Impact of redox agents on the extractability of gluten proteins during bread making

The gluten proteins gliadin and glutenin are important for dough and bread characteristics. In the present work, redox agents were used to impact gluten properties and to study gliadin-glutenin interactions in bread making. In control bread making, mixing increased the extractability of glutenin. The level of SDS-extractable glutenin decreased during fermentation and then further in the oven. The levels of extractable alpha- and gamma-gliadin also decreased during bread baking due to gliadin-glutenin polymerization. Neither oxidizing nor reducing agents had an impact on glutenin extractabilities after mixing. The redox additives did not affect omega-gliadin extractabilities during bread making due to their lack of cysteine residues. Potassium iodate (0.82-2.47 mumol/g of protein) and potassium bromate (1.07-3.17 mumol/g of protein) increased both alpha- and gamma-gliadin extractabilities during baking. Increasing concentrations of glutathione (1.15-3.45 mumol/g of protein) decreased levels of extractable alpha- and gamma-gliadins during baking. The work not only demonstrated that, during baking, glutenin and gliadin polymerize through heat-induced sulfhydryl-disulfide exchange reactions, but also demonstrated for the first time that oxidizing agents, besides their effect on dough rheology and hence bread volume, hinder gliadin-glutenin linking during baking, while glutathione increases the degree of covalent gliadin to glutenin linking. Keywords: Bread making; wheat gluten; gliadin-glutenin interaction; protein extractability.status: publishe

Heat-Induced Cross-Linking and Degradation of Wheat Gluten, Serum Albumin, and Mixtures Thereof

Some wheat-based food systems, such as cakes, cookies, and egg noodles, contain mixtures of animal and plant (gluten) proteins and are processed under (mildly) alkaline conditions. Although changes in these proteins during processing can affect end product quality, they have seldom been studied. This study investigated protein cross-linking and degradation during heating (0-120 min, pH 8.0, 50-130 °C) of (mixtures of) wheat gluten and bovine serum albumin (BSA). The decrease in protein extractabilities in sodium dodecyl sulfate containing buffer under (non)reducing conditions and the levels of (cross-linked) amino acids were measured. No indications for polymerization at 50 °C were found. Below 100 °C, BSA polymerized more readily than wheat gluten. Above 100 °C, the opposite was observed. The kinetics of heat-induced polymerization of a 1:1 gluten-BSA mixture were similar to that of isolated gluten, implying that gluten decelerated BSA denaturation. Severe heating (130 °C, >15 min) induced degradation reactions in gluten but not in BSA. At all conditions used in this study, disulfide (SS) bonds contributed to the extractability loss. In addition, above 110 °C, β-elimination of cystine led to non-SS cross-links. Intramolecular SS bonds more often transformed in intermolecular non-SS bonds in BSA than in gluten.status: publishe

Impact of redox agents on the physico-chemistry of wheat gluten proteins during hydrothermal treatment

The impact of the oxidants potassium bromate and potassium iodate and the reducing agent dithiothreitol (DTT) on the theological behaviour of 20% (w/v) gluten-in-water suspensions during thermal treatment was monitored with the rapid visco analyser (RVA). The suspensions were subjected to a linear temperature increase from 40 to 95 degrees C in 14 min, a holding step of 40 min at 95 degrees C, a cooling step (7 min) with a linear temperature decrease to 50 degrees C, and a final holding step at 50 degrees C (13 min). Potassium iodate (1.18 and 1.77 mu mol/g protein) and potassium bromate (1.52 and 15.2 mu mol/g protein) decreased RVA viscosities in the holding step and increased sodium dodecyl sulphate (SDS) protein extractabilities suggesting a greater heat resistance and decreased gliadin-glutenin cross-linking. In contrast, in the presence of DTT (1.65 and 3.30 mu mol/g protein) RVA viscosity increased at lower temperatures and lowered SDS extractabilities. It is postulated that low concentrations of reducing agent facilitate gliadin-glutenin cross-linking during heating while oxidants hinder gluten polymerization due to decreased levels of free sulphydryl groups and less flexibility of the glutenin chains. (c) 2006 Elsevier Ltd. All rights reserved.status: publishe

Mechanism of gliadin-glutenin cross-linking during hydrothermal treatment

The gluten proteins, gliadin and glutenin, are important for wheat flour functionality and they undergo changes during heat treatment involving sulfhydryl (SH) groups. To change the level of SH-groups during hydrothermal treatment, the oxidant, potassium iodate (2.1 mu mol/g protein) and the reducing agent dithiothreitol (DTT, 6.1 mu mol/g protein) were added to 20% (w/w) gluten-in-water suspensions at room temperature, at 90 degrees C and after 15 min at 95 degrees C, and the viscosity was measured by the Rapid Visco Analyser (RVA). Protein extractabilities after hydrothermal treatment were determined by size-exclusion and reversed-phase HPLC. DTT decreased maximal RVA viscosity and the levels of extractable alpha- and gamma-gliadin and this decrease was independent of the time of addition during hydrothermal treatment. In contrast, potassium iodate increased the levels of extractable alpha- and gamma-gliadin. Its impact was less when added at later times during RVA analysis. A SH-blocking agent (N-ethylmaleimide, 8.0 mu mol/g protein), added at room temperature to the gluten suspension, decreased RVA viscosity at 95 degrees C and increased the extractabilities of glutenin and alpha- and gamma-gliadin after hydrothermal treatment. Subsequent addition, at 90 degrees C, of a reducing agent (glutathione, 3.1 and 6.2 mu mol/g protein) recovered the control RVA profile and restored the control protein extractabilities after RVA analysis. This shows the importance of heat-induced gliadin-glutenin reactions for gluten viscosity and of the presence of free SH-groups for the polymerization of gluten proteins. A model explaining gliadin-glutenin polymerization through a sulfhydryl-disulfide exchange mechanism and demonstrating the effects of redox agents is put forward. (C) 2007 Elsevier Ltd. All rights reserved.status: publishe

Reaction kinetics of gliadin-glutenin cross-linking in model systems and in bread making

The gluten proteins gliadin and glutenin are important for wheat flour functionality in bread making, where, during baking, they polymerize through a heat-induced sulfhydryl-disulfide exchange mechanism. A model system was used to study the kinetics of this reaction. Thus, gluten was subjected to hydrothermal treatment with the rapid visco analyzer (RVA) with holding temperatures of 80, 90, and 95 degrees C. At these temperatures, omega-gliadin solubility did not change, but the solubilities of alpha- and gamma-gliadin in 60% ethanol decreased according to first-order reaction kinetics. All reaction rate constants increased with temperature. The activation energies for the heat-induced exchange reaction were 110 and 147 kJ/mol for alpha- and gamma-gliadin, respectively. Starch did not influence the reaction rates of the association of alpha- and gamma-gliadin with glutenin. During gluten-starch model bread baking, glutenin oxidized first, and when the internal crumb temperature reached 100 degrees C, alpha- and gamma-gliadin crosslinked to glutenin, again following first-order reaction kinetics. The experimental findings and similarities in temperature conditions and reaction kinetics suggest that the RVA system can be instrumental in understanding gluten behavior in concentrated food systems, such as bread making.status: publishe

Processing wheat proteins into sustainable materials

status: accepte

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. (C) 2009 Elsevier Ltd. All rights reserved.status: publishe

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). (C) 2009 Elsevier Ltd. All rights reserved.status: publishe

Beta-elimination reactions and formation of covalent cross-links in gliadin during heating at alkaline pH

The aim of this study was to increase insight in gluten polymerisation. While previous research on this topic focused on disulfide (SS) bonds, the present paper focuses on cross-links based on dehydro-protein formation through -elimination reactions. Gliadin, the monomeric fraction of gluten containing no free sulfhydryl (SH) groups, was heated for 120 min at pH 8.0 and 130 °C, and cross-link formation was evaluated by determining extractability in sodium dodecyl sulfate containing buffer, reaction products of -elimination reactions, and cross-links involving the latter. Heating decreased gliadin extractability. Reduction of SS bonds increased extractability of heated gliadin, but did not restore it to that of non-heated gliadin, suggesting contribution of both SS and non-SS bonds to gliadin cross-linking. Decreased SS levels and the presence of dehydroalanine and SH groups in heated gliadin samples indicated cleavage of SS bonds by -elimination reactions. Some of the formed free SH groups were then involved in oxidation and/or SH-SS interchange reactions leading to intermolecular SS bonds. In addition, amino acid analysis revealed formation of an irreversible non-SS cross-link between dehydroalanine and the free SH group of cysteine, namely lanthionine. In conclusion, non-SS bonds may well contribute to the gluten network under specified reaction conditions.status: publishe