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.     

MRI evaluation of local expansion in bread crumb during baking

The aim of this study was to undertake a dynamic and quantitative evaluation of the local evolution of porosity in a section of bread using magnetic resonance imaging (MRI). A prototype of an oven compatible with continuous MRI acquisition had been previously developed and installed in the probe of the imager. Oil microcapsules, which provide a very different MRI signal to that obtained from the dough, were also incorporated in the dough before proving, and their position in the bread was monitored throughout baking. Microcapsules delimited dough layers of which initial thickness ranged from 7 to 20 mm. Monitoring of the expansion of these layers is presented and discussed.Local expansion of the lower part of the bread predominated, contributing to 82% of the overall expansion at an oven air temperature of 130 °C. This was related to a slight escape of gas in this region, favored by one-dimensional heat transport. When the crust was formed early, as for an oven air temperature of 182 °C, local expansion did not cease with the crust setting and occurred to the detriment of the other regions, which were compressed.     

Dough/crumb transition during French bread baking

The modifications occurring during dough to crumb (D/C) transition of French bread (350 g) were studied in an instrumented pilot-scale oven for doughs with different contents of minor components, soluble, lipids and puroindolines. Internal temperature measurements showed that, for most compositions, complete D/C transition occurred between 55 and 70 °C, after 5 min of baking, and coincided with maximum loaf expansion. Differential scanning calorimetry (DSC) in excess of water performed on samples taken during baking (3 and 5 min) showed that starch gelatinization and melting developed continuously during D/C transition for various contents of the soluble fraction in dough. Dynamic thermomechanical analysis (DMA) on dough showed that dough stiffened between 60 and 70 °C, as seen by the increase of elastic modulus E′ by more than one decade, for all dough compositions. Relating these changes to the results of baking experiments, D/C transition was assigned first to gluten reticulation and, to a lesser extent, to continuous starch granule swelling.     

Formation and distribution of ice upon freezing of different formulations of wheat bread

The formation and distribution of ice upon the freezing of fresh breadcrumb were investigated using differential scanning calorimetry. Three types of wheat bread containing different amounts of sugar and dietary fiber were measured. Various frozen states were produced through freezing with different cooling rates (0.5, 1, 2, 5, 10, 20 and 30 °C/min) to −30 °C; they were then analyzed and compared by thawing with the same heating rate (10 °C/min) to 20 °C. All DSC heating traces exhibited dual endotherms in the temperature range for the melting of ice: The major transition was attributed to the ice formed in the large crumb pores (gas cells) and the minor event, which preceded the major endotherm, was assigned primarily to the ice formed in the nanometer-sized pores within the gluten-starch matrix. The size of ice crystals in the two classes of pores was estimated using the modified Gibbs–Thompson relation. The distributions of ice in these pores depended on the bread compositions. It is concluded that the complex crumb porosity plays an essential role in shaping the activities of water and ice in the breadcrumb.     

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.     

Fate of ACE-inhibitory peptides during the bread-making process: Quantification of peptides in sourdough,bread crumb,steamed bread and soda crackers

This study compared the concentration of angiotensin-converting enzyme (ACE) inhibitory peptides at different stages of the bread-making process, including kneading, proofing, and final products. Steamed bread, baked bread, and soda crackers were produced with 3–20% addition of rye malt sourdoughs to assess products differing in their thermal treatment. Eight tripeptides with known or predicted ACE-inhibitory activity were quantified by LC/MS in multiple reaction monitoring (MRM) mode. In wheat sourdough and rye-malt gluten sourdough, IPP was the predominant tripeptide at 58 and 473 μmol kg⁻¹, respectively, followed by LQP, IQP, and LPP. During the bread-making process, peptide concentrations were modified by enzymatic conversions at the dough stage and by thermal reactions during baking. The concentrations of IPP, LPP and VPP remained stable during dough preparation but decreased during thermal treatment; the concentrations of other peptides were changed at the dough stage but remained relatively stable during baking. The cumulative concentration of 8 ACE-inhibitory peptides in steamed bread and bread crumb exceeded 60 μmol kg⁻¹, while soda crackers contained less than 3 μmol kg⁻¹. The peptide levels in bread thus likely meet in vivo active concentrations.     

Lipids in bread making: Sources, interactions, and impact on bread quality

Lipids exhibit important functional properties in bread making, although they are present in lower levels than starch or protein. They originate from flour, in which they are endogenously present, or from added shortening and/or surfactants. This review discusses lipid sources and their interactions during the entire process of bread making from dough mixing to fermentation, proofing, baking and the stored product. The focus is on lipid interactions with starch and gluten proteins, their role in gas cell stabilisation and their impact on bread loaf volume, crumb structure and crumb firming. Widely accepted views on lipid functionality, although often opposing, are presented and critically discussed.     

Experimental data and modeling of the thermodynamic properties of bread dough at refrigeration and freezing temperatures

Thermodynamic properties of bread dough (fusion enthalpy, apparent specific heat, initial freezing point and unfreezable water) were measured at temperatures from −40 °C to 35 °C using differential scanning calorimetry. The initial freezing point was also calculated based on the water activity of dough. The apparent specific heat varied as a function of temperature: specific heat in the freezing region varied from (1.7–23.1) J g⁻¹ °C⁻¹, and was constant at temperatures above freezing (2.7 J g⁻¹ °C⁻¹). Unfreezable water content varied from (0.174–0.182) g/g of total product. Values of heat capacity as a function of temperature were correlated using thermodynamic models. A modification for low-moisture foodstuffs (such as bread dough) was successfully applied to the experimental data.     

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.     

Kinetics of crust formation during conventional French bread baking

64 breads were baked in a deck oven under conventional French baking conditions. Their hearth and surface temperatures were measured at various baking times using thermocouples and Infra-red thermometer; their moisture content was determined after sampling of the external layer, termed the “crust”, and by weighing breads. In addition, an image analysis method, based on saturation and colors, allowed determination of a mask of crust and the kinetics of its mean thickness. Associated with hydrothermal kinetics, it showed that the development of crust was achieved at a final local temperature of 160 °C and moisture of 5% for an average final thickness of 1.7 mm. DSC analysis of crust samples also underlined the significance of non gelatinized starch in the crust. This result was interpreted by representing the hydrothermal history of crust in a starch melting diagram.     

Comparison of the impact of dextran and reuteran on the quality of wheat sourdough bread

This study investigates the influence of in situ exopolysaccharides (EPS) and organic acids on dough rheology and wheat bread quality. Dextran forming Weissella cibaria MG1 was compared to reuteran forming Lactobacillus reuteri VIP. For in situ production of EPS, sourdoughs were supplemented with 15% sucrose. Control sourdoughs were prepared with the same strain but without sucrose. W. cibaria MG1 and L. reuteri VIP formed 5.1 and 5.8 g kg⁻¹ dextran and reuteran, respectively. Formation of EPS from sucrose led to production of high amounts of acetate by L. reuteri VIP, but only small amounts were detected in W. cibaria MG1 sourdough. EPS containing sourdough or control sourdough was incorporated at 10% and 20% in wheat dough. EPS significantly influenced the rheological properties of the dough, with dextran exhibiting the strongest impact. The addition of dextran enriched W. cibaria MG1 sourdough significantly increased CO₂ production, whereas increased acidity in reuteran containing dough reduced gas production. The quality of wheat bread was enhanced when 10% of L. reuteri-sucrose sourdough was added. The positive effect of reuteran was masked by increased acidification after 20% sourdough addition. Incorporation of dextran enriched sourdough (10% and 20%) provided mildly acidic wheat bread with improved bread quality.     

Wheat (Triticum aestivum L.) puroindoline functionality in bread making and its impact on bread quality

Wheat puroindolines (PINs) spontaneously adsorb at air/water interfaces and show excellent foaming properties. They can positively impact bread quality, in which the formation of stable foam is important for product quality. The impact of endogenous PINs on bread quality was studied by preparing gluten–starch blends from isolated gluten and starch fractions with different PIN levels, which allowed largely retaining the interaction between PINs and flour components. Our results indicate that blends with high PIN levels yielded more homogeneous crumb structures with fine gas cells than bread made with blends containing medium or low PIN levels. However, the mechanism by which PINs exert this crumb improving effect is not clear. Varying PIN levels impacted neither dough extensibility nor did it result in different PIN levels in dough liquor. Lipid removal yielded bread with a less homogeneous crumb gas cell distribution, indicating that lipids also are required to obtain good crumb structure.     

Dough and bread made from high- and low-protein flours by vacuum mixing: Part 2. Yeast activity,dough proofing and bread quality

The combined effects of reduced mixer headspace pressure and mixing duration on the yeast activity, proofing and quality of dough and bread made from both high-protein flour (HPF) and low-protein flour (LPF) were addressed in this study. Rheofermentometer analysis showed that a reduction in mixer headspace pressure up to 0.08 MPa did not affect the overall gassing power of yeast in either of the two dough matrices. An increase in mixing duration sped up the mass transfer rate of CO₂ at the initial stage of fermentation, leading to a faster expansion of dough volume at the beginning. Moreover, an increase in mixing time promoted dough development and gas inclusion, which resulted in a increased volume of dough and bread, as well as a softer texture of both baked bread and steamed bread. In general, reduced headspace pressure produced baked bread of smaller volume, denser structure and harder texture. On the other hand, vacuum mixing produced steamed bread with softer texture without significantly changing the bread’s volume and porosity.     

Nutritional value of bread: Influence of processing,food interaction and consumer perception

The nearly ubiquitous consumption of cereals all over the world gives cereals an important position in international nutrition. Besides the high starch content as energy source, cereals provide dietary fibre, nutritious protein and lipids rich in essential fatty acids. Important micronutrients present in cereals are vitamins, especially many B vitamins, minerals, antioxidants and phytochemicals. In general, cereals provide important amounts of most nutrients. However, processing may decrease or increase the levels of the bioactive components in grains and also modify the bioavailability of these components. In addition, interactions between bread and companion foods have effects on the nutritional quality. The aim of this paper is to review the existing literature on the effects of processing techniques and interactions with other food components in a mixed meal on the nutritional quality of bread. Furthermore, research findings on the consumer perception of bread in Belgium are included. This information can help health professionals and policy-makers to give clear and targeted advice about the positive effects of bread in the human nutrition during consultations and information campaigns.     

Effect of barley and oat flour types and sourdoughs on dough rheology and bread quality of composite wheat bread

The potential of sourdough to improve bread quality of barley and oat enriched wheat breads may depend on the characteristics of the added flour (cereal type, variety, extraction rate). We compared the effect of different barley flours and oat bran (substitution level 40%), unfermented and as sourdoughs (20% of total flour), on composite wheat dough and bread characteristics by combining empirical rheological analyses (DoughLab, SMS/Kieffer Dough and Gluten Extensibility Rig) with small-scale baking of hearth loaves. Whole grain barley flour sourdough increased resistance to extension (Rmax) of the dough and improved the form ratio of hearth loaves compared to unfermented whole grain barley flour. However, sourdough showed little effect on the breads prepared with sifted barley flour or oat bran. The breads made with oat bran showed highest bread volume, lowest crumb firmness and highest β-glucan calcofluor weight average molecular weight (MW). The heat treatment of oat bran inactivated endogenous enzymes resulting in less β-glucan degradation. High MW β-glucans will increase the viscosity of the doughs water phase, which in turn may stabilise gas cells and may therefore be the reason for the higher bread volume of the oat bran breads observed in our study.     

Effect of the drying rate on the complex viscosity of wheat flour dough transforming into crust and crumb during baking

This study aimed at characterizing the effect of hydrothermal dynamics on the dough rheology, in order to develop a complete dough viscosity model valid at different locations during baking. The dough rheology was characterised using dynamic mechanical thermal analysis (DMTA). Temperature and water content (WC) were monitored during DMTA. At high heating rates (15–30°C/min), relevant to the top crust, viscosity behaved as if WC was kept constant, in spite of dehydration (37%); such similarity was valid up to 80°C (stage A). Beyond, the viscosity decrease observed in the samples at constant WC was replaced by a long-lasting plateau (stage B, 3–4 × 10⁶ Pa.s), attributed to WC reduction below ∼37%. Above the boiling water temperature, the logarithm of viscosity increased linearly with decreasing WC (stage C). At lower heating rates (5°C/min), relevant to the bottom crust, viscosity was two-fold higher than that at higher heating rates, suggesting lower oven-rise. The viscosity decrease, observed at high temperatures (>80°C) for samples at constant WC, was not observed if drying occurred late (case of crumb beneath the crust); instead, viscosity increased up to levels close to that of the top crust (2–3 × 10⁷ Pa.s at WC∼20%). Despite these deviations, viscosity as a WC function was modelled with a unique equation set.     

Studies on the baking properties of wheat: Pigeonpea flour blends

Pigeonpea flour was substituted at levels of 0, 5, 10, 15,20, 25% to wheat flour and whole wheat meal for bread andChapatti making, respectively. Blends were prepared up to50% for cookie making. Increasing levels of pigeonpeas inthe blends significantly increased the protein and mineralcontent of the baked products. The bread from 10%pigeonpea flour blend with 2–3% vital gluten and 0.5%SSL had high loaf volume and loaf quality. Blends containing 15% pigeonpea flour were acceptable for Chapatti and 30% pigeonpea flour with 0.25% SSL wereacceptable for cookie making.     

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.     

Effect of bran on bread staling: Physico-chemical characterization and molecular mobility

High fibre breads were produced with the addition of durum wheat bran fractions (regular bran and a fraction extracted from the most internal bran layer) and their physico-chemical properties and water status were characterised during storage. Bran enriched breads exhibited similar properties during storage, they were harder, less springy and less cohesive than the control. Water status was strongly affected by bran addition, independently of bran composition: water activity, moisture and frozen water content (measured by Differential Scanning Calorimetry) were generally higher in the bran samples than in the control bread during storage. Amylopectin retrogradation was significantly larger in the presence of bran fractions. ¹H NMR mobility (T₂ number of populations and relaxation times) was different in the high fibre breads as compared to the control sample. The changes in protons mobility observed upon storage indicated an influence of bran on water/gluten/starch molecular domains and their dynamics, that may have affected the development of the gluten network resulting in different textural properties.