POLYMERIZATION OF A VEGETABLE PROTEIN, WHEAT GLUTEN, AND THE GLASS-SOFTENING TRANSITION OF ITS DRY AND REACTED STATE

The heat evolved as a result of irreversible chemical changes that occ ur on heating 2, 5.6, 15, 25, and 50 wt % water containing wheat glute n has been measured by differential scanning calorimetry. Further, the effects of the removal of water vapors on evaporation of the gluten's moisture on its polymerization have been investigated. The amount of moisture removed during heating and the heat of the reaction were meas ured and correlated with the extent of reaction. When the moisture and gases produced on heating gluten were prevented from escaping partial ly, the total heat evolved decreased, and the reaction exotherm shifte d to a higher temperature. These observations have been interpreted in terms of a chemical equilibrium that is attained within the short dur ation allowed by the heating rate. By varying the amount of water and the time allowed for its diffusion at both ambient temperature and 253 K, it is shown that the thermal effects observed are a consequence of two processes: the vaporization of water, which is endothermic, and t he condensation reaction, which is exothermic. The kinetics of this re action depends upon the amount of water present, which plasticizes the proteins. Increase in the molecular mobility due to the presence of w ater thus allows further bond formation by chemical reactions, but aft er the bond has formed, the mobility decreases and some of the water e scapes by vaporization because of the exothermic nature of the reactio ns, thus preventing further reaction. This negative feedback between t he physical process of plasticization by water and the chemical proces s of polymerization thus prevents the reaction from reaching completio n. The ultimately formed dry wheat gluten undergoes a glass-softening transition at 436 +/- 2 K. The shape of this endotherm, and so the imp licit molecular dynamics, depend upon the thermal treatment conditions of the gluten, but not its T-g onset. It has a broader distribution a fter reacting in excess water kept at 373 K than when it was degraded and charred by heating to 523 K. This distribution of relaxation times and other characteristics of its heat capacity relaxation are similar to those of synthetic polymers. Wheat gluten seems to be the only mat erial for which a T-g endotherm becomes observable in an experiment do ne to measure the reaction kinetics.