Aeration of bread dough influenced by different way of processing

The effect of steady shearing versus z-blade mixing on mechanical aeration and gas retaining ability of the dough during processing and subsequent proofing and bread baking stages was investigated. Reduction in moisture content led to reduction in both static and dynamic densities of z-blade mixed dough. At low moisture content, dough had higher consistency and tended to physically entrap more air bubbles upon processing, leading to a higher dough volume and, thereby a low density. The results showed that both processes led to similar mechanical aeration as measured by static dough density immediately after processing. Shearing at a low rotational speed, led to similar proofing dough volume as z-blade mixing did. Nevertheless, both dough expansion test and breadmaking trials showed a significant reduction in gas retaining ability of sheared dough, especially at higher rotational speeds. This is explained by the fact that higher shear rates could break up the gluten network and negatively influence gas retaining ability. The results revealed the influence of processing conditions; e.g. the type of deformation flow on dough aeration. Furthermore, it was shown that rotational speed in the shearing system influences the aeration and gas holding ability of the dough during proofing and baking processes.     

Application of epifluorescence light microscopy (EFLM) to study the microstructure of wheat dough: a comparison with confocal scanning laser microscopy (CSLM) technique

To study dough microstructure, epifluorescence light microscopy (EFLM) combined with digital image processing software was used, which enabled an improved image quality. A comparison was made between EFLM and confocal scanning laser microscopy (CSLM) methods. Both techniques were satisfactorily able to demonstrate changes in the dough microstructure upon different stages of z-blade mixing. Dough mixed for a shorter time (under-mixed) showed a heterogeneous structure with coarse protein domains and clusters of starch due to local segregation or de-mixing effect. Increasing mixing time (optimal mixing) led to development of interconnected gluten network covering starch granules throughout the dough, representing optimal development. Over-mixing led to formation of a homogeneous dough microstructure in which the gluten phase showed a fine distribution throughout the dough. Using a double staining method in the preparation of samples for both microscopic techniques it was possible to observe gluten network structures together with starch granules. Moreover, special features of image processing software described in this study enabled us to improve EFLM images and to obtain comparable images with CSLM. This could favour a low cost and a convenient microscopic observation of biomaterials.     

Mixing behaviour of a zero-developed dough compared to a flour–water mixture

The Z-blade mixing behaviour of zero-developed (ZD) doughs from the flours of two wheat cultivars of different gluten strength was compared to that of conventionally mixed dough made from the same flours. In farinograph experiments, use of ZD dough led to shorter development time (with less energy requirement), less stability time, and consequently earlier breakdown compared to conventional mixing of the corresponding flour–water mixture. Mixing of ZD doughs led to an almost similar decrease of glutenin macro-polymer (GMP) wet weight as that of doughs prepared from flour–water mixtures. However, comparison of wet weight of re-assembled GMP revealed that until time-to-peak (TTP) mixing, there was no difference in GMP recovery with respect to the starting material used in the z-blade mixing experiments. Beyond TTP, recovery of GMP in doughs prepared from both starting materials was reduced. The results of large-strain deformation rheology showed strong visco-elastic behaviour as characterised by the highest values of fracture properties (except ε_H), followed by a decline in those properties upon further mixing for doughs mixed from both flour–water mixture and ZD dough from both types of wheat cultivars. It was concluded that at mixing regimes before TTP, there was no difference between ZD doughs and flour–water mixtures in the mixer. When ZD dough is used as a starting material for dough preparation instead of flour, extra care should be taken not to over-mix the developing dough.     

Dough processing in a Couette-type device with varying eccentricity: Effect on glutenin macro-polymer properties and dough micro-structure

Dough mixing involves a combination of different deformation flows, e.g. shear and elongation. The complicated nature of mixing process makes it difficult to understand dough processing at a mechanistic level. A new Couette device allowed the effects of shear flow on the physical properties of glutenin macro-polymer (GMP) and micro-structure formation of the dough to be studied. Steady shear deformation using concentric Couette-type flow did not decrease GMP content or size of glutenin particles. Confocal scanning laser microscopy revealed the formation of interconnected gluten domains indicating the development of a gluten network. In an eccentric Couette configuration the results depended on the degree of eccentricity. A higher degree of eccentricity and a longer processing time led to considerable reduction in GMP content and size of glutenin particles. The micro-structural change in the narrow gap regions of the eccentric cell occurred early in processing, leading to a break up of large protein domains, and a microscopically more homogeneous dough. Transient high shear flow led to elongation and break up of the macroscopic gluten network. In low shear regions of the eccentric cell (wider gap settings), reformation or aggregation of protein domains was observed. Thus, the gluten aggregation–break up mechanisms are strongly influenced by the local flow profile in a conventional mixer. The impact of different types of shear flow must be taken into account in the design of dough mixers.     

Effects of vacuum mixing and mixing time on the processing quality of noodle dough with high oat flour content

The effects of different mixing parameters (vacuum mixing and mixing time) on oat (70% oat flour) and wheat noodle dough were investigated on the basis of textural properties and gluten formation. The results showed that at a vacuum degree of −0.06 MPa and mixing time of 10 min, oat and wheat dough sheets exhibited the highest resistance to extension and glutenin macropolymer (GMP) content, and had the most compact and uniform gluten network. Compared with wheat noodle dough, oat dough had lower resistance to extension, lower tightly bound water content, and higher GMP content. Microstructural examination showed that oat noodle dough had a more aggregated distribution of gluten protein compared with wheat noodle dough under the optimum mixing parameters. Furthermore, the poor binding ability of vital wheat gluten with water molecules caused the indexes of oat noodle dough to be more strongly affected by the changes in mixing parameters than wheat noodle dough.     

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.     

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.     

Migration of gluten under shear flow as a novel mechanism for separating wheat flour into gluten and starch

This paper describes a novel principle for the separation of wheat flour into starch and gluten in a concentrated medium. The process is based on the use of simple shear flow in a cone-and-cone device. The separation takes place in two steps. Initially, local segregation of gluten and starch phases occurs, leading to formation of macroscopically visible gluten patches distributed throughout the dough. This local segregation can be understood by considering the dough as a visco-elastic matrix containing an inert filler (starch). Further shearing leads to aggregation of those patches and migration (large-scale separation) towards the apex of the cone. As a result, the wheat dough is separated into a protein-poor fraction, containing less than 4% protein, and a protein-rich fraction containing almost 50% protein on a dry weight basis. However, under the process conditions used, upon a very long shearing, a redistribution of the aggregated gluten structures in the starch phase was observed, demonstrating a processing limit for the separation performance. Compared to traditional processing, the separation process presented shows opportunities for producing high quality gluten accompanied with significant water savings. Considering the fact that simple shear flow in steady rate is less harmful to gluten quality, such a separation process could benefit gluten quality.     

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.     

The effect of sodium chloride on gluten network formation and rheology

Gluten samples were obtained from two wheat flours with different levels of total protein in the presence or absence of sodium chloride (2% flour base). The dynamic oscillation rheology, large extensional deformation, confocal laser scanning microscopy (CLSM), transmission electron microscopy (TEM) and chemical analysis of disulfide bond linkages and the ratio of polymeric glutenins and monomeric gliadins were used to investigate the effect of salt on the structure and rheological properties of gluten. CLSM and TEM images showed that NaCl caused the gluten to form fibrous structure. The presence of NaCl increased non-covalent interactions and β-sheet structure, measured by FTIR, in gluten proteins. The gluten matrix formed with salt resulted in higher tan δ values corresponding to a less elastic network when measured using oscillatory rheometry. Large deformation extensional measurements showed that the maximum force to fracture were lower for the gluten samples prepared in the presence of NaCl. The results from this study indicate that changes in the solvent quality due to the presence of NaCl during dough mixing result in different molecular conformation and network structure of gluten proteins which contributed to the differences in the rheological properties.     

Rheological Properties of Dough During Mechanical Dough Development

During mechanical development dough is subjected to both shear and extensional deformations. Thus, it is expected that both flow conditions contribute to the development of dough. In order to monitor rheological changes, occurring during mixing, shear and extensional properties of dough prepared with two flours of different strength and various levels of mixing energy were determined using fundamental rheological methods. Rheological measurements included: small deformation and large deformation (shear test), planar extensional flow and a combined shear/extensional flow test, namely extrusion test. Results obtained in this research showed that, during mixing, dough develops with an increase in both apparent shear and extensional viscosities. For all the tests, plots of the measured rheological properties as a function of the mixing energy resembled typical mixing curves. This indicated that the increase in the power drawn by the mixer motor is due to the increase in both apparent shear and extensional viscosities. After peak dough development these properties decreased synchronously with the mixing curves. Results from small deformation shear tests exhibited large variability, particularly when non-mixed and underdeveloped doughs were tested. This variability was associated with poor water distribution in the sample due to insufficient mixing. Results of large deformation tests, including shear, planar extensional flow and the extrusion test, were less variable and showed that mixing and type of flour affect the rheological properties of dough.     

Effect of Water Unextractable Solids on Gluten Formation and Properties: Mechanistic Considerations

A miniaturised set-up for gluten-starch separation was used to systematically study the effect of water unextractable solids (WUS) on the formation and properties of gluten. The results showed that WUS not only have a negative effect on gluten yield, but also affect gluten and glutenin macropolymer (GMP) composition and rheological properties. The negative effect of WUS on gluten yield could be compensated for to a large extent, but not completely, by increasing mixing time and mixing water. Adding xylanase can effectively counteract the effect of WUS. On the basis of these results we hypothesize that WUS interfere with gluten formation in both a direct and an indirect way. WUS interfere indirectly by competing for water and thus changing conditions for gluten development. This effect can be corrected for by the combination of adding more 0·2% NaCl solution during dough mixing and a longer mixing time. The particulate nature of WUS requires that the direct effect occurs through an interaction between WUS particles and gluten particles. Both effects of WUS can be counteracted through the use of xylanase.     

The effect of mixing on glutenin particle properties: aggregation factors that affect gluten function in dough

Previously we reported that the SDS insoluble gel-layer: the Glutenin Macro Polymer (GMP) can be considered as a gel consisting of protein particles. These glutenin particles have a size of about 10⁻¹–10² μm and consist of HMW-GS and LMW-GS only. In GMP isolates from flour, the particles are spherical. In isolates from dough, glutenin particles have lost this shape. This seems relevant, since mixing disrupts the particles and the mixing energy required for dough development correlated with the glutenin particle size in flour. The question studied in this paper is how changes at a glutenin particle level affected the subsequent process of gluten network formation during dough rest and if this could be used to explain resulting dough rheological properties. To this end, we studied how various mixing regimes affected the dough properties during and after resting (elasticity). We cannot fully explain the differences in the final dough properties observed using parameters such as the quantity of GMP in flour, the quantity of re-assembled GMP in dough and the size of re-assembled glutenin particles. However, other parameters were found to be important: (1) the Huggins constant K′ reflecting the tendency of glutenin particles to interact at level II of the Hyperaggregation model; (2) the composition of glutenin particles affecting the potential to form smaller or larger particles and (3) for over-mixed dough, covalent re-polymerisation at the so-called level I of hyperaggregation. Using these parameters we can better explain dough viscoelasticity after resting.     

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.     

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.     

How gluten properties are affected by pentosans

In the gluten-starch separation process gluten is formed first as a result of breakdown of the gliadin-glutelin structures during mixing followed by their re-agglomeration. To date the effect of pentosans and enzymes have not been studied separately. A simple modification of TNO Glutomatic system enables pentosans, enzymes, and other materials to be added after the mixing step allowing the effect of these additives to be studied separately. Using this technique, we observed that re-aggregation of gluten proteins starts immediately after the first mixing step during the dough dilution phase. Xylanase addition prior to dough mixing can lead to ‘overdose effects’ but these were not observed when xylanase was added later during the re-agglomeration phase. We were able to distinguish between physical and chemical effects of pentosans on gluten formation. The effect of water-extractable pentosans is only partly related to its viscosity, a ferulic acid (FA) related reaction is more important. Pentosans affect the affect the agglomeration by increasing the size of the glutenin macropolymer particles. When the water-extractable pentosan effect is prevented by xylanase or FA addition, aggregation during dilution is more extensive and the glutenin macropolymer has a lower average particle size with a resulting difference in gluten rheology.     

Hydration,polymerization and rheological properties of frozen gluten-water dough as influenced by thermostable ice structuring protein extract from Chinese privet (Ligustrum vulgare) leaves

The effects of thermostable ice structuring proteins (TSISPs) extracted from Chinese privet (Ligustrum vulgare) leaves on water molecular state, dehydration of gluten proteins, secondary structure of proteins, glutenin subunit of glutenin macropolymer (GMP) and rheological properties of gluten doughs during frozen storage were investigated by nuclear magnetic resonance (NMR), attenuated total reflectance-Fourier transform infrared reflectance (ATR-FTIR), reversed phase-high performance liquid chromatography (RP-HPLC) and dynamic rheometry. After frozen storage for 5 weeks, the control sample showed dehydration of gluten proteins and mobility of water molecules in gluten dough increased, significantly indicating ice formation and water redistribution. Secondary structure of gluten proteins changed significantly, α-helix decreased and β-sheet increased. Glutenin subunits depolymerized, indicated by the decrease in high molecular weight glutenins/low molecular weight-glutenins (HMW/LMW) ratio. The decrease in elastic moduli (G′) and viscous moduli (G′') showed the deterioration of rheological properties of gluten dough. The addition of TSISPs inhibited the dehydration of gluten proteins, decrease in α-helix, increase in β-sheet and HMW/LMW ratio, resulting in improved rheological properties of gluten dough.     

Evidence that pentosans and xylanase affect the re-agglomeration of the gluten network

In the gluten-starch separation process gluten is formed first as a result of breakdown of the gliadin-glutelin structures during mixing followed by their re-agglomeration. To date the effect of pentosans and enzymes have not been studied separately. A simple modification of TNO Glutomatic system enables pentosans, enzymes, and other materials to be added after the mixing step allowing the effect of these additives to be studied separately. Using this technique, we observed that re-aggregation of gluten proteins starts immediately after the first mixing step during the dough dilution phase. Xylanase addition prior to dough mixing can lead to ‘overdose effects’ but these were not observed when xylanase was added later during the re-agglomeration phase. We were able to distinguish between physical and chemical effects of pentosans on gluten formation. The effect of water-extractable pentosans is only partly related to its viscosity, a ferulic acid (FA) related reaction is more important. Pentosans affect the affect the agglomeration by increasing the size of the glutenin macropolymer particles. When the water-extractable pentosan effect is prevented by xylanase or FA addition, aggregation during dilution is more extensive and the glutenin macropolymer has a lower average particle size with a resulting difference in gluten rheology.     

Rheological properties of kafirin and zein prolamins

The possibility of forming dough from kafirin was investigated and laboratory prepared kafirin was formed into a viscoelastic dough system. Measurements with Contraction Flow showed that dough systems prepared from kafirin and from commercial zein had the required extensional rheological properties for baking of leavened bread. The extensional viscosity and strain hardening of the kafirin and zein dough systems were similar to those of gluten and wheat flour doughs. The kafirin dough system, however, unlike the zein dough system rapidly became very stiff. The stiffening behaviour of the kafirin dough system was presumed to be caused by cross-linking of kafirin monomers. SDS-PAGE showed that the kafirin essentially only contained α- and γ-kafirin, whereas the zein essentially only contained α-zein. Since γ-kafirin contains more cysteine residues than the α-prolamin it is more likely to form disulphide cross-links, which probably caused the differences in stiffening behaviour between kafirin and zein dough systems. Overall the kafirin dough system displayed rheological properties sufficient for baking of porous bread. Kafirin like zein appears to have promising properties for making non-gluten leavened doughs.     

Influence of process parameters on yield and composition of gluten fractions obtained in a laboratory scale dough batter procedure

The influence of process parameters during the dough formation step on wheat flour gluten agglomeration and composition in a laboratory scale gluten–starch separation process was studied. In the process, in which a dough was transformed into a batter then poured over a set of vibrating sieves (400, 250 and 125 μm), increasing water contents, mixing times and speeds during dough development all had a positive effect on gluten agglomeration as indicated by an increased gluten protein recovery on the 400 μm sieve. This showed the importance of optimal gluten hydration and development at the dough making stage of the process. The total level of gluten recovered on the three sieves was not affected significantly by the variables. Changes in gluten agglomeration behaviour coincided with changes in the carbohydrate composition of the gluten fractions. When the gluten protein recovery on the 400 μm sieve increased, the arabinose and xylose contents of the fractions decreased, while the starch content increased.