| Whey proteins are widely used in a variety of food products due to their functional and nutritional properties. This study focuses on modifying whey protein gel properties at acidic conditions by enzymatic treatment. Enzymes offer a powerful tool to modify biopolymers in general, and proteins in specific. Transglutaminase enzyme is used in this study to induce &egr;-(gamma-glutamyl)lysine bonds between whey protein molecules at alkaline or neutral conditions, followed by subsequent acidification using glucono-delta-lactone (GDL) to form gels at the desired acidic pH.; We examine the viability of beta-lactoglobulin and alpha-lactalbumin for crosslinking and conclude that alpha-lactalbumin is easily crosslinked in its native state, while beta-lactoglobulin needs partial or complete denaturation to undergo the enzymatic catalysis. Denaturation of beta-lactoglobulin is done either by raising the pH to 8, or by thermal treatment (80°C for 1 hr) or by chemical denaturation using dithiothreitol (DTT). Crosslinks induced by transglutaminase increase the molecular weight of the whey proteins considerably to exceed 107 Da.; In the first part of this study, we investigate the cold-set gel formation by initially conducting the enzymatic reaction at pH 8 and 50°C then following up with acidulation by GDL to pH 4. The resulting gel exhibits superior rheological properties with higher elastic modulus and substantially higher fracture/yield stress and strain compared to cold-set gels with no enzyme.; In the second part of this study, we examine an alternative route for crosslinking, in which we preheat the whey protein first at 80°C for 1 hr at pH 7, and then conduct the crosslinking at 50°C. This procedure induces both disulfide and &egr;-(gamma-glutamyl)lysine bonds.; In the third part of this study, we investigate the relative roles of physical and chemical interactions that affect the properties of protein polymers and cold-set gels at pH 4, prepared using the same protocol as in the second part of the thesis.; In the fourth part of the thesis, we investigate the conformational characteristics of beta-lactoglobulin---the main constituent of whey proteins---subject to enzymatic crosslinking, using Fourier Transform Infrared (FTIR) spectroscopy.; In the fifth part of the study, we derive a transglutaminase-catalyzed polymerization model of beta-lactoglobulin based on probabilistic approach of non-linear polymers. Derived equations show critical gelation conversion of 5.8%.; In the last part of the study, we present a brief discussion of the effect of transglutaminase on hydrophobic associations in chemically (using DTT) denatured whey proteins. (Abstract shortened by UMI.)... |
