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.     

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.     

Improved viscoelastic zein–starch doughs for leavened gluten-free breads: Their rheology and microstructure

Gluten-free bread was prepared from commercial zein (20 g), maize starch (80 g), water (75 g), saccharose, NaCl and dry yeast by mixing above zein's glass transition temperature (T_g) at 40°C. Addition of hydroxypropyl methylcellulose (HPMC, 2 g) significantly improved quality, and the resulting bread resembled wheat bread having a regular, fine crumb grain, a round top and good aeration (specific volume 3.2 ml/g). In model studies, HPMC stabilized gas bubbles well. Additionally, laser scanning confocal microscopy (LSCM) revealed finer zein strands in the dough when HPMC was present, while dynamic oscillatory tests showed that HPMC rendered gluten-like hydrated zein above its T_g softer (i.e. |G*| was significantly lower). LSCM revealed that cooling below T_g alone did not destroy the zein strands; however, upon mechanical impact below T_g, they shattered into small pieces. When such dough was heated above T_g and then remixed, zein strands did not reform, and this dough lacked resistance in uniaxial extension tests. When within the breadmaking process, dough was cooled below T_g and subsequently reheated, breads had large void spaces under the crust. Likely, expanding gas bubbles broke zein strands below T_g resulting in structural weakness.     

Simulating the formation of bread crust in a DMTA rheometer

Water evaporates very fast from the surface layers of dough, enhanced by high heating rates at the beginning of baking. The rheological properties of the surface layers play an important role during the baking process, especially in local and overall expansion and water loss. The aim of this study was to determine the rheological properties of bread dough in the heat-moisture dynamics of the baking process, especially in surface drying and delayed drying conditions. The DMTA method was used in tensile mode in order to expose the samples to fast dehydration to simulate real bread crust. The degree of starch gelatinization was demonstrated by the disappearance of the “Maltese cross” (polarized light microscopy). Temperature and water content were monitored during baking. The modulus evolution depended on both the heat and moisture evolution (i.e. immediate or delayed in the present study). In contrast to reports in the literature, starch gelatinization was observed even under drying conditions. Nevertheless, comparison between samples under drying and under delayed drying conditions suggested that water content prevailed in the rheological changes.     

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.     

Effects of Emulsifiers on Pita Bread Quality

The purpose of this study was to investigate the effects of emulsifiers [sodium stearoyl-2-lactylate (SSL), glycerol mono stearate (GMS-90), di-acetyl tartaric acid esters of monoglycerides (DATEM), S-570, S-1170, S-1670, P-1670] on pita bread quality. Three flour types (soft wheat flour with 10·0% protein, hard red winter wheat flour with 11·6% protein, and hard red spring wheat flour with 14·4% protein) and three emulsifier concentrations (0·25%, 0·50%, 0·75%, flour basis) were used. Pita breads were baked in an air impingement oven. Loaves were scored on the first and second days, and the influence of the more effective emulsifiers on shelf life stability were investigated after 5 days. Analysis of variance indicated that the flour of moderate protein content (11·6%) resulted in the best overall product quality. The high protein flour (14·4%) gave a dough with high water absorption and bread loaves with dark crust color. On the other hand, the low protein flour (10·0%) had low water absorption. The loaves were uniform in texture and thickness for top and bottom layers, and had a light crust color. All emulsifiers improved the tearing quality of the product. The low concentration (0·25%) of SSL, S-1170, S-1670, and S-570 with flour of moderate protein content (11·6% protein) resulted in a good quality pita bread. No significant differences occurred amongst breads containing those emulsifiers for ability to roll and fold and tearing quality after 5 days of storage at room temperature.     

Soy ingredients stabilize bread dough during frozen storage

Bread with 48.5% soy ingredients was assessed for quality during frozen storage of the dough. Soy protein was hypothesized to prevent water migration during frozen storage, thereby producing dough that would exhibit fewer structural changes than traditional wheat bread. Wheat and soy bread were baked from dough that was fresh or frozen (−20 °C, 2 or 4 wks). Dough and bread were assessed for physical properties including moisture content, percent “freezable” and “unfreezable” water, dough extensibility, and bread texture. The bread was subjected to an untrained sensory panel. The soy bread was denser, chewier, and had a higher moisture content than wheat bread. When baked from fresh or frozen dough, soy bread was rated “moderately acceptable” or higher by 70% of panelists. Soy minimized changes in dough extensibility and resistive force to extension, leading to minimal changes in bread hardness. Although consumers could distinguish between bread baked from soy dough that was fresh or frozen for 4 wks, sensorial and textural data suggested that the rate at which the quality of the soy dough deteriorated was slower than that of wheat dough. In conclusion, the dough of consumer-acceptable soy bread retained quality characteristics during frozen storage slightly better than wheat dough.     

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.     

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.     

Dynamic viscoelastic, calorimetric and dielectric characteristics of wheat protein isolates

Dynamic oscillatory rheology of two wheat protein isolate (Prolite 100 and Prolite 200) doughs (≈48% moisture content, wet basis) were studied over a frequency range of 0.1–10 Hz during temperature sweep from 20 to 90 °C at a heating rate of 2 °C/min. Both doughs behaved similarly during heating; showed a threshold value and increased sharply, thereafter. Prolite 200 dough had a higher elastic modulus (G′) and lower phase angle (δ) whereas Prolite 100 showed a distinct gel point at 52.2 °C followed by significant increase up to 90 °C. Rheological data of doughs after isothermal heating at 90 °C for 15 min followed by cooling to 20 °C resulted in strong mechanical strength. However, Prolite 100 dough showed more viscoelastic characteristics with significant transformation from liquid-like to solid-like behavior after heating than Prolite 200. Thermal analysis of isolates indicated distinct endothermic peaks in wider temperature range (50–130 °C) at various moisture levels. Lower temperatures could be associated with denaturation of various fractions of proteins whereas higher temperature linked to glass transition temperature of isolates. SDS–PAGE did not show any clear distinction among protein subunits between two isolates. Dielectric measurements of isolates at frequencies from 500 to 3000 MHz and temperature range between 30 and 80 °C indicated Prolite 200 had higher dielectric constant (ε′) and loss factor (ε″) than Prolite 100. Isolates showed significant changes in dielectric properties above 50 °C indicating protein denaturation and supported rheological and calorimetric data.     

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.     

Surface rheological properties of bread dough components in relation to gas bubble stability

The surface rheological properties of dough (components) were determined in order to estimate the effect of these properties on disproportionation and coalescence of gas bubbles in bread dough. Three different systems were studied as a model for the gas-dough interface: a diluted aqueous dough dispersion, gluten and wheat lipids spread on water. The surface dilational modulus, E, and tanϑ of these systems were determined as a function of frequency using a modified Langmuir trough. Values of E and tanϑ found were: 35–100 mN/m and 0·7–0–2, resp., for dough dispersions, 20–45 mN/m and 0·4–0·15, resp., for gluten, and 20–90 mN/m and 1·3–0·1, resp., for lipids in the frequency range tested at room temperature. On the assumption that the gas-dough interface is comparable either to the surface of the dough dispersions tested or to a water surface with spread gluten, it was shown that disproportionation of gas bubbles in dough can be retarded but not prevented. Wheat lipids present in the right concentration in the surface can prevent this foam stabilising mechanism to a larger extent. The surface dilational modulus as well as the surface tension during continuous expansion of dough dispersions were also determined at 45°C. The surface dilational modulus of a dough dispersion at 45°C was 7–25 mN/m, which was approximately 5 times smaller than at room temperature. Results of surface tension measurements during continuous expansion in a Langmuir trough showed that values for surface tension were only slightly higher than at equilibrium (ca. 2 mN/m) at 45°C and at deformation rates of the surface comparable to those at oven rise. These results suggest that thin dough films at higher temperatures will be less stable than at room temperature. Implications in relation with coalescence in dough are discussed. No significant differences in surface rheological properties of dough dispersions of wheats with different bread-making qualities were found in the sinusoidal oscillation tests nor in the continuous expansion tests. Surface rheological properties, therefore, appear not to be the main factor responsible for differences in baking quality amongst different wheats.     

Optimisation of rheological properties of gluten-free doughs with HPMC,psyllium and different levels of water

Hydrocolloids have traditionally been investigated as an alternative to gluten for making good quality products for coeliac patients. This study investigated the interactions between hydroxypropylmethylcellulose (HPMC) (2–4 g/100 g of flour), psyllium (0–4 g/100 g of flour) and water level (90–110 g/100 g of flour) in gluten-free breadmaking. Psyllium incorporation reduced the pasting temperature and compliance values, and increased elastic (G′) and viscous (G″) moduli values. In contrast, HPMC addition had no important effects on pasting properties and compliance values, but also increased G′ and G″ values. Psyllium inclusion reduced bread specific volume and increased bread hardness, while there were hardly differences in the bread specific volume and hardness between the percentages of HPMC studied. In addition, when the dough hydration level was increased, there was a decrease in the influence of hydrocolloids on dough rheology and specific volume and hardness of breads.     

Improving the quality of nutrient-rich Teff (Eragrostis tef) breads by combination of enzymes in straight dough and sourdough breadmaking

The growing interest in the benefits of wholegrain products has resulted in the development of baked products incorporating less utilised and ancient grains such as, millet, quinoa, sorghum and teff. However, addition of wholegrains can have detrimental effects on textural and sensory bread product qualities.Enzymes can be utilised to improve breadmaking performance of wholegrain flours, which do not possess the same visco-elastic properties as refined wheat flour, in order to produce a healthy and consumer acceptable cereal product.The effects of Teff grain on dough and bread quality, selected nutritional properties and the impact of enzymes on physical, textural and sensory properties of straight dough and sourdough Teff breads were investigated.Teff breads were prepared with the replacement of white wheat flour with Teff flour at various levels (0%, 10%, 20%, and 30%) using straight dough and sourdough breadmaking. Different combinations of enzymes, including xylanase and amylase (X + A), amylase and glucose oxidase (A + GO), glucose oxidase and xylanase (GO + X), lipase and amylase (L + A) were used to improve the quality of the highest level Teff breads. A number of physical, textural and sensory properties of the finished products were studied. The nutritional value of breads was determined by measuring chemical composition for iron, total antioxidant capacity, protein, fibre and fat. The obtained results were used to estimates intakes of nutrients and to compare them with DRIs.The incorporation of Teff significantly (P < 0.05) improved dietary iron levels as 30% Teff breads contained more than double the amount of iron when compared to corresponding wheat bread (6 mg/100 g v 2 mg/100 g). Addition of Teff also significantly (P < 0.05) improved total antioxidant capacity from 1.4 mM TEAC/100 g to 2.4 mM TEAC/100 g. It was estimated that an average daily allowance of 200 g of Teff enriched bread would contribute to DRIs in the range of 42-81% for iron in females, 72-138% for iron in males; 38-39% for protein in males, 46-48% for protein in females; and 47-50% of fibre in adults.The major challenge was encountered in producing the highest level of Teff bread with good textural and sensory attributes. Increasing the level of Teff significantly (P < 0.05) increased dough development time, degree of softening, crumb firmness and bitter flavour whilst decreasing the dough stability, specific loaf volume and overall acceptability of the bread. Teff breads produced with the addition of enzyme combinations showed significant improvements (P < 0.05) in terms of loaf volume, crumb firmness, crumb structure, flavour and overall acceptability in both straight dough and sourdough breadmaking.     

Expansion capacity of doughs: methodology and applications

An instrument for measuring the expansion capacity of dough was developed based on the application of a known negative pressure and measurement of the height reached by the dough using a dough height tracker. At a negative pressure of 74 cm of Hg, the same maximum heights were reached after expansion at all stages of processing from after mixing to end of proof. For this negative pressure (74 cm Hg), the expanded dough heights measured immediately after mixing for doughs from 9 hard and 10 soft wheat flours coincided closely with the heights reached by corresponding baked loaves (r=0.99). Pizza doughs were also found to give a good correlation with baked pizza height (r=0.78, significant at 1% level). The method was used to obtain information about the timing of the effects of bromate addition and flour lipid extraction during processing. An increase in dough expansion capacity from bromate addition was observed only after the final proofing stage. Gas cell fineness of the bread crumb, measured by CrumbScan software, was decreased by bromate addition but gas cell elongation was increased. Effects of lipid removal were different. An increase in expansion capacity occurred at all processing stages and gas cell fineness of the bread crumb was increased. Expansion capacity appears to be an inherent property of a dough and may have potential as a rapid measurement to predict baking performance.     

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.     

Interrelation between mechanical and biological aeration in starch-based gluten-free dough systems

Alternative aeration and gas stabilization strategies are required for the production of starch-based cellular food systems, such as gluten-free bread. In the present study, density and temperature were monitored in mixing experiments without yeast, aiming at maximum mechanical aeration. Additionally, the same trials were performed with subsequent biological aeration, including yeast fermentation and baking. As a result, the gas volume fraction was elevated to 21%, instead of 6% with conventional kneading. Reducing the water content from 120% to 90% (flour/starch weight base) raised dough viscosity and temperature without affecting the state of aeration. The bread volume was strongly influenced by the dough temperature after mixing (R² = 0.98), since it depended on yeast activity. The implemented process is suitable to aerate starch-based dough systems mechanically and enables the production of gluten-free bread with high volume and fine pores.     

Improved viscoelastic zein–starch doughs for leavened gluten-free breads: Their rheology and microstructure

Gluten-free bread was prepared from commercial zein (20 g), maize starch (80 g), water (75 g), saccharose, NaCl and dry yeast by mixing above zein's glass transition temperature (T_g) at 40°C. Addition of hydroxypropyl methylcellulose (HPMC, 2 g) significantly improved quality, and the resulting bread resembled wheat bread having a regular, fine crumb grain, a round top and good aeration (specific volume 3.2 ml/g). In model studies, HPMC stabilized gas bubbles well. Additionally, laser scanning confocal microscopy (LSCM) revealed finer zein strands in the dough when HPMC was present, while dynamic oscillatory tests showed that HPMC rendered gluten-like hydrated zein above its T_g softer (i.e. |G*| was significantly lower). LSCM revealed that cooling below T_g alone did not destroy the zein strands; however, upon mechanical impact below T_g, they shattered into small pieces. When such dough was heated above T_g and then remixed, zein strands did not reform, and this dough lacked resistance in uniaxial extension tests. When within the breadmaking process, dough was cooled below T_g and subsequently reheated, breads had large void spaces under the crust. Likely, expanding gas bubbles broke zein strands below T_g resulting in structural weakness.     

Effect of extrusion cooking of sorghum flour on rheology,morphology and heating rate of sorghum–wheat composite dough

Addition of a gluten-free flour such as sorghum has negative impact on the quality of wheat dough for bread making. One of the methods which can be used to promote the quality of sorghum-wheat composite dough is to extrude the sorghum flour before incorporation. In this regard, to produce a dough with appropriate bakery properties sorghum flour was extruded at 110 °C and 160 °C die temperature with 10%, 14% and 18% feed moisture. The effect of extruded sorghum flour incorporation (10%) on rheological (farinography and stress relaxation behavior), morphological and temperature profile of sorghum-wheat composite dough were evaluated. Extrusion cooking altered the sorghum-wheat composite dough properties through partial gelatinization of starch granules. Addition of extruded sorghum flour increased the water absorption and dough development time but it decreased the dough stability. Native sorghum-wheat composite dough showed viscoelastic liquid-like behavior whereas addition of sorghum flour extrudate changed dough to a more viscoelastic solid-like structure. Maxwell model was more appropriate than Peleg model to describe the viscoelasticity of the sorghum-wheat composite dough. Extrusion cooking decreased composite dough elasticity and viscosity. Sorghum extrudate increased the heating rate of composite dough crumb during baking. Addition of extruded sorghum flour formed a non-uniform and less compact dough structure. As a result, dough containing extruded sorghum flour had a good potential for producing a high-yielding bread in a short time of baking.     

Extensional rheological properties in mixed and fermented/rested dough and relationships with steamed bread quality

In this study, sixteen wheat varieties for cultivation in China were examined for the flour characteristics using the farinograph, extensograph and rheofermentometer, uniaxial extensional rheology employing the extensograph and the Kieffer extensibility rig and biaxial extension by uniaxial compression of mixed dough with and without yeast, rested and fermented dough, and steamed bread quality including specific volume and texture properties. Three statistical analysis methods including Pearson correlation, principle component and stepwise multiple regression analysis were carried out to correlate dough properties with steamed bread quality. Biaxial extension viscosity was positively correlated with texture properties (hardness and chewiness) of steamed bread (r = 0.521–0.685, p < 0.05). Based on the correlation coefficients and the model (r² = 0.852, p = 0.003) obtained using stepwise multiple regression analysis, the best predictors for specific volume of steamed bread were the maximum resistance to extension of rested dough (r = 0.664, p < 0.01) and total work for breakage of fermented dough (r = 0.662, p < 0.01). Principal component analysis of rheological properties of fermented dough and flour characteristics provided more useful information for discriminating wheat flour quality and help breeders to select most convenient wheat flour for the steamed bread making.