Staling of frozen partly and fully baked breads. Study of the combined effect of amylopectin recrystallization and water content on bread firmness

Partly baked (PB) and fully baked (FB) breads were frozen at −18 °C for 7, 21, 63, 92, 126 and 188 d and were analysed after its thawing (FB) or thawing and final baking (PB). The starch retrogradation, the moisture content and the firmness were measured as properties closely related to the aging of bread. The temperature of glass transition of the maximally freeze-concentrated state, T_g′, was also measured and established in (−18 ± 0.8) °C. This value cannot ensure molecular immobility in both types of bread during its frozen storage at (−18 ± 2) °C. Consequently, the rearrangements of starch component molecules, needed for its recrystallization and for the diffusion of water during frozen storage, could take place and could justify the changes observed in the bread. PB bread showed a significant decrease in firmness with frozen storage, while the firmness of the FB bread did not change significantly, although an increase when compared with the control, not frozen bread, was detected. A regression study led to the conclusion that the combined effect of starch component crystallization and water loss could explain the firming evolution and that both variables exerted an effect of similar intensity on crumb firmness.     

Impact of the chilling conditions on the kinetics of staling of bread

Breads were chilled at room temperature or using a vacuum chilling process. The staling rate was studied after partial baking and after the final baking. Vacuum chilled breads (VCBs) exhibited a higher moisture loss than conventionally chilled bread (CCB). A higher enthalpy of melting of amylopectin crystals was observed for VCB in comparison to CCB.VCB had a negative effect on the texture of the bread. The hardness of the crumb of VCB was increased by 20% at the end of staling and the kinetics of staling was 10% faster for VCB in comparison to CCB. After 12 days of storage at 4 °C, a second and final baking was applied to the bread. The staling rate of VCB was almost two times that of CCB. The staling rate of CCB was unchanged in comparison to the staling rate after partial baking. Data on the enthalpy of melting of amylopectin showed that the same amount of amylopectin was formed at the end of staling for CCB after part baked process and after full baking process. Beside, the VCB showed a higher amount of recrystallized amylopectin (10%) at the end of the staling which followed the partial baking.     

Staling of fresh and frozen gluten-free bread

The difficulty in finding gluten-free bread and its high price make it necessary to prolong its shelf life to facilitate its availability. Freezing is an interesting alternative. The storage of bread at over zero temperatures, 20 °C and 4 °C, showed faster staling at refrigerator temperatures. A good relationship between crumb firmness and the extent of starch recrystallization was obtained, although the effect of water loss was also detected. The study of freezing and frozen storage at −14 °C and −28 °C for 7 days showed a substantial effect of the storage temperature on gluten-free bread quality and shelf life. Breads stored at −28 °C retained a quality similar to that of fresh breads while a marked deterioration of the breads stored at −14 °C was observed. This effect, the strongest on bread texture, was a result of starch recrystallization. The glass transition, T_g’ and onset of ice melting, T_m’ of the maximally freeze-concentrated bread crumb were −37.1 ± 0.6 °C and −19.3 ± 0.2 °C respectively. The higher amount of unfrozen water at −14 °C could explain the acceleration of reactions responsible for bread staling during frozen storage. The use of storage temperatures below T_m’ is recommended to retain high quality of the gluten-free bread during frozen storage.     

Impact of the baking kinetics on staling rate and mechanical properties of bread crumb and degassed bread crumb

This paper presents a study on the impact of baking conditions on crumb staling. Breads were baked at 220 °C, 200 °C and 180 °C corresponding to 6, 8 and 10 min to rise the temperature to 98 °C in the crumb (heating rates 13, 9.8 and 7.8 °C/min respectively with an initial temperature of 20 °C). A new protocol has been developed, consisting in baking a slab of degassed dough in a miniaturized oven to mimic the baking conditions of conventional bread making. Texture tests were done during staling on degassed crumb and on conventional crumb. Calorimetry tests showed that during storage, amylopectin recrystallisation occurred before crumb stiffening. A first order kinetics model was used to fit the evolution of the crumb texture (Young's modulus) and of the recrystallisation of amylopectin. The results showed that the hardening of the crumb during staling occurred after retrogradation of amylopectin. In addition, the staling rate was faster for faster baking kinetics. A mechanical model showed that the relative Young modulus is proportional to the square of the relative density of the crumb.     

Crystallinity changes in wheat starch during the bread-making process: Starch crystallinity in the bread crust

The crystallinity of starch in crispy bread crust was quantified using several different techniques. Confocal scanning laser microscopy (CSLM) demonstrated the presence of granular starch in the crust and remnants of granules when moving towards the crumb. Differential scanning calorimetry (DSC) showed an endothermic transition at 70 °C associated with the melting of crystalline amylopectin. The relative starch crystallinity, as determined by X-ray and DSC, from different types of breads was found to lie between 36% and 41% (X-ray) and between 32% and 43% (DSC) for fresh bread crust. Storage of breads in a closed box (22 °C) for up to 20 days showed an increase in crust crystallinity due to amylopectin retrogradation both by X-ray and DSC. However, DSC thermograms of 1-day old bread crust showed no amylopectin retrogradation and after 2 days storage, amylopectin retrogradation in the crust was hardly detectable. ¹³C CP MAS NMR was used to characterize the physical state of starch in flour and bread crumb and crust. The intensity of the peaks showed a dependence on the degree of starch gelatinization.     

Impact of different baking processes on bread firmness and starch properties in breadcrumb

The influence the quality and shelf life of baked product has previously been reported to be effected by the time and temperature of the baking process. In this study, dough was baked at 219 °C by using different ovens (conventional, impingement or hybrid) or with doughs of different sizes (large or small) for varying times. During baking the temperature profile at the dough center was recorded. Texture, thermal properties and pasting characteristics of baked product with reference to baking conditions were investigated. Small breads baked in the hybrid oven had the highest heating rate (25.1 °C/min) while large breads baked in conventional oven had the lowest heating rate (6.0 °C/min). When the data are viewed as a function of heating rate in this study, the enthalpy of amylopectin recrystallization, rate of bread firmness and the amount of soluble amylose were all-lower at the slower heating rate. The differences observed in product firmness following storage are potentially a consequence of the extent of starch granule hydration, swelling, dispersion and extent of reassociation; all of which are affected by the heating rate during baking.     

Wheat Bread Quality as Influenced by the Substitution of Waxy and Regular Barley Flours in Their Native and Cooked Forms

The substitution of wheat flour with barley flours altered the bread loaf volume, colour and bread crumb firmness. These changes were found to be dependent on the barley cultivar, substitution level and flour treatment. In native form, Phoenix barley flour at 15% substitution produced breads with bigger loaf volume and softer crumb than Candle barley flour. However, when the barley flours were heat-treated (pan-cooked in excess water and then dried) before substitution, Candle barley flour produced better quality breads in terms of loaf volume, crumb firmness and crust colour than the Phoenix counterparts. The baking functionality of Candle flour was markedly improved when added after heat treatment.     

Impact of long-term frozen storage on the dynamics of water and ice in wheat bread

Frozen storage of bread has a substantial impact on the dynamics of water and ice in the crumb and crust. In this study, the impact was characterized using wheat bread stored at −18 °C for a long term of ∼4 months. The frozen bread incurred a considerable loss of the crumb water that migrated out and formed ice crystals on the bread surface. Such a moisture decrease underwent more rapidly for the bread stored without intact crust, suggesting the specific role of crust during frozen storage. Moisture also redistributed significantly within the frozen crumb, resulting in an elevated crumb heterogeneity of freezable water. This redistribution of freezable water was accompanied by a progressive recrystallization of the crumb-borne ice crystals, which were measured to grow into bulk sizes using a modified calorimetric procedure for analyzing the crumb samples at their as-frozen states.     

Quality and microbial characteristics of part-baked Sangak bread packaged in modified atmosphere during storage

The effect of part-baking and modified atmosphere packaging (MAP) on quality and microbial stability of part-baked flat bread (Sangak) was investigated. Part-baked Sangak bread was packaged in 100% CO₂, 50% CO₂:50% N₂, 25% CO₂:75% N₂ and air as a control and stored at 25 °C for 21 days. Color, firmness, moisture, density and microbiological analyses of the package breads were carried out at 3-day intervals during storage. No significant effect of MAP was found on moisture, color and texture during the storage of part-baked bread, while the total aerobic plate count (APC) and the mold and yeast count (M + Y) were dependent on the concentration of CO₂ in the package headspace. With increasing concentration of CO₂, microbial counts were decreased. Density of part-baked bread was also somewhat influenced by the MAP. The observed reduction in the density of control samples during storage was less than those in MAP. The results of the present study demonstrated that it is possible to prolong the shelf-life of Sangak bread from a few days to about 21 days by using bake-off technology (BOT) and MAP under high CO₂ concentration.     

Effect of the thermostable xylanase B (XynB) from Thermotoga maritima on the quality of frozen partially baked bread

The effect of the recombinantly produced xylanase B (XynB) from Thermotoga maritima MSB8 on the quality of frozen partially baked bread (FPBB) was investigated. Addition of XynB to wheat flour dough resulted in a significant increase in dough extensibility (L), swelling (G), and a decrease in dough resistance to deformation (P), configuration. Bread crumb characteristics were studied by differential scanning calorimeter (DSC) and dynamic-mechanical analysis (DMA). The results show that addition of XynB leads to improvements in the bread quality of FPBB and retards bread staling compared to the control. The greatest improvements were obtained in specific volume (+35.2%) and crumb firmness (−40.0%). The control FPBB was significantly firmer in texture and higher in amylopectin recrystallization than the bread with XynB. During frozen storage of FPBB with and without XynB for 8 weeks, the crumb firmness increased gradually and the specific volume slightly decreased with the frozen storage time. The ΔH values of freezable water (FW) endothermic transitions increased with frozen storage time for all samples. However, addition of XynB lowered the ΔH values indicating a decrease in FW. Therefore, XynB is useful in improving the quality of FPBB. DMA was also used to monitor the shrinking behavior of the samples. Addition of XynB increased the contraction during chilling but significantly diminished the total shrinking and frozen-state shrinking of the bread crumb during the freezing process.     

Volume change of bread and bread crumb during cooling,chilling and freezing,and the impact of baking

During baking, bread dough undergoes an expansion followed by a slight contraction at the end of baking. The contraction during baking has been evidenced by some authors. However, there is a limited amount of literature about the contraction of the crumb during the chilling phase and also during the freezing phase in the case of freezing. A study has been carried out to better understand the impact of the baking degree on the contraction of the crumb during chilling after baking and during freezing. The volume of the samples has been evaluated with a laser volumeter. Breads (70 g dough) were baked until reaching 75 °C, 85 °C, 95 °C, 98 °C and then 98 °C for 10 min. Results showed that a longer baking resulted in a lower contraction of the bread. The volume change was between 25% and 2.5% for baking at 75 °C—0 min dwell and 98 °C—10 min dwell, respectively. The contraction was compared to the contraction of degassed bread crumb samples, which was more important. SEM pictures showed that the degree of baking also corresponded to a very different structure of the crumb. For the longer baking, the starch granules were fully gelatinized and no ghosts of starch granules were visible. The magnitude of the contraction was thus associated with the degree of baking and with the degree of starch granule destructuration.     

Improvement to baking quality of frozen bread dough by novel zein-based ice nucleation films

Freezing deteriorates the baking quality of frozen bread dough. This study revealed the protective effects of zein-based ice nucleation films (INFs) on the baking quality of frozen dough. INFs were prepared by immobilizing biogenic ice nucleators on the surface of zein films, which consequently revealed ice nucleation activity and increased the ice nucleation temperature of water from −15 °C to −6.7 °C. By using these films to wrap frozen dough during five freeze/thaw cycles, the specific volume of bread was increased by up to 25% compared to the bread from control frozen dough. The reason was attributed to 40% more viable yeast cells preserved by INFs. In addition, zein-based INFs also reduced the water loss by frozen dough resulting in higher water content in bread crumb. Combining the protective effects on both specific volume and water content from zein-based INFs, the obtained bread showed 68% lower firmness and fracturability and 2.4 times higher resilience compared to the control. The INFs were also superior in that for zein-based INFs, biogenic ice nucleators showed desirable affinity with the surface to sustain at least fifteen repetitive uses on freezing water.     

Effect of physicochemical and empirical rheological wheat flour properties on quality parameters of bread made from pre-fermented frozen dough

The objective of this study was to examine the influence of flour quality on the properties of bread made from pre-fermented frozen dough. The physicochemical parameters of 8 different wheat flours were determined, especially the protein quality was analysed in detail by a RP-HPLC procedure. A standardized baking experiment was performed with frozen storage periods from 1 to 168 days. Baked bread was characterised for specific loaf volume, crumb firmness and crumb elasticity. The results were compared to none frozen control breads. Duration of frozen storage significantly affected specific loaf volume and crumb firmness. The reduction of specific loaf volume was different among the used flours and its behaviour and intensity was highly influenced by flour properties. For control breads wet gluten, flourgraph E7 maximum resistance and RVA peak viscosity were positively correlated with specific loaf volume. However, after 1–28 days of frozen storage, wet gluten content was not significantly influencing specific loaf volume, while other parameters were still significantly correlated with the final bread properties. After 168 days of frozen storage all breads showed low volume and high crumb firmness, thus no significant correlations between flour properties and bread quality were found. Findings suggest that flours with strong gluten networks, which show high resistance to extension, are most suitable for frozen dough production. Furthermore, starch pasting characteristics were also affecting bread quality in pre-fermented frozen dough.     

Volume, texture, and molecular mechanism behind the collapse of bread made with different levels of hard waxy wheat flours

Physico-chemical properties of bread baked by partially replacing normal wheat (Triticum aestivum L.) flour (15, 30, and 45%) with two hard waxy wheat flours were investigated. Substitution with waxy wheat flour resulted in higher loaf volume and softer loaves. However, substitution at >30% resulted in excessive post-bake shrinkage and a ‘key-hole’ shape with an open crumb structure. Bread crumb microstructure indicated a loss of starch granule rigidity and fusing of starch granules. The cells in the interior of the bread did not become gas-continuous and as a result, shrunk as the loaf cooled. Soluble starch content was significantly higher in bread crumb containing waxy wheat flour than in control bread. Debranching studies indicated that the soluble starch in bread made with 30-45% hard waxy wheat flour was mostly amylopectin. Incorporation of waxy wheat flour resulted in softer bread immediately after baking but did not retard staling upon storage.     

Permeability of crust is key to crispness retention

Bread loses crispness rapidly after baking because water originating from the wet crumb accumulates in the dry crust. This water accumulation might be increased by the dense and low permeable character of the bread crust. Our objective was to investigate the influence of permeability of the crust on water uptake in the crust and on crispness retention. To achieve this objective, we increased the permeability of the control bread crust by creating small channels through the crust. The water vapour permeability of the crust with and without channels was measured using a newly developed method for brittle materials. Two further properties were measured over time: crispness of bread by analysing acoustic properties and water content of the crust. Control bread crust had low water vapour permeability and functioned as a barrier, leading to increased uptake of water in the crust. Water uptake was halved, however, if the water vapour permeability of the crust was doubled. As a consequence, crispness retention increased eight-fold; breads stored for four hours were as crispy as control breads stored for 30 min. We can conclude, therefore, that permeability of crust is key to crispness retention.     

Factors influencing acrylamide formation in rye,wheat and spelt breads

In this study, a simple strategy for acrylamide (AA) reduction in white and dark wheat and spelt and rye breads, including the impact of flour basic composition, flour extraction rate, type of technology and baking time and temperature was addressed. Moreover, the correlation between AA formation in breads and total phenolic compounds (TPC) and antioxidant capacity (AC) of flours and breads was calculated. The studies showed an impact of flours origin on AA formation in breads with the following rank: wheat bread ≤ spelt bread < rye bread. There was no statistically significant effect of flour extraction rates and their chemical components on AA formation in breads baked at 200 °C/35 min. However, a weak effect was noted for wheat and spelt breads baked at higher temperature. In contrast, a positive correlation between AA in wheat, spelt and rye breads baked at both applied conditions (200 °C for 35 min or 240 °C for 30 min) and AC of white and dark flours was noted. The same finding was noted between AA formation and TPC and AC of bread and its crust. The provided data indicated that AA mitigation strategy should be based on the selection of lower baking temperature with longer baking time as the main important factor amongst others.     

Combining local pressure and temperature measurements during bread baking: insights into crust properties and alveolar structure of crumb

Improvements in both the miniaturisation and heat compensation of pressure transducers made it possible to measure pressures as low as 5 kPa inside bread dough during baking (ΔT = 80 °C). Additional calibration was found to be necessary to decrease it below 0.18 kPa according to the variations in temperature encountered during baking. Two probes with both a thermocouple and a miniature pressure transducer were used to reveal pressure gradients inside bread dough during baking and post-cooling. During baking, increase in pressure (up to 1.1 kPa) was mainly attributed to the mechanical restrictions exerted on the dough by the stiffened surface layers. Pressure build-up due to the stiffening of bubble walls could not be detected. Various effects of the rupture in the bubble walls are reported. Sudden falls in pressure observed up to 70 °C were attributed to the bubble coalescence phenomenon. Evidence of an open porous structure was provided by the balance in pressure through the dough before the end of baking and the almost simultaneous lowering of pressure (−0.15 kPa) throughout the crumb during cooling. The slight lowering of pressure during post-cooling was also evidence of lower permeability of the crust compared to the crumb.     

Effects of pressure treatment of hydrated oat,finger millet and sorghum flours on the quality and nutritional properties of composite wheat breads

Breadmaking achievement using grains alternative to wheat and rye is a challenging task for cereal technologists, since most of the available innovative breads are characterised by poor crumb and crust characteristics, slight flavour and fast staling. To improve texture, mouth-feel, acceptability and shelf-life of breads prepared by using minor and/or under-utilised cereals, gluten and/or polymeric substances that mimic the viscoelastic properties of gluten, are required. Recent studies reported that high hydrostatic pressure (HP) treatment may represent an efficient non-thermal technique to promote the dough structure formation of composite cereal matrices. In the present study the effects of HP on the techno-functional and nutritional properties of oat-, millet-, and sorghum- based breads were evaluated compared to their unpressured- and gluten-added conventionally made counterparts. HP-treated (350 MPa, 10 min) wheat, oat, millet and sorghum batters were added to the bread recipe, replacing 50%, 60% and 40% of untreated wheat flour, respectively. Data from bread analyses revealed non significant physico-chemical impairment, and superior nutritional and sensory profiles in most quality features when HP treatment was applied to dough batters, compared with conventional/gluten-added samples. Specifically, HP breads deserved better sensory scores and exhibited higher antiradical activities despite a reduction in specific volume (wheat and oat) and faster staling kinetics (millet and sorghum) that were explicit in some composite samples.     

Influence of formulation and process on the aromatic profile and physical characteristics of bread

This work consisted of two complementary sets of experiments in which breads differing in their recipe and/or process were characterised according to their odorant perception, volatile compound composition and physical properties. The results revealed that commercial partially baked and/or partially baked frozen breads were characterised by a different odour compared to commercial conventional, fully baked frozen and frozen dough breads, which were perceived similarly. These differences were explained by their variable aromatic composition. By analysing breads based on the same recipe but from different processes, it was demonstrated that adding a freezing stage before dough proofing or at the end of the conventional process, as well as after partial baking, did not influence bread aroma. Likewise, partial baking had no effect on bread odour and aromatic profile. Thus, the aromatic differences between commercial conventional, fully baked frozen and frozen dough breads on the one hand, and commercial partially baked breads on the other hand, were due to their different formulations. Concerning bread physical properties, the recipe also influenced bread crust/crumb ratio and density. Moreover, adding a partial baking stage to the process led to breads with a more compact crumb.     

The effect of chitosan oligosaccharides on bread staling

The aim of this work was to investigate the effects of chitosan oligosaccharides and chitosan on the rate of staling and properties of bread crumb and crust. Rates of crumb firming varied with storage time. The possible mechanisms including prevention of amylose–lipid complexation, acceleration of dehydration from both starch and gluten, adsorption of chitosan onto the starch surface and increase of moisture migration rate from crumb to crust are proposed and analysed. Chitosan oligosaccharides and low molecular weight chitosan increase bread crumb staling rate to a much lesser extent than does middle molecular weight chitosan.