Mechanism of gas cell stabilization in breadmaking. II. The secondary liquid lamellae

The study for the first time demonstrates that flour lipids at their natural levels do not affect dough rheology as measured by bubble inflation, thus indicating the presence of liquid lamellae as an independent secondary gas cell stabilizing mechanism in bread dough. The liquid lamellae, stabilized by adsorbed surface active compounds, plays its role during the later proving and early baking stage, when discontinuities occur in the gluten–starch matrix surrounding gas bubbles. To study this secondary stabilizing mechanism, different lipid fractions were added incrementally to the defatted flours. No effects were observed on the rheological properties of the dough. However, large effects on the loaf volume were measured. The additives used were the total flour lipid and its polar and non-polar fractions and the fatty acids palmitic, linoleic and myristic. Polar lipids and palmitic acid had positive or little effect on loaf volume, respectively. Non-polar lipid, linoleic and myristic acids had negative effects on loaf volume. The different effects of the lipid fractions are thought to be related to the type of monolayer that is formed. Polar lipid and palmitic acid form condensed monolayers at the air/water interface whereas non-polar lipid, linoleic and myristic acids form expanded monolayers.     

Pseudocereals and teff in complex breadmaking matrices: Impact on lipid dynamics

The use of pseudocereals and ancient grains for breadmaking applications is receiving particular attention since they involve nutrient dense grains with proven health-promoting attributes. Dilution up to 20% of the basic rye/wheat flour blend by accumulative addition of amaranth, buckwheat, quinoa and teff flours (5% single flour) did positively impact either some dough visco-metric and visco-elastic features, or some techno-functional and nutritional characteristics of mixed bread matrices, and induced concomitant dynamics in lipid binding over mixing and baking steps. A preferential lipid binding to the gluten/non gluten proteins and to the outside part of the starch granules takes place during mixing, in such a way that the higher the accumulation of bound lipids during mixing, the higher the bioaccessible polyphenol content in blended breads. During baking, lipids bind to the gluten/non gluten proteins at the expenses of both a free lipid displacement and a lipid migration from the inside part of the starch granules to the protein active sites. It was observed that the higher the decrease of free lipid content during baking, the higher the pasting temperature and the lower the total setback on cooling and the dynamic moduli, but the higher the specific volume in blended breads.     

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.     

Composition and surface properties of dough liquor

The composition and surface properties of dough liquor isolated by ultracentrifugation have been characterised. Addition of ascorbate had no effect and salts only a limited effect, on the yield, protein content and composition of the dough liquor. Fourier transform infrared spectroscopy (FT-IR) revealed the presence of proteins, lipids, starch oligosaccharides together with the non-starch polysaccharide, arabinoxylan. At high dilution the dough liquor air:water interface was dominated by protein, with surface tensions of around 55 mN/m and high surface elasticity. As the concentration was increased, surface tensions dropped to around 40 mN/m for undiluted dough liquor. This was accompanied by the interface becoming less elastic, and indicated that dough liquor lipids were interacting and disrupting the protein films in concentrated dough liquor. Dough liquors from de-fatted flours remained elastic and gave surface tension values of around 50–55 mN/m even at low dilution, indicating that removal of the lipids gave rise to a purely protein stabilised interface. Addition of salt to the dough had the greatest effect on the surface properties, both reducing surface tension and reducing surface elasticity, probably because the charge screening effect of the salt improved the dispersion of lipids in the dough liquor, thus enabling it to disrupt the protein films more effectively. These results indicate that the aqueous phase of bread doughs lining the gas cells would give rise to a mixed protein:lipid interface. Such interfaces are unstable, and would contribute to the instability of the foam structure of risen dough. In addition they show that dough ingredients may modify gas cell stability (and hence may affect crumb structure), by altering the composition and properties of the aqueous phase of doughs.     

Mechanism of gas cell stabilization in bread making. I. The primary gluten–starch matrix

Expansion of dough and hence bread making performance is postulated to depend on a dual mechanism for stabilization of inflating gas bubbles. Two flours were used in this study, one from the wheat variety Jagger (Jagger) and the other from a composite of soft wheat varieties (Soft). Thin liquid lamellae (films), stabilized by adsorbed surface active compounds, act as an auxiliary to the primary gluten–starch matrix in stabilizing expanding gas cells and this mechanism operates when discontinuities begin to appear in the gluten–starch matrix during later proving and early baking stages. Contributions of the liquid lamellae stability to dough expansion were assessed using flours varying in their lipid content. Incremental addition of natural lipids back into defatted flour caused bread volume to decrease, and, after reaching a minimum, to increase. Strain hardening is a key rheological property responsible for stabilizing the primary gluten–starch matrix. Jagger gave higher test-bake loaf volume than Soft and higher strain hardening index for dough. The different lipid treatments were found to have negligible effects on strain hardening index. Image analysis of crumb grain revealed that differences in number of gas cells and average cell elongation with different lipid treatments were insignificant. The evidence agrees with a dual mechanism to stabilize the gas cells in bread dough. To understand dough rheology at a molecular level, rheological properties of doughs were varied by addition of flour protein fractions prepared by pH fractionation. Fractions were characterized by SE-HPLC and MALLS. The molecular weight distribution (MWD) of fractions progressively shifted to higher values as the pH of fractionation decreased. Mixograph dough development time paralleled the MWD. However, the strain hardening index and the test-bake loaf volume increased with increasing MWD up to a point (optimum), after which they declined. At a given strain rate, the behavior at the optimum is thought to result from slippage of the maximum number of statistical segments between entanglements, without disrupting the entangled network of polymeric proteins. Shift of MWD to molecular weight higher than the optimum results in a stronger network with reduced slippage through entanglement nodes, whereas a shift to lower molecular weights will decrease the strength of the network due to a lesser number of entanglements per chain.     

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.     

Gas Cell Stabilisation and Gas Retention in Wheat Bread Dough

Gas cell stabilisation and gas retention are of considerable interest because of their technological significance in bread making. We review recent studies in relation to the stabilisation of gas cells and the mechanisms of gas retention, and discuss how these may be affected by the liquid phase of dough. The possibility is discussed of the involvement of surface active materials, such as proteins and pentosans dissolved in the dough aqueous phase, and, perhaps more importantly, non-starch polar lipids in the formation and stabilisation of gas cells. There is accumulating evidence for the hypothesis that liquid films play a critical role in the mechanisms of gas retention in dough. The hypothesis proposes that two closely related, consecutive stages are involved in dough expansion. During the first stage, the expanding gas cells remain discrete until discontinuities develop in the starch–protein matrix, leaving areas containing only a liquid film. The timing and the degree to which such discontinuities occur is largely dependent on gluten proteins. The second stage involves an increase in the surface area of the liquid film as discontinuities become increasingly frequent during expansion. Failure of the lamellar film to maintain the rate at which new surface area is generated leads to the rupture of this film and, consequently, the loss of gas retention. Consideration is also given to the role of bakery fat in gas retention and to additional factors that affect gas retention in wholemeal doughs, in particular the physical disruption of the foam structure of such doughs by components of the outer layers of the grain.     

The final established physicochemical properties of steamed bread made from frozen dough: Study of the combined effects of gluten polymerization,water content and starch crystallinity on bread firmness

The gluten polymerization behavior, water content, starch crystallinity and firmness of Chinese steamed bread made from frozen dough were investigated and their correlations were also established in this study. The decreased degree of gluten polymerization in steamed bread was observed by the enhanced SDS-extractable proteins (SDSEPs) upon frozen storage. Less incorporation of glutenin in the glutenin–gliadin crosslinking of steamed bread mainly contributed to the decreased degree of gluten polymerization. The decreased moisture of steamed bread had a significant negative correlation with the sublimated water in frozen dough (r = −0.8850, P < 0.01). Frozen storage also induced an increase in starch crystallinity and bread firmness. A multiple linear regression model with SDS-extractable proteins, water content and melting enthalpy of starch crystals of steamed bread accounted for 86% of the variance in the natural logarithm of firmness and further revealed that starch crystallinity mainly contributed to bread firmness.     

Study of the mechanism of improvement due to waxy wheat flour addition on the quality of frozen dough bread

Waxy wheat flour (WWF) was substituted for 10% regular wheat flour (RWF) in frozen doughs and the physicochemical properties of starch and protein isolated from the frozen doughs stored for different time intervals (0, 1, 2, 4 and 8 weeks) were determined to establish the underlying reasons leading to the effects observed in WWF addition on frozen dough quality. Using Nuclear Magnetic Resonance (NMR), Differential Scanning Calorimeter (DSC) and X-ray Diffraction (XRD) among others, the gluten content, water molecular state, glutenin macropolymer content, damaged starch content, starch swelling power, gelatinization properties, starch crystallinity and bread specific volume were measured. Compared to RWF dough at the same frozen storage condition, 10% WWF addition decreased dry gluten and glutenin macropolymer contents and T₂₃ proton density of frozen dough, but increased the wet gluten content, T₂₁ and T₂₂ proton density. 10% WWF addition also decreased damaged starch content, but increased starch swelling power, gelatinization temperature and enthalpy, crystallinity of starch and bread specific volume of frozen dough. Results in the present study showed that the improvement observed due to WWF addition in frozen dough bread quality might be attributed to its inhibition of redistribution of water molecules bound to proteins, increase in damaged starch content and decrease in starch swelling power.     

燕麦粉对强筋面粉品质特性及其面包品质的影响

面包逐渐成为中国人营养早餐的选择,但面包中膳食纤维含量较低。燕麦是一种食疗兼备的特色杂粮作物,富含膳食纤维。为拓宽燕麦的应用范围、提高面包的营养价值,以优质强筋小麦品种藁优2018和燕麦粉为材料,研究了燕麦粉对小麦面粉面团流变学特性及淀粉糊化特性的影响,并进行了燕麦面包实验室制作和质构分析。结果表明,随着燕麦粉添加比例的增加,面团吸水率逐渐增加,面团稳定时间和粉质质量指数先减小后增加;面团拉伸能量、拉伸长度、拉伸阻力、最大拉伸阻力均逐渐减小。说明燕麦粉对面团拉伸特性各参数均有弱化作用。在藁优2018小麦面粉中添加10%的燕麦粉,既能保证面包的感官品质,又能满足面包营养最大化。     

The role of gluten in a sugar-snap cookie system: A model approach based on gluten–starch blends

The cookie making properties of dough made from blends of commercial wheat starch and gluten were determined. Higher gluten levels decreased dough piece weight, its density, stickiness and hardness. The largest spread was obtained when no gluten was added. However, this resulted in cookies of unacceptable structure. Higher gluten contents increased spread onset time, decreased cookie spread but generally had little impact on set time while additional water lowered spread onset time and likewise had no statistically significant impact on set time. The results showed that the final cookie diameter is quite dependent on the spread onset time which itself depends on the amount of water available to the non-gluten constituents in the system. Size-exclusion high performance liquid chromatography showed that during baking, proteins aggregated. This indicated that during the process the added gluten acquired the necessary mobility for interaction. However, because increasing levels of gluten increasingly decreased the relative level of water available to itself, and because the set time, and, hence, the set temperature, did not depend on the gluten level, we concluded that cookie dough setting was not determined by an ‘apparent’ glass transition. Furthermore, more protein aggregation went hand in hand with less spread.     

Starch isolation method impacts soft wheat (Triticum aestivum L. cv. Claire) starch puroindoline and lipid levels as well as its functional properties

Wheat (Triticum aestivum L.) kernel hardness is a major quality characteristic, which has been ascribed to the presence of puroindolines a and b. These proteins occur in higher levels at the surface of water-washed starch granules from soft wheat cultivars than at that of starch from hard wheat cultivars. In the present study, prime starch was isolated from flour from soft wheat (cultivar Claire) using a dough ball or batter based separation method. Starch isolated with the dough ball method contained lower levels of puroindolines, as well as of other starch granule associated proteins and lipids than that isolated with the batter method. Similar patterns of puroindoline and lipid levels after starch isolation can presumably be related to (polar) lipid binding by puroindolines. Both isolated starch fractions showed comparable differential scanning calorimetry thermograms, whereas higher levels of starch surface associated components restricted starch swelling. Necessary controls demonstrated that the observed differences did not arise from artefacts associated with hydration, fractionation or freeze-drying in the experimental protocols. Apparently, proteins and lipids at the starch granule surface impact water absorption and, as such, starch swelling, but they do not affect starch granule internal phenomena such as melting of the crystalline amylopectin chains.     

Effect of glutathione on wheat dough properties and bread quality

Reduced glutathione (GSH), released from lysed yeast cells, is well-known for weakening dough structure. However, its influence on bread texture and staling has not yet been completely elucidated. Herein, this study aims at assessing the effects of GSH on dough properties and bread quality, especially bread staling, using Rheofermentometer analysis, texture profile analysis (TPA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), and low-field nuclear magnetic resonance (LF-NMR). The results revealed that GSH substantially decreased the dough height and gas-retention capacity during fermentation. The weakened dough structure was attributed to GSH-induced disulfide bond cleavage in gluten proteins and the depolymerization of glutenin polymers. The addition of 0.005%–0.01% GSH resulted in acceptable bread quality. However, bread supplemented with 0.015%–0.03% of GSH exhibited a coarser and more open crumb microstructure, as well as high level of hardness and low resilience during aging. GSH promoted crumb water loss and drove the water shift from the immobilized to the bound state. The DSC and XRD analyses further confirmed that GSH promoted starch retrogradation and recrystallization. These results suggest that apart from the gluten structure, GSH also altered gluten−starch interactions and water redistribution, ultimately decreasing bread quality and accelerating bread staling.     

Scanning Electron Microscopic Study of Dough and Chapati from Gluten-reconstituted Good and Poor Quality Flour

Hard and soft wheat flours, which were used in the study, resulted in good and poor quality chapatis respectively. Gluten was isolated and interchanged among the two whole wheat flours and studied by scanning electron microscopy for its influence on structural characteristics of dough and its relation to chapati-making quality. Microscopic observations clearly indicated that larger gluten strands covered starch granules in hard wheat flour dough, while gluten was short and starch granules exposed in dough prepared from soft wheat flour. Greater film forming ability of gluten in hard wheat flour dough manifested in long and bulky starch strands interwoven with protein matrix in its chapati crumb. Higher moisture retention and starch gelatinization as a consequence of greater film forming ability of gluten in hard wheat flour resulted in pliable and soft textured chapati.     

Variation in polar lipids located on the surface of wheat starch

It is unknown whether starch isolated before dough development has the same surface lipid composition as starch isolated after dough development. The abundance of starch surface polar lipids is related to the physical hardness of the endosperm, but the variation in specific lipid classes and molecular species is unknown. The objective of this study was to determine the variation in polar lipids present on the surface of wheat starch granules. The experimental wheat lines used are, within each set, near-isogenic to each other but vary in endosperm hardness. Starch was isolated using two different processes: a dough and a batter method. Direct infusion electrospray ionization tandem mass spectrometry was used to identify and quantitatively determine the polar lipid species in wheat flour and on starch. Wide ranges in starch surface polar lipid concentrations were observed between the starch isolation methods. Starch isolation method provided a greater source of variation than did wheat kernel hardness. When dough is optimally mixed, lipids originally on the surface of wheat starch are dissociated, whereas in a batter system, starch surface lipids stay associated with the starch surface. The predominant starch surface polar lipids were digalactosyldigylcerol (DGDG), monogalactosyldigylcerol (MGDG) and phosphatidylcholine (PC) polar lipid classes.     

Impact of lipases with different substrate specificity in wheat flour separation on the properties of the resultant gluten

Wheat gluten was isolated in a laboratory dough-batter flour separation process in the presence or absence of lipases differing in hydrolysis specificity. The obtained gluten was blended with wheat starch to obtain gluten-starch (GS) blends of which the water and oil binding capacities were investigated. Furthermore, GS blends were mixed into dough and processed into model breads, of which dough extensibility and loaf volume were measured, respectively. In comparison to GS blends prepared with control gluten, oil binding capacity was higher when GS blends contained gluten isolated with Lecitase Ultra (at 5.0 mg enzyme protein/kg flour), a lipase hydrolyzing both non-polar and polar lipids. Additionally, dough extensibility and total work needed for fracture were lower for dough prepared from GS blends containing gluten isolated with Lipolase (at 5.0 mg enzyme protein/kg flour), a lipase selectively degrading non-polar lipids. In GS blend bread making, this resulted in inferior loaf volumes. Comparable GS blend properties were measured when using control gluten and gluten isolated with YieldMAX, a lipase mainly degrading N-acyl phosphatidylethanolamine. In conclusion, properties of GS blend model systems are altered when gluten prepared in the presence of lipases is used to a degree which depends on lipase specificity and concentration.     

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.     

Use of chemical redox agents and exogenous enzymes to modify the protein network during breadmaking – A review

During breadmaking, a continuous protein network is formed which confers visco-elasticity to dough. The properties of this protein network are highly dependent on the characteristics of the gluten proteins of the wheat flour. A good quality (highly elastic) gluten network retains the carbon dioxide that is produced by the yeast, giving dough and bread with optimal properties. However, the properties of the gluten proteins can differ substantially between wheat flours and are highly dependent on genetic, environmental and post-harvest conditions. Deficiencies in wheat quality for breadmaking can be overcome by incorporating exogenous components which alter the functionality of the gluten proteins during breadmaking. These include additives (e.g. potassium bromate, iodate, chlorine dioxide and chlorine, azodicarbonamide, ascorbic acid and peroxides) and enzymes affecting protein crosslinking. Transglutaminase, glucose oxidase, hexose oxidase and laccase all promote the formation of covalent bonds between gluten proteins and, hence, can serve as alternatives to chemical bread improvers.     

A suspension model of the linear viscoelasticity of gluten doughs

We regard gluten dough as a mixture of gluten, starch and water. We show that stress intensification around the starch particles enables one to describe the rapid strain softening of dough at low strains. The starch is in the form of a combination of A-particles (close to oblate spheroids) and B-particles (almost spherical). This suggests that a suspension theory should be able to account for the linear viscoelastic properties of doughs. We develop a new representation for the prediction of the linear viscoelastic properties of a viscoelastic matrix (gluten) with embedded oblate spheroids and spherical particles. The calculations are compared with experiments on gluten mixes derived from an Australian Baker’s flour. We note that the non-sphericity of the A-particles is very important in stiffening the gluten matrix and also that the effective volume fraction of the starch is greater than that calculated by assuming a starch density of 1.4 g/ml.     

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.