Measurement of dough specific volume in chemically leavened dough systems

Gas production and gas retention properties of doughs are pivotal to the manufacture of bread of good quality, but these properties are rarely measured directly in fermenting dough due to a paucity of suitable instrumentation. A digital image analysis-based method was used to measure the dynamic specific volume (DSV) of various chemically leavened dough systems. Sodium bicarbonate (1.4–4.2 g per 100 g of flour) in combination with equivalent neutralizing amounts of the leavening acidulants glucono-delta-lactone, potassium acid tartrate, adipic acid or sodium acid pyrophosphate consistently increased the specific volume of bread dough so that void fractions in the dough spanned between 5 and 67% at ordinary fermentation temperatures. The relationship between the specific volume of dough at the end of fermentation and the actual gas evolved (measured independently) was essentially linear and was characterized by a slope that provided a good index of the actual gas-trapping properties of dough. Therefore, the use of the DSV technique in conjunction with chemical leaveners offers the possibility of obtaining quantitative, real time information on the gassing capacity of the leavening system and the gas-holding capacity of the dough.     

Water Sorption and Dielectric Relaxations of Wheat Dough (Containing Sucrose, NaCl, and their Mixtures)

Dielectric relaxations of wheat doughs with different water contents and effects of sucrose, NaCl, and their mixture on relaxation temperatures were investigated using dielectric analysis (DEA). All ingredients were dissolved in distilled water used to prepare wheat flour doughs to optimum consistency. Before analysis, samples were stored at room temperature in vacuum desiccators over a_w range of 0·225–0·753. Dynamic DEA measurements were made at a heating rate of 2 °C/min from 40 °C below and above the observed relaxation zone. The frequencies used were 0·1, 0·5, 1, and 5 Hz. Steady state water contents varied from 3·21 to 10·89 g H₂O/100 g dm over a_w range used for the plain dough (flour+water). Added ingredients increased sorption of doughs. The tan δ of DEA showed an α-relaxation (glass transition) in all doughs at all frequencies used. The relaxation peak temperature, taken as T_g, increased with increasing frequency. Added sucrose decreased the T_g of doughs, as well as added NaCl. A dramatic depressing effect of NaCl on T_g was probably due to an increase in conductivity of doughs.     

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.     

Thermal, Dynamic-mechanical, and Dielectric Analysis of Phase and State Transitions ofFrozen Wheat Doughs

The physical state of wheat dough at sub-zero temperatures was investigated using Differential Scanning Calorimetry (DSC), Dynamic-mechanical Analysis (DMA) and Dielectric Analysis (DEA). Also, the water sorption properties of freeze-dried dough were investigated. DMA and DEA measurements were made at a heating rate of 1 °C/min from −150 to 10 °C. Before the measurements, samples were equilibrated at −25 °C for 15–20 min to allow maximum ice formation, and then cooled at 1 °C/min to −150 °C. The frequencies used were 0·1, 0·5, 1, 2·5, 5, 10, and 20 Hz for DMA and 0·1, 0·5, 1, 5, 10, 20, 50, 100 and 1000 Hz for DEA. The DSC thermograms were obtained for annealed samples by scanning from −80 to 20 °C at 5 °C/min, and they showed only ice melting starting at −18 °C. The tan δ of DMA and DEA showed an α-relaxation (glass transition), two low temperature relaxations (β and γ) and melting of ice. At the higher frequencies (>0·5 Hz), the α-relaxation coincided with melting of ice, and all relaxation temperatures (α, β and γ) increased with increasing frequencies as measured by DEA.     

Dielectric Relaxations of Frozen Wheat Doughs Containing Sucrose, NaCl, Ascorbic Acid and Their Mixtures

The effects of sucrose, NaCl, and ascorbic acid on dielectric relaxations of frozen wheat doughs were investigated using dielectric analysis (DEA). All ingredients were dissolved in distilled water used to prepare wheat flour doughs to optimum consistency using a 10 g bowl Micro-Mixer. DEA measurements were made at a heating rate of 1 °C/min from −150 to 10 °C. Before the measurements, samples were equilibrated at −30 °C for 15 min to allow maximum ice formation, and then cooled at 1 °C/min to −150 °C. The frequencies used were 0·1, 0·5, 1, 5, 10, 20, 50, 100 and 1000 Hz. The dissipation factor (tan δ) of DEA showed an α-relaxation (glass transition), two low temperature relaxations (β and γ) and ice dissolution. Added NaCl had a markedly depressed the glass transition temperature (T_g′) and onset of melting of ice temperature (T_m′), probably because of the higher conductivity of the frozen material, and the decreased transition temperatures of the unfrozen solute phase. At the higher frequencies, the α-relaxation coincided with melting of ice, and all relaxation temperatures (α, β and γ) increased with increasing frequencies.     

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.     

Continuous Spectrophotometric Assay and Properties of Ascorbic Acid Oxidising Factors in Wheat

A continuous spectrophotometric assay was developed to measure ascorbic acid oxidation in crude Na₂SO₄ extracts of flour. The rate of ascorbic acid oxidation in flour extracts measured using this method was similar to the rate in flour-water suspensions and 2–4 fold less than the rate in dough measured using an indophenol-xylene extraction method. Flour extracts appeared to contain two ascorbic acid oxidising factors; one with optimal activity at pH 6·3 and 30 °C and the other with optimal activity at pH 10 and 40 °C. The pH 6·3 factor had properties similar to those of ascorbate oxidase (EC 1·10·3·3) in its pH and temperature stability, strong inhibition by NaN₃, KCN and diethyldithiocarbamate, inactivation by proteases, and greater stereospecificity towards -ascorbic acid than -isoascorbic acid. The pH 6·3 factor was most concentrated in the pollard milling fraction of wheat and was lowest in flour. The pH 10 factor had several properties indicating non-enzymic oxidation of ascorbic acid; it was not inactivated by proteases, it was inhibited poorly or not at all by the above ascorbate oxidase inhibitors, and it had low specificity for stereoisomers of ascorbic acid.     

Influence of reducing agents on properties of thermo-molded wheat gluten bioplastics

The present work aims to study the influence of reducing agents of sodium bisulfite, sodium sulfite and thioglycolic acid on the equibiaxial extensional deformation of glycerol plasticized wheat gluten and the properties of gluten bioplastics thermo-molded at 125 °C. Moisture absorption, weight loss and water uptake, uniaxial tensile properties (Young's modulus, tensile strength, elongation at break and tensile set), and morphology observations were performed to characterize the physical properties of the thermo-molded gluten bioplastics. The results showed that reducing agents facilitated the viscous flow and restrained the elastic recovery of the plasticized gluten while not hindering the crosslinking reaction of gluten proteins during thermo-molding. On the contrary, reducing agents do not significantly influence moisture absorption, Young's modulus, tensile strength and the morphology of the gluten bioplastics thermo-molded at 125 °C. It is shown that reducing agents are highly effective for tailoring the flow viscosity of the plasticized gluten dough and the mechanical properties of thermo-molded gluten bioplastics.     

The impact of heating and cooling on the physico-chemical properties of wheat gluten–water suspensions

The rapid visco analysis (RVA) system was used to measure rheological behaviour in 20% (w/v) gluten-in-water suspensions upon applying temperature profiles. The temperature profiles included a linear temperature increase, a holding step, a cooling step with a linear temperature decrease to 50 °C, and a final holding step at 50 °C. Temperature and duration of the holding phase both affected RVA viscosity and protein extractability. Size-exclusion and reversed-phase HPLC showed that increasing the temperature (up to 95 °C) mainly decreased glutenin extractability. Holding at 95 °C resulted in polymerisation of both gliadin and glutenin. Above 80 °C, the RVA viscosity steadily increased with longer holding times while the gliadin and glutenin extractabilities decreased. Their reduced extractability in 60% ethanol showed that γ-gliadins were more affected after heating than α-gliadins and ω-gliadins. Enrichment of wheat gluten in either gliadin or glutenin showed that both gliadin and glutenin are necessary for the initial viscosity in the RVA profile. The formation of polymers through disulphide bonding caused a viscosity rise in the RVA profile. The amounts of free sulphydryl groups markedly decreased between 70 and 80 °C and when holding the temperature at 95 °C.     

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.     

Pasting Profiles and Solubility of Native and Cross-Linked Corn Starch inDimethylsulfoxide-Water Mixtures

The pasting profiles of native and cross-linked corn starch in dimethylsulfoxide (DMSO)–water mixtures were investigated with a Rapid Visco Analyser. The temperature profile included an isothermal step of 1 min at 30 °C, followed by a linear gradient to 75 °C in 3 min and finally an isothermal step of 20 min at 75 °C. The profiles were characterised by an initial peak (at 75 °C) followed by a trough, much as is the case with profiles of such starch in water. Increasing the DMSO concentration in the DMSO–water mixture from 70–97·5% (by weight), without changing the starch concentration, resulted in lower end-viscosities. The RVA pasting profiles of a series of sodium trimetaphosphate (STMP) and epichlorohydrin (ECH) cross-linked corn starches were investigated in 92·5% DMSO at different dry matter (d.m.) contents. At high starch concentrations (>60% d.m.), the end viscosity increased with the degree of cross-linking. At lower starch concentrations (<6% d.m.), the more highly cross-linked starches yielded lower peak- and end-viscosities than starches with lower degrees of cross-linking. The solubilisation of the starches in 92·5% DMSO was complete for the native starch and decreased considerably for the highly cross-linked starches. Light microscopy showed that the cross-linked starches, upon heating in DMSO–water, were converted to a biphasic system consisting of insoluble granule remnants and a starch solution.     

Fatty Acid Composition of Three Rice Varieties Following Storage

The petroleum ether extractable lipids (PEE-L) and aqueous propan-1-ol extractable lipids (PWE-L) of three varieties of rice were determined gravimetrically and characterised by fatty acid profiles. The content of PEE-L (22·5–28·2 mg g⁻¹) was higher than that of PWE-L (7·4–11·5 mg g⁻¹) in brown rice with the situation reversed in milled rice (3·0–4·5 mg g⁻¹vs. 7·2–8·7 mg g⁻¹). The ratio of unsaturated fatty acid to saturated fatty acid was about two times higher in PEE-L than that in PWE-L for both brown and milled rice reflecting the selective complexation of saturated fatty acids. Rice storage at 37 °C resulted in some minor but statistically significant changes in the fatty acid profile. In the case of brown rice, the only notable changes were a reduction in the amounts of oleic and linoleic acids in the aqueous propan-1-ol extractable fatty acid fraction (PWE-FA) following storage at the higher temperature. Milled rice of all three varieties showed a decrease in linoleic acid content of PEE-L following storage at 37 °C for 4 and 7 months compared to storage at 4 °C. There was no change in fatty acid contents of PWE-L of milled rice when stored at 4 and 37 °C for 4 and 7 months. This implies that the PWE-L (or bound lipids) were more stable than PEE-L (or free lipids) during storage.     

Macromolecular Features of Starches Determined by Aqueous High-performance Size Exclusion Chromatography

Starches from various botanical sources, having different amylose–amylopectin levels, were treated with 95% (v/v) dimethylsulphoxide and then dissolved by microwave heating in a high pressure vessel (MWHPV). This procedure led to an almost complete dissolution of the starch sample when it was subsequently taken up in water. Various treatment times were tested, and optimal conditions corresponded to 35 s heating time in MWHPV (maximum temperature 143 °C). Under these conditions, degradation of the polymers was not noticeable and the degree of solubilisation of starch in the samples was in the range 87 to 100%. High-performance size exclusion chromatography (HPSEC) in tandem with multi-angle laser light scattering and refractive index detection was used to investigate apparent weight-average molar mass ( _w) and z-average gyration radius ( _G) of the starch components. Apparent _wand _Gvalues for samples with different amylose contents (between ≈0 and 65·8%) were between 2·2±0·2×10⁸and 2·4±0·2×10⁷g/mol and between 250±10 and 163±5 nm, respectively. These values were higher than those reported previously^1,2probably due to our more complete but less severe solubilisation procedure. When the heating time was increased from 35 to 90 s (maximum temperature 211°C), the _wand _Gdecreased, demonstrating possible polymer degradation due to temperature. Partial precipitation of high amylose samples after 24 h storage was detected by modifications in chromatograms, compared with freshly prepared samples.     

Dependence of Extensograph Parameters on Wheat Protein Composition Measured by SE-HPLC

Size-exclusion HPLC measurements were made on the total protein from flours of 158 wheat lines grown at five locations (31 at all sites and an extra one at three locations). Closely spaced cuts in the chromatograms were made at elution times differing by 0·4 min. The percentage of the protein that eluted up to each time was correlated with Extensograph dough maximum resistance (R_max) and dough extensibility (Ext). For R_max, maximal correlation (r=0·489*** for all locations) was obtained for the percentage of protein eluting before 13·2 min, corresponding to estimated molecular weights equal to and greater than 250 000. In the case of Ext, the highest correlation (0·859*** for all locations) was obtained for the percentage of protein in the flour which eluted before 16·8 min, which included essentially all the flour polymeric protein with estimated molecular weights above 52 000. The different dependencies of R_maxand Ext provide a basis for manipulating these parameters in predictable ways in breeding programs.     

Starch gelatinization and amylose–lipid interactions during rice parboiling investigated by temperature resolved wide angle X-ray scattering and differential scanning calorimetry

Starch gelatinization and formation of crystalline amylose–lipid complexes during the heat/moisture treatment step in rice parboiling were studied with temperature resolved wide angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC) using flour from Puntal (24% apparent amylose) and Jacinto (12% apparent amylose) rice samples [66, 40 or 25% moisture content (mc)]. Temperature resolved WAXS showed that the crystallinity index (CI, i.e. the relative amount of A-type crystals) of non-parboiled rice flours at 66% mc, monotonically decreased between 65 °C (Puntal) and 70 °C (Jacinto) and 90 °C (both Puntal and Jacinto). These temperatures were in agreement with the respective onset and conclusion temperatures of the M₁ endotherm measured by DSC. At 40% mc, the CI decreased monotonically from 65 °C (Puntal) and 70 °C (Jacinto) until 105 °C. In DSC both M₁ and M₂ endotherms were present. The conclusion temperature of the M₂ endotherm was higher than 105 °C. At 25% mc, the CI decreased very gradually and A-type crystals were no longer present at 145 °C. Under these conditions, no DSC endotherms were detected. No type II amylose–lipid complexes were formed during heating at 66% mc. In contrast, at 40 and 25% mc, V_h-type crystals were formed from 100 and 130 °C, respectively. Non-parboiled white rice flour had a strong A-type pattern. Mildly parboiled rice had a clear A-type, with a weak V_h-type and B-type pattern. Severe parboiling resulted in partially crystalline systems with superimposed A-type, V_h-type and B-type crystals. It was concluded that the rice variety, the combination of mc and the moisture distribution in the rice kernel and the temperature during parboiling all impact the level and the types of crystals in the parboiled rice.     

Properties of Frozen Wheat Doughs at Subzero Temperatures

The subzero properties of wheat doughs were measured by dynamic mechanical thermal analysis (DMTA) and dielectric thermal analysis (DETA) over the temperature range −90 to +40 °C and by¹H-nuclear magnetic resonance (NMR) T₂relaxation over the range −45 to 0 °C. The experiments revealed two transitions in the dough: one independent of frequency at −10 °C (attributed to ice melting) and one dependent on frequency at −30 °C (attributed to a glass transition). The glass transition temperatures measured by DMTA moved to higher temperatures during frozen storage when the optimal water content of dough was used. A reduction in the water content eliminated this phenomenon. A similar effect of water reduction was observed by NMR studies, in which amplitude ratios and decay times were used to calculate the phase transitions. However, the glass transition recorded by NMR was independent of frozen storage with optimal water content. The changes of water state in frozen doughs were studied by ultracentrifugation (the amount of liquid phase) and NMR (freezable water based on liquid amplitude ratios). Frozen storage increased the liquid phase in dough with optimal water content. Thus, ice crystals are growing during frozen storage resulting in the concentration of polymers and a higher glass transition observed by DMTA. The increase of liquid phase during storage was substantially lower when the water content of dough was decreased. Ice crystals» growth can be minimised by reducing water content. The experiments were carried out with four different flours. The measurement of glass transition temperature by DMTA, DETA or NMR did not reveal great differences in doughs made from different flours. The amount of liquid phase was strongly flour dependent.     

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.     

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.     

Rheofermentometer parameters and bread specific volume of frozen sweet dough influenced by ingredients and dough mixing temperature

Response surface methodology described the effects of salt, lactic acid, shortening, and exogenous trehalose and dough mixing temperature (DMT) and their interactions on the three rheological and fermentation parameters. These included maximum dough height (Hm), maximum height of gas release (Hm′) and CO₂ production, measured by the Rheofermentometer F3, and bread specific volume (Sp. Vol.) of frozen sweet dough. The models could estimate the four parameters with R² values of 0.76, 0.69, 0.93, and 0.59, respectively. Salt significantly influenced all four parameters in a negative way. DMT affected positively the Hm and Sp. Vol. of bread. Lactic acid affected Hm only, but its interactions with other variables influenced all four parameters. Shortening level affected Hm′ and CO₂ production positively and Sp. Vol. negatively. The added exogenous trehalose improved Hm, Hm′, and CO₂ production significantly, but not the Sp. Vol. of bread. Among the three Rheofermentometer parameters, Hm showed the highest correlation with Sp. Vol. (R² = 0.75). DMT for the maximum Hm and Sp. Vol. varied with the level of other ingredients. Trehalose alone could not overcome the challenges in a sweet frozen dough system to improve the Sp. Vol., and its combined effects with other ingredients will need to be evaluated to restore the impaired gas retention of the frozen sweet dough.     

Rheofermentometer parameters and bread specific volume of frozen sweet dough influenced by ingredients and dough mixing temperature

Response surface methodology described the effects of salt, lactic acid, shortening, and exogenous trehalose and dough mixing temperature (DMT) and their interactions on the three rheological and fermentation parameters. These included maximum dough height (Hm), maximum height of gas release (Hm′) and CO₂ production, measured by the Rheofermentometer F3, and bread specific volume (Sp. Vol.) of frozen sweet dough. The models could estimate the four parameters with R² values of 0.76, 0.69, 0.93, and 0.59, respectively. Salt significantly influenced all four parameters in a negative way. DMT affected positively the Hm and Sp. Vol. of bread. Lactic acid affected Hm only, but its interactions with other variables influenced all four parameters. Shortening level affected Hm′ and CO₂ production positively and Sp. Vol. negatively. The added exogenous trehalose improved Hm, Hm′, and CO₂ production significantly, but not the Sp. Vol. of bread. Among the three Rheofermentometer parameters, Hm showed the highest correlation with Sp. Vol. (R² = 0.75). DMT for the maximum Hm and Sp. Vol. varied with the level of other ingredients. Trehalose alone could not overcome the challenges in a sweet frozen dough system to improve the Sp. Vol., and its combined effects with other ingredients will need to be evaluated to restore the impaired gas retention of the frozen sweet dough.