, showing a microstructure without any damage due to ice crystals growth and similar to that observed in the samples from positive temperature stored parbaked bread. 2004 Elsevier Ltd. All rights reserved. Keywords: Par-baked bread; HMPC; Frozen storage; Positive temperature storage; Microstructure
1. Introduction The important economic losses resulting from bread staling have encouraged the search of methods for controlling or at least delaying the phenomenon. Among the proposed techniques, the addition of enzymes, emulsifiers and hydrocolloids have been extensively studied (Ba´rcenas, Haros, & Rosell, 2003; Jime´nez & *
Corresponding author. Tel.: +34 963 900022; fax: +34 963 636301. E-mail address: [email protected] (C.M. Rosell).
0260-8774/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.11.027
Martı´nez-Anaya, 2001; Martı´nez, Andreu, & Collar, 1999; Mettler & Seibel, 1993; Rosell, Haros, Escriva´, & Benedito de Barber, 2001; Twillman & White, 1988). A different approach for extending the shelf life of the bread consists in baking the bread until the crumb is formed, and to stop the baking before the Maillard reactions starts on the crust. This partially baked bread can be kept frozen or at room temperature. Final baking is done before consumption. In this way, it is possible to adjust the bread baking to the rate of selling. In addition, the partial baking and the further frozen
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storage allow the consumers to have fresh bread at any time of the day, widen the number of possible retailers, and also facilitate the work of the retailers (Fik & Surowka, 2002; Leuschner, OÕCallaghan, & Arendt, 1999; Vulicevic, Abdel-Aal, Mittal, & Lu, 2004). Frozen storage of the partially baked bread is an expensive process due to the high costs involved in the cold chain for frozen products. The use of positive temperature storage would reduce the refrigeration load of the process, including freezing and storage of the partially baked bread. Moreover, keeping the product at a positive temperature would suppress the damage caused by ice crystals during freezing. Nevertheless, keeping the bread at positive temperatures exposes the bread to an increased risk of deterioration during storage such as microbial and fungi growth or enzymatic reactions. Bread is a product of daily consumption, highly demanded, so sometimes long-storage of partially baked bread is not necessary and positive temperature storage could be an acceptable alternative to freezing. Amylopectin retrogradation is known to be one of key phenomena involved in bread staling. Although staling is accelerated at low temperatures, retrogradation should not represent a major problem since the retrograded amylopectin is melted during the second baking (Leuschner, OÕCallaghan, & Arendt, 1997). The addition of hydrocolloids to the bread formulation leads to better quality and extended shelf life (Ba´rcenas, Benedito, & Rosell, 2004; Davidou, Le Meste, Debever, & Bekaert, 1996; Ribotta, Pe´rez, Leo´n, & An˜o´n, 2004; Rosell, Rojas, & Benedito de Barber, 2001; Sharadanant & Khan, 2003). This effect has been described with the derivatives of cellulose, such as carboxymethyl cellulose and hydroxypropylmethylcellulose, which have reliable properties due to their synthetic origin. Hydroxypropylmethylcellulose (HPMC) has been successfully used in the straight dough process (Armero & Collar, 1996; Guarda, Rosell, Benedito, & Galotto, 2004; Rosell, Rojas, et al., 2001), due to its ability to act as an emulsifier, and to its capacity in strengthening the cell walls and in enhancing water retention (Bell, 1990; Dziezak, 1991; Sarkar & Walker, 1995). Therefore the effect of the HPMC on the quality of partially baked bread could be of interest. The aim of this study was to compare the subzero (frozen) and positive temperature storage conditions of partially baked bread, and to evaluate the use of the HPMC as a bread improver and antistaling agent for the baked products obtained with these technologies.
2. Materials and methods Commercial wheat flour (14% moisture content, 12.5% protein) was purchased from local market. Alveographic properties of the wheat flour were 54 mm of tenacity, 144 mm of extensibility and 199 · 10 4 J of
93
deformation energy. Compressed yeast was used as a starter. HPMC (Methocel K4M) was given by Dow Chemical (France). 2.1. Interrupted breadmaking process The bread recipe consisted in wheat flour (6.5 kg), compressed yeast (2%, flour basis), salt (2%, flour basis), and water (up to optimum consistency of 500 Brabender Units) was used in this study. HPMC when added was used at 0.5% concentration (flour basis). All the ingredients were mixed, rested for 10 min, divided in portions (150 g), kneaded and mechanically sheeted and rolled. Bread dough proofing was carried out in a proving cabinet at 28 C and 85% relative humidity up to the optimum dough volume increase. Proofed loaves were partially baked at 165 C for 7 min. Partially baked loaves (or par-baked loaves) were cooled at room temperature until the center of the crumb reached 40 C. 2.2. Storage of the partially baked loaves at subzero and positive temperatures In the case of frozen storage, partially baked bread was placed into a freezer at 35 C for a faster cooling and kept until the crumb center reached 6 C. The products were wrapped in polypropylene bags, and placed in a storage chamber at 25 C for 42 days. For the final baking, unwrapped frozen partially baked breads were thawed at ambient temperature for 30 min, baked at 195 C for 14 min, and finally cooled at ambient temperature for 60 min. In the case of positive temperature storage, partially baked bread was kept in a temperature controlled cabinet at 2 C, until that temperature was reached in the crumb core. Positive temperature stored partially baked breads were wrapped and kept at 2 C for 10 days. Final baking and cooling were performed as described before; with the exception that baking was only for 10 min, instead of the 14 min required in the case of frozen parbaked bread, due to the additional heat necessary to complete the thawing. For the staling studies, fully baked bread was packed again in polypropylene bags and stored at 25 C for 24 h. 2.3. Cryo scanning electron microscopy (cryo-SEM) A Jeol JSM-5410 scanning electron microscope equipped with a CT-1500 C cryo-unit (Oxford Instruments) was used. The sample was placed on the cryospecimen holder, and cryo-fixed in slush nitrogen (6 210 C), then transferred to the cryo-unit in the frozen state, where it was fractured, sublimated (15 min at 90 C) and sputter coated with gold (4 min, 2 mbar). Finally, the sample was transferred the microscope where it was observed at 15 kV and 130 C.
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2.4. Organoleptic properties evaluation Sensory analysis was carried out by an untrained panel and scored on a scale of 1 (least favourable) to 9 (most favourable). The attributes evaluated were visual appearance, aroma, taste and texture. For each one of attributes the average of the judges response was calculated. Bread was considered acceptable if their mean values were above 5 (neither like nor dislike). 2.5. Technological evaluation of the bread The technological parameters for determining bread quality included volume (rapeseed displacement), weight, specific volume and width/height ratio of the central slice. The water content was measured following the standard method (44-15A, AACC, 1995). The crumb hardness was carried out in a texture analyser TA-XT2i (Stable Microsystems, Surrey, UK). A 2 cm thick slice was compressed with a 25 mm probe up to 50% at 100 mm/min speed. 2.6. Image acquisition and analysis The crumb grain of the loaves was assessed using a digital image analysis system (Crowley, Grau, & Arendt, 2000). Images were taken from the centre of the bread slice and were captured using a Hewlett Packard flatbed scanner (HP ScanJet 4400c, Hewlett Packard, USA) supporting HP Precisionscan Pro 3.1 software (Hewlett Packard, USA). A single 40 mm · 40 mm square field of view was evaluated for each image. The clear colours were adjusted to 160 units and the dark ones to 120 units, while the middle colours to 2.2. Images were scanned fullscale in 256 grey levels at 150 dpi (dots per inch). Data were processed using SigmaScan Pro 5 software (Jandel Corporation, US). The crumb grain features chosen were number of cells, mean cell area and cell to total area ratio. 2.7. Statistical analysis In order to assess significant differences among samples, a multiple comparison analysis of samples was performed using the Statgraphics Plus 5.0 program. FisherÕs least significant differences (LSD) test was used to describe means with 95% confidence. 3. Results and discussion 3.1. Microstructure of the fully baked bread from partially baked bread stored at different temperatures and in the presence and absence of HPMC The micrographs of the cell walls of the fully baked bread from partially baked bread stored at frozen and
positive temperatures obtained by cryo scanning electron microscopy are displayed in Fig. 1. There was no difference between the bread from refrigerated parbaked bread stored for 0 and 10 days (Fig. 1a and b). In both micrographs of the cell walls well defined starch granules, filaments from the protein network that hold the bread structure, and numerous cavities that constitutes the gas phase of the crumb were observed. The cell walls of the bread from frozen par-baked bread without storage (Fig. 1c) had a similar picture to the positive temperature stored baked goods. The micrographs of those products were comparable to those obtained by Rojas, Rosell, Benedito de Barber, Pe´rez-Munuera, and Lluch (2000) in analysing the structure of the bread crumb obtained by conventional process. The microstructure of the bread from par-baked bread stored for 42 days at frozen temperatures (Fig. 1d) had instead a more compact appearance, showing less cavities of minor size. Moreover, it was not possible to distinguish the two characteristic phases of the solid crumb; those are the continuous or solid one formed by the protein network and the leached molecules of the starch polymers, and the discontinuous one constituted by the starch granules. In addition, in the cell walls of this sample it was possible to observe some places of dry and coarse appearance. In the presence of HPMC (Fig. 1e–h), the constituents of the crumb seemed to be intimately linked due to the presence of an agglutinating material that could be the HPMC. This was even more evident in the samples that were not stored which appear to be covered by a veil-like film. In all the samples containing HPMC, the starch granule structure was better defined. It should be stressed out that in the presence of HPMC the frozen samples without and with frozen storage (Fig. 1g and h) had similar microstructure than their counterparts from positive temperature stored par-baked bread (Fig. 1e and f). These results indicate that positive temperature storage and freezing did not affect crumb microstructure. However, it is evident that frozen storage promoted changes in crumb microstructure. It is possible that the growth of the ice crystals during the frozen storage damages the structure of the crumb constituents, as has been observed in bread dough (Berglund, Shelton, & Freeman, 1991; Naito et al., 2004; Ribotta et al., 2004; Varriano-Marston, Hsu, & Mahdi, 1980). On the other hand, the appearance of the samples containing HPMC indicates that this hydrocolloid was integrated in the structure of the crumb and interacts with its constituents exerting a shielding effect. The formation of physical entanglements or real links between the HPMC and the gluten proteins or between the HPMC and the starch granules has been already suggested (Armero & Collar, 1998; Collar, Armero, & Martı´nez, 1998).
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Fig. 1. Cryo-SEM micrographs showing the effect of storage temperature and HPMC (3500·). (a,e) wheat bread from partially baked bread after positive temperature storage and full baking; (b,f) wheat bread from partially baked bread after 10 days of positive temperature storage and full baking; (c,g) wheat bread from partially baked bread after freezing, thawing and full baking; (d,h) wheat bread from partially baked bread after freezing, 42 days of frozen storage, thawing and full baking. Control: a–d; HPMC: e–h. The arrows show starch granules (G), cavities (C), protein network (N), and damaged zones (DZ).
3.2. Quality attributes of the fully baked bread obtained from partially baked bread stored at different temperatures and in the presence and absence of HPMC The quality attributes of the fully baked loaves obtained from positive temperature stored/frozen par-
baked bread in the presence and absence of HPMC is summarised in Table 1. In general, the loaves from frozen par-baked bread had lower specific volume than those from positive temperature stored par-baked bread. This could be a consequence of the damage of the protein network that was promoted by the growth of the
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Table 1 Quality attributes of wheat bread from partially baked bread stored at both subzero (frozen) and positive temperatures after full baking Positive temperature stored Specific volume (cm3/g) Width/height ratio Moisture content (%) Hardness (g) Number of cells Mean cell area (mm2) Cell to total area ratio
Frozen
Control
HPMC
Control
HPMC
3.98 ± 0.07a,b 1.48 ± 0.01a 34.39 ± 0.02a 268.0 ± 23.4b 618 ± 30a 0.80 ± 0.04a 0.31 ± 0.02a
4.33 ± 0.07c 1.57 ± 0.03a 35.59 ± 0.03c 202.4 ± 24.2a 646 ± 32a 0.68 ± 0.03a 0.27 ± 0.01a
3.86 ± 0.14a 1.59 ± 0.03a 35.32 ± 0.03b 422.3 ± 23.4c 667 ± 33a 0.54 ± 0.02a 0.27 ± 0.02a
4.06 ± 0.06b 1.51 ± 0.03a 37.09 ± 0.03d 283.7 ± 13.8b 607 ± 30a 0.75 ± 0.04a 0.29 ± 0.02a
Positive temperature storage: 10 days; frozen storage: 42 days. Means within rows followed by the same letter were not significantly different (P < 0.05).
ice crystals during frozen storage. Damaged protein would not be strong enough to hold the crumb bringing about a reduction of the bread volume, as has been observed in the bread obtained from frozen dough (Bhattacharya, Langstaff, & Berzonsky, 2003; Havet, Mankai, & Le Bail, 2000; Ribotta et al., 2004; Seguchi, Nikaidoo, & Morimoto, 2003; Sharadanant & Khan, 2003). The presence of HPMC produced a significant increase in the specific volume of loaves from frozen and positive temperature stored par-baked bread. The improving effect of the HPMC on the volume of bread obtained by conventional process has already been reported (Armero & Collar, 1996; Guarda et al., 2004; Rosell, Rojas, et al., 2001). This result could be attributed to the ability of the hydrocolloid chains to release water molecules associated to them in a hydrated system such as a bread dough, and the subsequent interaction among them, leading to a polymeric network when are exposed to high temperatures (Bell, 1990; Dziezak, 1991; Haque, Richardson, Morris, Gidley, & Caswell, 1993; Sarkar & Walker, 1995). The HPMC network shields the bread dough from the gas release during baking, allowing a better development of the loaves and in consequence, great volume. The water content of the loaves from positive temperature stored par-baked bread (Table 1) was significantly (P < 0.05) lower than that of those from their frozen counterparts, likely due to differences in the duration of the final baking or to condensations during thawing of frozen samples. Concerning the effect of the HPMC on the moisture content, the baked products containing HPMC had significantly (P < 0.05) higher moisture content than those in absence of HPMC. The crumb hardness (Table 1) was significantly (P < 0.05) higher in the samples from frozen par-baked bread than in those from their positive temperature stored counterpart. Considering the amylopectin retrogradation as the main responsible of the crumb hardening (Gray & Bemiller, 2003) and that the optimum temperature for its retrogradation ranges from 3 to 5 C (Slade & Levine, 1987), it should be expected higher
crumb hardness in the sample from positive temperature stored par-baked bread. However, it should be taken into account that the par-baked bread requires a second baking to obtain bread. Melting of the already retrograded amylopectin is occurring during this second baking (Leuschner et al., 1997). Therefore, the amylopectin retrogradation that is expected during the storage of par-baked bread at positive temperatures is annulled during the second baking. The high crumb hardness observed on the loaves from frozen samples probably results from the mechanical damage of the crumb structure due to the mechanical damage provoked by crystals during freezing plus recoarsening during the frozen storage. This explanation is based on the Kou and Chinachoti (1991) findings that described a relationship between the mechanical damage of the crumb structure and its hardness. Those authors also described that the crumb structure undergoes an irreversible damage when compressed during a prolonged period, and there is a positive relationship between the degree of compression and the crumb hardness. Conversely, the growth of the ice crystals during the storage at frozen temperatures could promote the rupture of the starch granules, with the subsequent leaching of intragranular amylose, favouring in consequence the formation of complexes between intra and intergranular amylose that are responsible of the crumb hardness in the immediate hours after baking (Hug-Iten, Escher, & Conde-Petit, 2003). Other plausible explanation for the higher hardness of the samples from frozen par-baked bread could be their lower specific volume, since there is an inverse relationship between the hardness and volume (Armero & Collar, 1996). The presence of HPMC produced a significant reduction of the crumb hardness; this phenomenon was more obvious in the bread from frozen par-baked bread, which had similar hardness than the samples from positive temperature stored control. These results support the hypothesis described in the microstructure observations, in that the HPMC could protect the crumb constituents against the ice crystals damage. The hydrocolloids have been widely used in the ice cream
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3.3. Sensory quality of the fully baked bread obtained from partially baked bread stored at different temperatures and in the presence and absence of HPMC The sensory analysis of the fully baked breads from positive temperature stored and frozen par-baked bread in the absence and presence of HPMC is shown in Table 2. There were no significant (P < 0.05) differTable 2 Sensory quality of wheat bread from partially baked bread stored at both subzero (frozen) and positive temperatures after full baking Appearance
Taste
Texture
Positive temperature stored Control 7.1 ± 1.3a 6.7 ± 1.2a HPMC 6.8 ± 1.3a 6.6 ± 1.3a
5.8 ± 1.9b 6.0 ± 1.5b
5.6 ± 1.9b 6.2 ± 1.5b
Frozen Control HPMC
6.8 ± 1.2a 6.2 ± 1.6a,b
7.1 ± 1.3a 6.9 ± 1.3a
6.9 ± 1.2a 7.0 ± 1.5a
Aroma
7.0 ± 1.1a 6.4 ± 1.1a
Positive temperature storage: 10 days; frozen storage: 42 days. Means within columns followed by the same letter were not significantly different (P < 0.05).
ences regarding appearance and aroma. In general, the taste was better in the samples from frozen par-baked bread than that in those from positive temperature stored par-baked bread, but in the presence of HPMC this preference disappeared. Concerning the texture, loaves from frozen samples were better scored and no differences were detected in the presence of HPMC. The microstructure results revealed the damage of crumb constituents in the bread from frozen storage in absence of HPMC, so that a high score in texture was not expected. A possible explanation could be that damaged crumb was appreciated as a less elastic crumb, which could be pleasant to the judges whose perceived a wide variety of attributes associated to texture. In fact, likely the higher score of the bread from frozen samples concerning taste could be related to the better texture perceived by the panel. The presence of HPMC did not modify the sensory quality of the loaves, although it has been described that HPMC addition improved the sensory attributes of the typical Chilean bread (Guarda et al., 2004). 3.4. Staling of the fully baked bread obtained from partially baked bread stored at different temperatures and in the presence and absence of HPMC The staling of those loaves was followed by measuring the crumb hardness increase during the 24-h storage at 25 C (Fig. 2). The loaves from frozen par-baked bread showed higher hardness increase than those from positive temperature stored par-baked bread. This rapid staling could be also attributed to the damage produced by the growth of the ice crystals on the crumb structure. The possible rupture of both the starch granules and the protein network that hold the crumb, could promote the diverse phenomena responsible of the crumb hardening, such as the formation of linkages between gluten
Control c
450
Hardness increase (g)
production for delaying or reducing the growth of ice crystals during storage. Caldwell, Goff, and Stanley (1992) demonstrated using scanning electron microscopy that ice creams containing hydrocolloids had smaller ice crystals than the control, before and after the storage at fluctuating temperatures. It has been proposed that the hydrocolloids modify the kinetic properties of the non-frozen phase, because their concentration in the liquid phase surrounded the ice crystals limits the water diffusion from the small to the big crystals. In consequence, the hydrocolloids limit both the Ostwald ripening and the recrystallization phenomena (Goff, 1992), which could support that the HPMC also protects the crumb structure during freezing and frozen storage. In addition, the low crumb hardness of the products containing HPMC can be attributed to their high moisture content, since several studies reported the inverse relationship between the moisture content and the hardness (Fik & Surowka, 2002; He & Hoseney, 1990; Hug-Iten et al., 2003; Rogers, Zeleznak, Lai, & Hoseney, 1988). On the other hand, the microstructure analysis could support a possible interaction between HPMC and the bread constituents, namely amylose, which interferes with the hardening process during the first hours after baking. The lower hardness of the bread containing HPMC agrees with previous results obtained with conventional breadmaking process (Armero & Collar, 1996; Guarda et al., 2004; Rosell, Rojas, et al., 2001). Concerning, other quality attributes of the bread, such as width/height ratio and the grain structure of the crumb, there were no significant differences among the samples tested (Table 1).
97
400 350 300 250
Control a,b
HPMC b HPMC a
200 150 100 50 0 Positive temperature storage
Subzero temperature storage
Fig. 2. Hardness increase during ageing of wheat bread from partially baked bread stored at subzero (frozen) or positive temperatures. Frozen storage: 42 days; positive temperature storage: 10 days. Ageing conditions: 24 h at 25 C.
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proteins and starch (Martin, Zeleznak, & Hoseney, 1991), the water redistribution between crumb and crust (Piazza & Masi, 1995) and among the polymers present in the crumb (Chen, Long, Ruan, & Labuza, 1997), and the amylopectin recrystallization (Schoch & French, 1947; Xie, Dowell, & Sun, 2004). The addition of HPMC reduced the crumb hardening of both samples (positive temperature stored and frozen) during staling. Different mechanisms could be responsible for this antistaling effect, such as the higher moisture content present in the samples containing the polymer, and the possible interaction of the HPMC with the polysaccharides, proteins and other bread constituents that hinder the formation of complexes between them. In the loaves from frozen par-baked bread, the HPMC effect could be also due to the previously described effect of the HPMC limiting the diffusion of water molecules and in consequence the growth of the ice crystals, which in turn decreases the damage on the crumb structure. The antistaling effect of the HPMC has been already reported in the bread obtained by conventional breadmaking process (Armero & Collar, 1998; Guarda et al., 2004). Finally, it should be stressed out that the bread is a very complex system, even when obtained from a basic formulation, for which the staling mechanisms are not completely understood. Therefore, the addition of positive temperature storage and frozen storage and the presence of bread improvers as HPMC increase the complexity of the problem.
4. Conclusions The addition of HPMC to the par-baked bread recipe resulted in baked products of better quality and with low hardening rate of the crumb, without affecting the sensory attributes. The HPMC had a protective effect against the damage promoted by freezing and frozen storage, which was supported with the scanning electron microscopy analysis of the crumb structure. The micrographs revealed an intimate interaction between the HPMC chains and the constituents of the bread crumb. The positive temperature storage seems to be a good alternative to the frozen storage, in fact in this case, parbaked bread led to better specific volume, softer crumbs and lower hardening rate in comparison with the bread from frozen par-baked. However, it would be necessary to perform some additional studies on the sensory quality and the microbiology safety in long storage periods.
Acknowledgments This work was financially supported by Ministerio de Educacio´n y Ciencia Projects (AGL2001-1273,
AGL2002-4093) and Consejo Superior de Investigaciones Cientı´ficas (CSIC), Spain. M.E. Ba´rcenas would like to thank her grant from Universidad de las Ame´ricas, Puebla, Me´xico. Authors also thank Dr. I. Perez-Munuera for her technical assistance in the microstructure analysis.
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