Studies on the lipids of flour V. —Effect of air on lipid binding

Lipid distribution in mechanically developed doughs was found to be sensitive to relatively small amounts of air present in the dough mixer atmosphere. In doughs mixed to high work levels, less than 5 % air present in the nitrogen-air mixture fed into the mixing bowl was sufficient to cause a significant decrease in bound lipid. Although flour pigments were readily bleached by small amounts of air, lipid peroxides did not increase significantly until at least 50% air was present. Polyunsaturated free fatty acids were most susceptible to peroxidation although, at high work levels in air, peroxides were found in all lipid classes. While lipids bound during nitrogen mixing were readily released by subsequent mixing in air, the effect of air on lipid binding was not reversed by further mixing in nitrogen. Addition of peroxidised lipid to the dough did not prevent lipid binding in nitrogen-mixed doughs and it was concluded that a mechanism of lipoxidase-coupled site oxidation was responsible for the effect of air on lipid binding rather than the direct action of the lipid peroxides themselves.     

Isolation and characterisation of wheat flour proteins. III.—Amino-acid composition of Sephadex-gel fractionated wheat flour proteins

Wheat flour proteins were extracted with 0·05M -acetic acid, 0·01M -sodium pyrophosphate buffer, pH 7·0, and 0.05M -acetic acid after pyrophosphate buffer, and were fractionated by Sephadex-gel filtration. Seventeen amino-acids were determined by ion-exchange Chromatography in hydrolysates of 19 protein extracts and fractions. Concentrations of amino-acids with ionisable cationic chains and with hydroxylic groups, and of cystine, glycine, alanine, valine, leucine and methionine were higher, and of dicarboxylic acids, and their amides, and of proline were lower, in the pyrophosphate- than in the acetic acid-extracted proteins. The distribution of amino-acids in Sephadex-gel separated fractions depended on the elution volume of the fraction. It was affected also by the system used in extracting the proteins from the flour and by the presence of salt-dispersible proteins in the acetic acid-extractable proteins.     

Studies on the lipids of flour IV.—Factors affecting lipid binding in breadmaking

The binding of specific lipids during the high-speed mixing of doughs and in the resulting bread was studied in relation to mixer atmosphere. the presence of air was found to cause a fall in the linoleic acid content of the triglyceride lipids, an effect apparently related to the reduction in lipid binding. Replacement of the free lipid of flour (the main source of triglyceride linpleate) by extra shortening fat caused a large increase in bound lipid in doughs mixed either in air or in nitrogen, together with the drop in bread quality and volume observed previously. It was concluded that the natural free lipid of flour plays an important part in modern breadmaking processes both in responding to the atmosphere in the dough mixing chamber and in its effect on the binding of shortening triglyceride during dough development.     

The effect of lipoxygenase action on the mechanical development of wheat flour doughs

The mechanical development of dough has been followed by measurement of the relaxation time of dough samples after mixing to various levels of work input. Parallel determinations of free and bound lipid have also been made. When doughs were mixed in air, addition of full-fat, enzyme-active soya bean flour (subsequently referred to as “soya flour”) resulted in an increase in relaxation time, particularly at higher work levels. The magnitude of this improvement increased with increasing work input and was dependent on the rate of work input. Addition of soya flour also enabled a higher level of mechanical work to be introduced before dough breakdown occurred. When doughs were mixed under nitrogen, or when the soya flour was heat denatured, no change in the rheological properties compared with the respective control doughs was found. The release of bound lipid, which occurred during dough development in air in the presence of soya flour, could also be induced by adding purified lipoxygenase to the dough, together with linoleic acid as a substrate. This resulted in rheological changes similar to those observed using soya flour. However addition of enzymically pre-peroxidised lipid to doughs mixed in nitrogen was without effect on relaxation times. These findings suggest that lipid release is linked with structural changes in dough protein and provide further support for a mechanism of coupled oxidation of protein -SH groups by lipoxygenase.     

Effects of oxidizing agents and defatting on the electrophoretic patterns of flour proteins during dough mixing

3 ) or ascorbic acid at levels of 50, 100 and 150 mg/kg. The effects of mixing, lipid extraction and additives on dough proteins were studied using electrophoresis. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis results for the 50 % 1-propanol insoluble fractions of both varieties showed that the relative band intensities of the flours were more intense than those of the optimum mixed control doughs. KBrO₃ did not affect the extractability of protein in optimum mixed dough, while ascorbic acid reduced the extractability. Generally, overmixing caused a decrease in relative band intensities. The reductions in the relative band intensities were especially noticeable in the oxidant-added doughs. The relative band intensities of the defatted samples were more intense than those of the undefatted ones. Received: 7 December 1999 / Revised version: 14 February 2000     

Stability of the molecular weight distribution in wheat flour proteins during dough making

The dissociating solvent 'AUC, consisting of urea (3 M), acetic acid (0.1 M), and cetyllrimethylammonium bromide (0.01 M) dissolved in water, facilitates comparison of the molecular weight distributions in wheat flour and doughs by means of gel filtration on Sephadex G-200. Dough making raises protein extract-ability from about 95% to nearly 100%. the increase is found in the high molecular weight (glutenin) fractions. No other measurable change is found in the molecular weight distribution, even after over-mixing or after the use of several chemical additives. Hence, intermolecular disulphide reactions between glutenins and the other proteins appear to be insignificant in normal doughs. However, thiols and sodium sulphite do shift the distribution to lower values and give a new peak corresponding to a molecular weight between 80,000 and 100,000. Continuous speetrorphotometric measurement of the effluent from a gel column can be made accurately by using a siphoning cell of simple design.     

Effects of dough mixing and oxidising improvers on free reduced and free oxidised glutathione and protein-glutathione mixed disulphides of wheat flour

Changes in free reduced glutathione (GSH), free oxidised glutathione (GSSG) and protein-glutathione mixed disulphides (PSSG) during dough mixing were monitored. After a rapid decrease in the GSH content and an increase in the GSSG content, the contents of both GSH and GSSG decreased progressively, whereas the PSSG content increased, as a simple flour-water dough was mixed. Total glutathione (GSH plus GSSG plus PSSG) levels in simple flour-water doughs and doughs treated with ascorbic acid or potassium bromate remained essentially constant during dough mixing, indicating that the reactions in which glutathione is involved are simple oxidations of sulphydryl (SH) groups to disulphide (SS) bonds and SH/SS interchange reactions. Yeast contributed high levels of GSH and GSSG to doughs, but analysis of dough aqueous phases (liquors) and the similarity of the PSSG contents of simple and yeasted flour-water doughs suggested that the yeast GSH and GSSG were largely unavailable to react with flour proteins. The GSH content of ascorbic-acid-treated and yeasted dough decreased rapidly on wetting the flour, the magnitude of the decrease indicating that the ascorbate oxidation system oxidised the yeast intracellular GSH as well as the flour GSH. With further mixing, the GSH content of the ascorbate dough remained constant at low levels similar to those of the simple flour-water dough. The GSSG content of the ascorbate dough increased rapidly on wetting the flour, but declined as dough mixing continued. The PSSG content of the doughs increased markedly as dough mixing proceeded to the optimum and then stabilised. The increase in the PSSG content lagged behind the rapid oxidation of GSH to GSSG. Potassium bromate caused a pattern of changes similar to those observed for ascorbate, but the changes in GSH and PSSG contents were smaller in magnitude. The results indicate that the changes in the different glutathione pools and the effects of oxidising bread improvers are rather more complex than envisaged previously, particularly the effects on PSSG.     

The effect of lipoxygenase action on the mechanical development of doughs from fat-extracted and reconstituted wheat flours

The mechanism by which soya lipoxygenase enzyme action improves the Theological properties of wheat flour doughs during mechanical development in air has been investigated further. Free-lipid extraction, reconstitution and replacement experiments have shown that the rheological effect of lipoxygenase action, which is consistent with an oxidative improvement of the dough proteins and may also result in extended mixing tolerance, only occurred in the presence of an oxidisable, polyunsaturated, free-lipid substrate. Addition of this substrate in an oxidised state (produced either by autoxidation or enzyme-oxidation) to doughs mixed from fat-extracted flour under nitrogen resulted only in a small rheological improvement, greater for the autoxidised than the enzyme-oxidised lipid, but in no way comparable with the large rheological effect of lipoxygenase action during dough mixing in air. Furthermore, the presence of an antioxidant, nordihydroguaiaretic acid (NDGA), during dough development, although greatly inhibiting peroxide formation, only marginally impaired the rheological improvement due to lipoxygenase action. Additional evidence is therefore provided for a coupled oxidation mechanism being responsible for the rheological effect, since lipoxygenase-catalysed oxidation actively occurring in the dough during mixing appears to be the fundamental requirement, irrespective of whether the primary oxidation products lead to lipid peroxides or oxidised NDGA.     

Rheological properties of wheat flour doughs I.—Method for determining the large deformation and rupture properties in simple tension

A method and an apparatus are described for determining the large deformation and rupture properties of wheat flour doughs in simple tension. Dough rings are submerged in a liquid of matching density to prevent the dough from deforming under its own weight and are stretched at constant rates of extension until rupture occurs. Stress-strain curves obtained at different temperatures and extension rates on doughs prepared from a medium strength flour and a weak flour are presented, and the effects of rest period, mixing time, and mixing atmosphere on dough properties are discussed.     

The formation and properties of wheat flour doughs

Among the cereal flours, only wheat flour will form a viscoelastic dough when mixed with water. The viscoelasticity appears to be because the gluten proteins are water compatible and thus will swell and interact. The gluten protein's large molecular size and low charge density appear to allow them to interact by both hydrogen and hydrophobic bonds. Wheat flour doughs are also unique in their ability to retain gas. This property appears to result from a slow rate of gas diffusion in the dough. The third major unique property of wheat flour doughs is their ability to set in the oven during baking, and thereby to produce a rigid loaf of bread. Although not clearly understood, this appears to be a heat-induced crosslinking of the gluten proteins.     

Dough aeration and rheology: Part 3. Effect of the presence of gas bubbles in bread dough on measured bulk rheology and work input rate

The effects of dough aeration on measured bulk rheology of dough were investigated by preparing doughs with various gas contents and assessing their rheology under large deformation biaxial extension using the SMS Dough Inflation System. Doughs were mixed to various gas contents using a laboratory‐scale mixer, the Tweedy 1, at various mixer headspace pressures. In order to consider the effect of oxidation processes, doughs were mixed in air, in nitrogen and in a controlled oxygen‐nitrogen atmosphere in which the partial pressure of oxygen was kept constant. In the latter two cases a flow rate of gas through the mixer was maintained, while the doughs mixed in air had a static headspace. Dough aeration was quantified using density measurements. The dough rheology tests were performed under two different modes: constant volumetric air flow rate; and constant strain rate at two levels, 0.1 and 0.05 s⁻¹. Analysis of the stress–strain data of an inflating dough bubble using an exponential model enabled the strain hardening index, failure strain and failure stress to be determined. The strain hardening index, failure strain and failure stress decreased with gas content, indicating that gas bubbles in dough disrupt the integrity of dough structure. The faster strain rate tended to give higher values of all parameters and to be more discriminating, while testing at a constant volumetric air flow rate gave lower values. Doughs mixed under a constant partial pressure of oxygen resulted in the strain hardening index being minimally affected by the gas content, although failure strain and failure stress decreased with increasing gas content in the dough. The flow of gas through the mixer appeared to affect dough aeration and rheology, and the rate of work input was found to increase with increasing mixer headspace pressures. These suggest that dough aeration not only affects the rheology of static dough through the physical presence of gas bubbles following mixing, it also affects the development of its rheological properties within the mixer, through the physical presence of the bubbles affecting the rate of work input that develops the doughs, and also through the turnover of air that supplies oxygen to facilitate this development. Copyright © 2005 Society of Chemical Industry     

Effect of soybean proteins on gluten depolymerization during mixing and resting

BACKGROUND: Gluten and soy proteins interact as a consequence of dough mixing; however, there is no evidence of the effect of soy protein on gluten depolymerization. The aim of this study was to assess the depolymerizing effect of soy protein on gluten network after mixing and resting of mixed doughs. Therefore, the changes in glutenin macropolymer (GMP) content, protein composition and free sulfhydryl content were evaluated. RESULTS: The protein profile from gluten–soybean blends, obtained by multistacking SDS‐PAGE, showed differences when compared to gluten profile. Soy and gluten proteins were extracted together with SDS buffer, which showed that soy proteins remained associated to insoluble wheat proteins even after hand‐washing the dough to obtain gluten. GMP content was determined to analyze the effect of soy protein on GMP gel formation. Protein content of GMP obtained from flour mixes and doughs was increased by inactive soy flour because soy proteins became insoluble and precipitated together with GMP. Active soy flour decreased GMP content due to gluten depolymerization. CONCLUSION: Soy proteins were associated to wheat protein through physical interaction and covalent and non‐covalent bonds during mixing and resting. These interactions produced large and medium‐size polymers. This fact increased SDS solubility of insoluble gluten proteins, producing a weakening of the gluten network. Physicochemical status of soy protein in the product had a great influence on how wheat–soy proteins interact. Copyright © 2007 Society of Chemical Industry     

Effects of processing on biochemical and rheological properties of wheat gluten proteins.

Protein solubility increases during mixing at water absorption suitable for bread doughs. The changes that result from heat treatment may be much greater than the differences normally found among wheat varieties. Sheeting brings about a reduction in the gluten content and increase in gel protein content of doughs. This may be due to increase in temperature during sheeting as a result of work input to the dough resulting in protein denaturation. Drying at higher temperatures causes denaturation of pasta dough protein. In addition, it has been shown that high temperatures contribute to the formation of the protein network. During extrusion processing proteins are denatured and chemical bonds are weakened as result of heat and shear through the extruder.     

Small and large deformation rheology for hard wheat flour dough as influenced by mixing and resting

ABSTRACT:  The effects of mixing and resting on the physicochemical properties of doughs prepared with strong and weak hard wheat flours were investigated, specifically concerning aspects related to their rheological behavior and molecular mobility. Small deformation dynamic tests showed that, during the initial resting period, the complex modulus G * decreased and phase angle decreased for undermixed dough, whereas overmixed dough showed opposite trends. G * values for optimally mixed dough did not vary during the resting period investigated. This was more obvious for the strong dough. Large deformation tests more clearly showed differences among optimal, under-, and overmixed dough, and also between doughs prepared with strong and weak flour. Optimally mixed dough exhibited the highest peak stress and strain for both samples. In addition, the peak stress of dough prepared with the strong flour was higher than that of dough prepared with weak flour. Inconsistent results between small and large deformation tests implied that small and large deformation tests reflected different structural aspects of dough. NMR measurements were performed to estimate the relaxation properties of the sample upon resting. Decreased water mobility during resting, indicated by decreasing T ₁ relaxation time, was possibly attributed to increasing molecular interactions caused by continued hydration. Evidence of additional molecular interactions created by mixing was also observed.     

Extraction and fractionation of wheat flour proteins

With the aid of a 1.5% sodium dodecyl sulphate solution (SDS) the proteins of two Dutch wheat flours were separated into an SDS-soluble and an SDS-insoluble fraction. The SDS-soluble fraction was fractionated with the aid of ethanol precipitation and gel filtration chromatography into albumins, globulins, gliadins, glutenins II and III, and glutelins I, II, III and IV. The SDS-insoluble proteinaceous material was separated into glutenins I and glycoproteins. The protein fractions were identified with the aid of SDS-polyacrylamide gel electrophoresis and amino acid analysis. The glutenins I and II consist of A, B and C subunits. Their molecular weights ranged from about 600 000 to 10 million. The glutenins III consist only of B and C subunits and their molecular weights ranged from 300 000 to 600 000. A hypothesis explains how the glutenins III are the precursors for glutenins I and II and how they contribute to gluten's structure. The glutelins I consist of α β and γ subunits and their molecular weights ranged from about 600 000 to several millions. The glutelins II consist also of α β and γ subunits with a molecular weight of about 300 000 to 600 000, while the molecular weights of the glutelins III and IV ranged from 10 000 to 200 000. All glutelins are insoluble in 70 % aqueous ethanol and 5 M urea, but soluble in SDS and 0.1M NaOH. Only the glutelins III and IV are soluble in 0.1M HAc. The globulins consist of a heterogeneous fraction of components, their molecular weights arranged from 98 000 to 10 000. The gliadins form a heterogeneous but homologous group of polypeptides with an average molecular weight of 35 000. The albumins consist of components with molecular weights lower than 15 000.     

Enhancement of soluble dietary fibre, polyphenols and antioxidant properties of chapatis prepared from whole wheat flour dough treated with amylases and xylanase

BACKGROUND: Chapati preparation involves various processing steps such as mixing the flour into dough, sheeting and baking. During these processing steps, flour components are likely to undergo changes in their nutrient and polyphenol composition and their antioxidant properties due to phenol‐mediated crosslinking of proteins and carbohydrates. Therefore, in the present study, changes in nutritional, nutraceutical and antioxidant properties of chapatis prepared from doughs treated with amylases and xylanase were determined. RESULTS: An increase in insoluble dietary fibre content and a decrease in soluble polyphenol content were observed during preparation of control chapatis from whole wheat flours. However, significant increases in soluble dietary fibre and soluble polyphenol contents were observed in chapatis prepared from amylase‐treated doughs compared with control chapatis. Extracts of chapatis prepared from amylase‐ and xylanase‐treated doughs showed better antioxidant properties than extracts of control chapatis. Among these enzyme treatments, chapatis prepared from amylase‐treated doughs showed better antioxidant properties than chapatis prepared from xylanase‐treated doughs. High‐performance liquid chromatography analysis of extracts of chapatis prepared from doughs treated with amylases showed the presence of potential antioxidant phenolic acids such as caffeic, gentisic and syringic acids in addition to the phenolic acids present in control chapatis. CONCLUSION: Treatment of doughs with amylases increased the contents of soluble dietary fibre and soluble polyphenols as well as improving the antioxidant properties of chapatis. Copyright © 2011 Society of Chemical Industry     

Extraction and fractionation of proteins of sorghum kernels

Using multi-step extraction, sorghum flour was shown to contain 28 % albumin and globulin, 30 % pro-lamin and 12 % soluble glutelin. The remaining 30 % proteins are difficult to extract and may be bound. Alkaline copper sulphate and sodium sulphite were not very effective in extracting the protein, nor were dilute acetic acid and sodium pyrophosphate buffer. A virtually complete extraction of proteins could be achieved by 8 M urea with 0·1 M sodium dodecyl sulphate; 8M urea alone extracted 60% of the total protein. Sephadex gel (G 100) filtration showed that the normal solvent extracts contain three major fractions, one a low molecular weight, dialysable fraction, one of high molecular weight proteins elected around the void volume, and an intermediate fraction. 8 M urea extracts yielded three main peaks and six minor peaks. From a standard curve, approximate molecular weights of these main fractions were estimated as 82,000, 46,000 and 2000, respectively. Disc electrophoresis on polyacrylamide gel gave compatible results. Single bands were observed in dialysed aqueous, saline and alcoholic extracts, two sharp bands in the case of sodium hydroxide extracts. Extraction with 8 M urea gave three main and five minor bands on disc electrophoresis.     

Changes in urea-dispersibility of proteins during maturation

The quantity of proteins involved in gluten formation in wheat harvested at various stages of development of the grain was inversely related to absorbence at 280 mμ of 3 M -urea extracts. The amount of protein, as determined by the biuret procedure, in the 3·0 M -urea extracts did not change during maturation. Thus the decreasing absorbence (280 mμ) with increasing maturity between 27 to 17 days pre-ripe seems to indicate a gradual increase in molecular weight and complexity of the synthesised proteins. The changes in formation of gluten proteins were accompanied by a decrease in water- and salt-dispersible proteins, and paralleled improvement in bread-making properties of the flours.     

Fractional extraction of cereal flour proteins

Wheat (Wichita), rye, barley, oat and maize flours were successively extracted with 0.04 m sodium chloride solution (3 × ), water (3 × ), 70% ethanol (3 ×) and 0.1 N acetic acid. The fractions and residues were analysed for nitrogen and subjected to starch-gel electrophoresis. Saline extractions chiefly removed the albumins and globulins of all five cereals. The prolamins of wheat, rye and barley appeared mostly in the three aqueous extracts but also, particularly, those of very low mobility, occurred in the alcoholic solutions. The acetic acid-soluble fractions of wheat, rye and barley were shown by electrophoresis under reducing conditions to be crosslinked by S.S bonds; those of rye and barley showed some characteristic differences from glutenin. Oats and maize were practically devoid of this fraction. Most of the protein of oats and maize was insoluble. Electrophoresis patterns after reduction were obtained from insoluble fractions, though there were differences among the genera. Both overall protein solubility and the quantity of the acetic acid-soluble fraction were correlated with baking quality. The presence of covalent crosslinks other than S.S in glutelin fractions is suspected.     

Effects of processing on biochemical and rheological properties of wheat gluten proteins

Protein solubility increases during mixing at water absorption suitable for bread doughs. The changes that result from heat treatment may be much greater than the differences normally found among wheat varieties. Sheeting brings about a reduction in the gluten content and increase in gel protein content of doughs. This may be due to increase in temperature during sheeting as a result of work input to the dough resulting in protein denaturation. Drying at higher temperatures causes denaturation of pasta dough protein. In addition, it has been shown that high temperatures contribute to the formation of the protein network. During extrusion processing proteins are denatured and chemical bonds are weakened as result of heat and shear through the extruder.