Strain Hardening Properties and Extensibility of Flour and Gluten Doughs in Relation to Breadmaking Performance

The mechanical properties of flour–water doughs and hydrated gluten of different wheat cultivars were determined. Measurements were performed at small deformations (dynamic measurements) as well as at large deformations (biaxial extension measurements). Results of dynamic measurements of flour doughs related poorly to breadmaking quality. For hydrated gluten doughs, all having the same water content, it was found that glutens from wheat cultivars with good baking quality had higher values for the storage modulus,G, and lower values for the loss tangent. The relevant type of deformation around an expanding gas bubble is biaxial extension. Wheats with a good baking performance exhibited greater strain hardening and greater extensibility. The differences in strain hardening observed at 20 °C were also present at 55 °C. No clear effects of NaCl or emulsifiers on the biaxial extension properties of flour dough were found. Extensograms as well as Alveograms from the flour doughs showed that, in general, good baking flours exhibited stronger resistance to extension and a greater extensibility, but differences found were not directly related to the results of the baking tests. The results indicate that the baking performance of dough is related to a combination of at least three different rheological characteristics.     

Rheological and mechanical properties of bioplastics based on gluten- and glutenin-rich fractions

The present work aims to investigate the dynamic rheology at small strains and the equibiaxial extensional deformation at large strain of the glycerol plasticized dough of gluten- and glutenin-rich fractions and their mixture as well as the uniaxial tension deformation behavior of the compression molded bioplastics. The influence of glutenin-to-gluten ratio (GGR) on the rheological properties of the glycerol plasticized doughs and the crosslinked bioplastics were investigated. The results showed that the glutenin dough exhibits higher moduli and lower loss factor and equibiaxial deformability in comparison with the gluten dough. Addition of a glutenin-rich fraction to the gluten dough can improve elasticity at small deformation and extensional deformational stress at large deformations but result in reductions in extensibility of the compression molded bioplastics.     

Rheological and mechanical properties of bioplastics based on gluten- and glutenin-rich fractions

The present work aims to investigate the dynamic rheology at small strains and the equibiaxial extensional deformation at large strain of the glycerol plasticized dough of gluten- and glutenin-rich fractions and their mixture as well as the uniaxial tension deformation behavior of the compression molded bioplastics. The influence of glutenin-to-gluten ratio (GGR) on the rheological properties of the glycerol plasticized doughs and the crosslinked bioplastics were investigated. The results showed that the glutenin dough exhibits higher moduli and lower loss factor and equibiaxial deformability in comparison with the gluten dough. Addition of a glutenin-rich fraction to the gluten dough can improve elasticity at small deformation and extensional deformational stress at large deformations but result in reductions in extensibility of the compression molded bioplastics.     

Large Deformation Properties of Short Doughs: Effect of Sucrose in Relation to Mixing Time

Large deformation rheological properties of short doughs of various composition prepared under various mixing times were determined in uniaxial compression. Sucrose-syrup doughs exhibited prominent yielding and flow behaviour. Their apparent biaxial extensional viscosity decreased with increasing sucrose content. The stress-strain curves for the sugar-free doughs indicated a stronger elastic contribution to deformation than did those for the sucrose-syrup doughs. The deformability of the former doughs increased with increasing water content. Regardless of dough type, mixing time had a pronounced effect on dough consistency. In addition, it drastically changed the shape of the stress-strain curve for a sugar-free dough. These results are discussed in terms of the structure of short doughs. It is concluded that sucrose delays, if not inhibits, gluten development and promotes formation of a non-fat continuous phase, whereas mixing promotes formation of a continuous fat phase.     

Rheological Properties of Short Doughs at Large Deformation

Large deformation rheological properties of short doughs of various composition were determined in uniaxial compression. Apart from a sugar-free dough, all doughs studied showed pronounced yielding and flow behaviour. Yielding occurred at very small strain, indicating strong strain dependence. At small strain, systems were predominantly solid-like; at large strain they behaved more like strain-rate thinning liquids. The doughs showed large differences in apparent biaxial extensional viscosity, depending on fat and sucrose contents. It is concluded that yielding behaviour is strongly influenced by intact flour particles that act as defects in the material. The occurrence of such particles is determined by the presence of sucrose, which delays gluten development through its effect on solvent quality. The sucrose also facilitates formation of a non-fat continuous phase, since it increases the quantity of solvent.     

On the relationship between large-deformation properties of wheat flour dough and baking quality

Baking performance for bread and puff pastry was tested for Six European and two Canadian wheat cultivars and related to the rheological and fracture properties in uniaxial extension of optimally mixed flour–water doughs and doughs to which a mix of bakery additives was added. Extensive baking tests were performed as a function of water addition for puff pastry and as a function of water addition and mixing time for bread. For optimum baking performance, puff pastry doughs required lower water additions than bread doughs. Baking performance of the flours differed for the two products. For puff pastry, higher volumes were obtained per gram of flour than for bread. Puff pastry volume was positively correlated with optimum bread dough mixing time, while bread volume was not. Instead, bread volume was positively correlated with gluten protein content.All doughs exhibited strain hardening, a more than proportional increase of the stress with the strain. For all doughs fracture, stress and strain increased with increasing displacement speed of the hook and decreasing temperature. Large differences were observed between the cultivars regarding stress, strain hardening, strain rate-dependency of the stress, fracture stress and fracture strain. At both 25 and 45 °C, addition of a mix of bakery additives resulted in a decrease of the stress at relatively small strains and a significant increase of the strain hardening coefficient. Fracture strains remained the same or increased as a result of addition of the mix. Differences between flours regarding the strain rate and temperature-dependency of the fracture strain remained. The weaker the dough, the stronger the strain rate and temperature-dependency of the fracture strain.Puff pastry volume was positively correlated with strain hardening and negatively with the strain rate-dependency of the stress. In short, the stronger the dough, the higher the puff pastry volume. For bread, it were not the strongest doughs that gave the highest loaf volumes, but those with intermediate dough strength. Low volumes for puff pastry and bread were found for doughs having a low fracture stress and low strain hardening coefficients. Loaf volumes of flours with high dough strength (i.e. high stress-level and high strain hardening) gave intermediate loaf volumes. We concluded that a high stress can hamper the extensibility of dough films between gas cells, thus limiting the expansion of gas cells during fermentation and baking and hence the loaf volume that can be obtained.     

Improvement of zein dough characteristics using dilute organic acids

The replacement of gluten in dough products poses a major challenge. Preparing zein doughs in dilute acetic acid and lactic acid, such as produced during sourdough fermentation, was investigated. Increasing acid concentrations (0.7, 1.3 and 5.4% [v/v]) increased zein extensibility and reduced the stress and related parameters. Preparation of zein-maize starch/-rice doughs in dilute organic acids improved dough properties to the extent that the doughs could hold air and be inflated into a bubble by Alveography. Further, they exhibited similar Stability (P), Distensibility and deformation energy (W) to wheat flour dough. Confocal laser scanning microscopy revealed an ordered linear fibril network in zein and zein-rice flour doughs prepared in the dilute acids, which became uniform with increasing acid concentration. SDS-PAGE showed that the acids did not hydrolyse or polymerise the zein. FTIR indicated that the acidic conditions slightly increased the proportion of α-helical conformation in the zein doughs, possibly as a result of deamination. This conformational change may be responsible for the considerably improved zein dough properties. Zein doughs prepared in dilute organic acids show potential as a gluten replacement in gluten-free formulations.     

Stress–Relaxation Properties of Mixograph Semolina–Water Doughs from Durum Wheat Cultivars of Variable Strength in Relation to Mixing Characteristics,Bread- and Pasta-making Performance

Stress–relaxation behaviours of Mixograph semolina–water doughs prepared from Canadian durum wheat cultivars with diverse gluten strength were investigated and related to mixing characteristics, large deformation properties, and bread- and pasta-making quality. Semolina from «strong» (S) and «moderately strong» (MS) durum wheat cultivars required a longer Mixograph mixing time (4–5 min) and higher work input (140–196 Arbitrary Units) to mix to peak dough resistance (PDR) than «weak» (W) and «very weak» (VW) durum cultivars (2–3 min and 80–117 AU). Extensigraph maximum resistance to extension (R_max/E ratio) and Alveograph P/L (tenacity to length ratio) values were higher for doughs from S cultivars than for MS, W, and VW cultivars. Doughs from S cultivars exhibited higher storage modulus (G′) and lower tan δ values at all frequencies, and slower rates of stress relaxation as compared to MS, W, and VW cultivars. Stress relaxation (times to relax 50% (t₅₀) and 75% (t₇₅) of initial stress) indicated that stronger doughs, which had higher proportions of glutenins, took longer to reach these iso-relaxation states, regardless of their initial relaxation modulus value. The parameters t₅₀and t₇₅were also strongly correlated with dough mixing properties, Extensigraph R_max/E, Alveograph P/L, mixing energy, mixing time and loaf volume obtained by a long and a short bread-making process. However, for S cultivars loaf volume was 10 to 20% lower than that expected of bread wheat of comparable protein content. Stress relaxation data demonstrated no simple correlation to pasta cooking quality indicating that stronger gluten did not translate into a superior pasta cooking quality. Results are interpreted in the context of multimodal networks and transient networks with reversible crosslinks.     

Influence of gliadin removal on strain hardening of hydrated wheat gluten during equibiaxial extensional deformation

The aim of the present work has been to study the equibiaxial extensional deformation of doughs of gluten- and glutenin-rich fractions containing 40 wt% water subjected to lubricated squeezing flow with four different crosshead speeds at room temperature. The gluten dough shows strain softening and hardening in succession whilst the dough where the gliadins have been removed by alcohol extraction does not show strain hardening behavior but breaks immediately after strain softening. The equibiaxial extensional viscosity decreases with increasing strain rate at given strains, appearing as strain rate thinning behavior, which is stronger in the glutenin dough than in the gluten dough. The large extensibility with strain hardening in the gluten dough is due to the presence of gliadins acting as both plasticizers and promoters for the more extensible networks.     

Grain of high digestible,high lysine (HDHL) sorghum contains kafirins which enhance the protein network of composite dough and bread

The aim of this study was to determine whether protein body-free kafirins in high digestibility, high-lysine (HDHL) sorghum flour can participate as viscoelastic proteins in sorghum-wheat composite dough and bread. Dough extensibility tests revealed that maximum resistance to extension (g) and time to dough breakage (sec) at 35 °C for HDHL sorghum-wheat composite doughs were substantially greater (p < 0.01) than for normal sorghum-wheat composite doughs at 30 and 60% substitution levels. Functional changes in HDHL kafirin occurred upon exceeding its T_g. Normal sorghum showed a clear decrease in strain hardening at 60% substitution, whereas HDHL sorghum maintained a level similar to wheat dough. Significantly higher loaf volumes resulted for HDHL sorghum-wheat composites compared to normal sorghum-wheat composites at substitution levels above 30% and up to 56%, with the largest difference at 42%. HDHL sorghum-wheat composite bread exhibited lower hardness values, lower compressibility and higher springiness than normal sorghum-wheat composite bread. Finally, HDHL sorghum flour mixed with 18% vital wheat gluten produced viscoelastic dough while normal sorghum did not. These results clearly show that kafirin in HDHL sorghum flour contributes to the formation of an improved protein network with viscoelastic properties that leads to better quality composite doughs and breads.     

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.     

Dough and bread made from high- and low-protein flours by vacuum mixing: Part 1: Gluten network formation

The effects of reduced headspace pressure on the development of gluten network in doughs made from both high-protein flour (HPF) and low-protein flour (LPF) were investigated. The effect of vacuum mixing was found to be dependent on both flour-type and mixing-time. A significant increase in dough extensibility was observed when the HPF dough was mixed under moderate vacuum of −0.04 MPa for 3 min as compared to the one mixed under atmospheric pressure for the same duration. This was attributed to the formation of a more extensive gluten network associated with an increased disulphide bond density and a significantly higher β-sheet to β-turn ratio. On the other hand, over-mixing was observed in the LPF dough that was mixed for 5 min under atmospheric pressure. Applying moderate vacuum of −0.04 MPa allowed the LPF dough to withstand longer mixing time, as indicated by its increased disulphide bond density and biaxial extensibility compared to the control dough mixed under atmospheric pressure. Results of this study suggest the potential of applying vacuum to reduce the mixing time required for high protein flour and to prevent the over-mixing of low protein flour.     

Microstructure formation and rheological behaviour of dough under simple shear flow

In a z-blade mixer, both shear and extensional deformations contribute to the development of dough structure. I effect of simple shearing versus z-blade mixing at similar levels of work input, on the microstructure and uniaxial extensional properties of two doughs prepared from flours of different strengths. With respect to microstructure, mixing initially increased the formation of coarse protein patches, leading to a heterogeneous dough structure with a high fracture stress (σ_max) and significant strain hardening. These parameters decreased with prolonged mixing. This was accompanied by loss of glutenin macro polymer (GMP) wet weight and formation of a more homogenous microstructure. Prolonged mixing typically led to an over-mixed state. In contrast, prolonged simple shearing did not affect GMP content or strain hardening and gave enhanced shear banding. Confocal scanning laser microscopy revealed that short-term simple shearing induced structure formation in the direction of the shear flow for both flour types, followed by formation of shear-banded gluten structures both parallel and perpendicular to the direction of shear flow. Uniaxial extension of dough oriented parallel or perpendicular to the shear field did not reveal anisotropy. Apparently, the observed heterogeneity on a scale of ‘mm’ was not relevant for this type of rheology. Nevertheless, a relative weakening of dough strength (reduced fracture stress) was observed as a function of long-term shearing. This seems to be related to a local segregation effect caused by differences in visco-elasticity between the gluten phase and the starch granules. The results of this study reveal important features of the dough processing and underline the importance of not only work input, but also the type of deformation applied.     

Distribution and function of LMW glutenins,HMW glutenins,and gliadins in wheat doughs analyzed with ‘in situ’ detection and quantitative imaging techniques

The different gluten subunits, gliadins, LMW glutenins, and HMW glutenins have been reported to play different key roles in different type of wheat products. This paper studied the interaction between gliadin, LMW and HMW glutenins in soft, hard and durum semolina flour doughs during different stages of mixing. In order to see how do the gluten subunits (gliadin, LMW glutenin and HMW glutenin) redistribute during mixing, dough samples were taken at maximum strength and 10 min after maximum strength. The doughs have been mixed with the same level of added water (55%), therefore they all have different strengths values due to their changes in proteins content. Oscillatory rheological measurements were performed on the doughs. It has been found that HMW glutenins are relatively immobile because of their less molecular mobility and do no redistribute themselves especially at high strength for doughs such as hard wheat flour. LMW glutenins and gliadins on the other hand redistribute themselves at even at high dough strengths forming a more stable network. In weaker doughs such as soft wheat, the breakdown of the three proteins subunits is responsible for the decay in dough strength. We have also visualized how the greater amount of LMW glutenins in semolina is in constant interaction with HMW glutenins and gliadins allowing the dough to maintain a stable strength for an extended mixing time. Finally, we have found the ‘in situ’ detection and quantitative analysis techniques to be more sensitive to the changes occurring in the gluten network of the dough than the oscillatory rheological analysis.     

Effect of sodium chloride on gluten network formation,dough microstructure and rheology in relation to breadmaking

Sodium chloride (NaCl) is an essential ingredient to control the functional properties of wheat dough and bread quality. This study investigated the effect of NaCl at 0, 1 and 2%, (w/w, flour base) on the gluten network formation during dough development, the dough rheology, and the baking characteristics of two commercial flours containing different levels of protein (9.0 and 13.5%) and with different glutenin-to-gliadin ratios. Examination of the dough structure by confocal microscopy at different stages of mixing show that the gluten network formation was delayed and the formation of elongated fibril protein structure at the end of dough development when NaCl was used. The fibril structure of protein influenced the dough strength, as determined by strain hardening coefficient and hardening index obtained from the large deformation extension measurements. NaCl had a greater effect on enhancing the strength of dough prepared from the low protein flour compared to those from the high protein flour. The effect of NaCl on loaf volume and crumb structure of bread followed a similar trend. These results indicate that the effect of NaCl on dough strength and bread quality may be partially compensated by choosing flour with an appropriate amount and quality of gluten protein.     

Extensional Flow Studies of Wheat Flour Dough. I. Experimental Method for Measurements in Contraction Flow Geometry and Application to Flours Varying in Breadmaking Performance

An experimental method is reported for extensional measurements on wheat flour doughs in contraction flow geometry. In this method the transient stress under constant extension rate was measured, followed by stress relaxation measurement after cessation of flow. Four different wheat cultivars (with large differences in bread volume) were used to evaluate the usefulness of the method and to find how different parameters relevant to baking quality could be extracted from the data. The plot of transient viscosity against time showed the effect known as strain hardening, appearing at a Hencky strain roughly independent of the strain rate but differing between the wheat varieties. The relaxation rate curves for the four wheat varieties were similar except at the highest strain rate, 7·40 s⁻¹. It was decided in the following case study to use the highest strain rate, 7·40 s⁻¹and to extract three parameters from the stress growth plot and three parameters from the stress relaxation rate plot. In the case study, flours of 17 wheat varieties were mixed in the Mixograph to maximum resistance before being subjected to the contraction flow measurement. The six parameters extracted from each wheat flour dough measurement, together with the corresponding protein content, Zeleny value, and bread volume, were evaluated by multivariate analysis. A model for predicting the bread volume from the contraction flow parameters explained 94·6% of the variation in bread volume, while a model with Zeleny values and protein contents included explained 97·2%.     

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.     

Effects of composition on dough development and air entrainment in doughs made from gluten-starch blends

Gluten and starch are the two main ingredients of a wheat flour dough and it is expected that the extent of air occlusion into the dough would be affected by differences in their relative ratios. The objectives of this paper were to investigate the hydration and development of gluten and how these key events in dough mixing affected air occlusion in gluten-starch doughs. For gluten-starch doughs of the same gluten content, decreasing the water absorption shortened development time and decreased dough density. For formulations of the same water absorption, decreasing the gluten content prolonged the time to development and increased dough density, reflecting less net air entrainment into the dough. The ratios of gluten, starch and water strongly influenced the development of the dough into a good gas-holding material, with the extent of gas entrainment during mixing being evident in measurements of both dough consistency and dough development time.     

Effects of laccase,xylanase and their combination on the rheological properties of wheat doughs

The effects of Trametes hirsuta laccase alone and in combination with Aspergillus oryzae and Bacillus subtilis xylanases on dough extensibility were studied using the Kieffer test to determine the dough extensibility (E_x) and the resistance to stretching (R_max). Laccase treatment resulted in dough hardening: the R_max of dough increased and the E_x at R_max decreased as a function of dosage (5–50 nkat/g flour). Xylanases softened flour and gluten doughs. Hardening by laccases and softening by xylanases was weaker in gluten doughs. Dough hardening, observed in the laccase treatments, decreased as a function of dough resting time. The softening effect occurred especially at higher laccase dosages (≈50 nkat/g flour). The softening phenomenon was related to the laccase-mediated depolymerization of the cross-linked AX network. In combined laccase and xylanase treatments, the effect of laccase was predominant, especially at low xylanase dosage, but when xylanase was added to flour dough at high concentrations, the hardening effect of laccase on dough was decreased. In combined laccase and xylanase treatments in gluten doughs, similar decreases in laccase-mediated hardening were not seen.     

Endogenous redox agents and enzymes that affect protein network formation during breadmaking – A review

During breadmaking, wheat gluten proteins form a continuous network which is stabilized by disulfide bonds and modified by thiol/disulfide interchange reactions. This gluten network results in visco-elastic dough that holds together the other dough components and assists in retaining carbon dioxide. Wheat flour contains several components, enzyme co-factors and enzymes which can affect the formation and properties of the gluten network and, hence, the dough and bread characteristics. We present a brief overview of our current knowledge of the fate of gluten proteins during breadmaking, and how they are affected by endogenous wheat components (e.g. glutathione, cysteine and NAD(P)(H)) and enzyme systems (e.g. tyrosinase, peroxidase, the NADP-dependent thioredoxin and glutathione enzyme systems, protein disulfide isomerase, lipoxygenase, catalase and dehydrogenases).