The Effect of Cysteine on Formation, Microstructure and Physico-Chemical Properties of Whey Proteins Based Gels and Emulsion Gels

Whey proteins (WP) gels are widely used in the food and pharmaceutical industries both to control and maintain the texture of products and as delivery systems of sensitive material and slow release agents. The combined manifestation of emulsification and gelation properties of WP provides unique opportunities for developing biological composites, in the form of emulsion gels that can encapsulate functional ingredients and drugs for controlled release and protective purposes, both for the food and pharmaceutical industries.;The availability and activation of the SH group of beta-Lactoglobulin (beta-lg) in heat-induced gelation of WP has been shown to play a major role in the gelation process and was shown to significantly affect the formation and properties of the gels formed. The gelation and properties of WP gels are also dependent on: protein composition and concentration, net charge on the surface of the proteins and on the ionic environment. The properties of emulsion gels are also govern by the lipid load, adsorption of protein-based structures at the oil/water interface, interactions between the latter and the protein-based network and properties of both proteins and lipids.;Recognizing the important role of active SH groups to the formation and properties of WP-based gels, the aim of the research was to investigate the effect of addition of a free thiol group which is not associated with beta-lg (in the form of free L-cysteine (Cys)) to individual WP (&agr;-lactalbumin (&agr;-la) and beta-lg) and different mixtures of these proteins on the properties of WP solutions and formation and properties of WP-gels and emulsion gels. Combined effect of Cys and protein concentration and composition, oil load, pH and the gelation temperature was also examined. A water-soluble model nutrient, riboflavin, was entrapped in Cys-fortified WP gels and emulsion gels and the time-dependent release of this compound into aqueous environment and into simulated gastric and intestinal fluids (SGF and SIF, respectively) was investigated.;Results indicated that Cys had a Cys:Protein (Cys:P) ratio-dependent effect on the properties of the different WP solutions, Gels and emulsion gels. In all cases, increasing Cys:P ratio lowered the denaturation temperature of the proteins by up to 18% and 22% for beta-lg and &agr;-la, respectively and did not allow renaturation of &agr;-la. Addition of Cys also allowed aggregation of the proteins at 25 °C and the aggregation rate at room temperature increased with increase in Cys:P ratio. &agr;-la solutions started aggregating at lower Cys:P ratios than the corresponding beta-lg solutions indicating that &agr;-la solutions are more sensitive to the addition of Cys than beta-lg solutions. Fortifying WPI solutions and emulsions with free Cys prior to the onset of gelation allowed, at a given protein concentration, lowering the gelation temperature and at a given temperature, lowering the critical protein concentration for gelation for both gels and emulsion gels. When gelled at 90 °C, the lowest protein concentration that allowed gelation was 4% with the addition of Cys, regardless of protein composition.;Addition of Cys changed the appearance, microstructure and properties of gels and emulsion gels in a Cys:P ratio-dependent manner. As Cys: P ratio was increased; gels were more brittle and less elastic and turned from gels that could be compressed to 80% deformation without breaking to brittle gels that broke at lower force and lower deformations. Gels also transitioned from "fine stranded" gels (transparent) to "particulated" (opaque) gels with increase in Cys:P ratios and their structure was more compact with a higher degree of crosslinking. Emulsion gels were softer and more brittle than their oil-free counterparts. Swelling ability of WP gels when immersed in water, SGF and SIF decreased with the addition of Cys by up to 93, 70 and 20%, respectively. Results indicated that swelling of emulsion gels immersed in water, SGF and SIF, was inversely proportional to Cys concentration and oil load. This could be attributed to the effect of Cys and oil on the microstructure of gels, which could decrease the diffusion of water and the expansion of protein network.;Results in this study present a food safe opportunity for using Cys and WP in different compositions and concentrations, oil loads, pH values and temperatures to modulate the mechanical properties of WP gels and emulsion gels and present an opportunity to lower the protein concentration and temperature needed to produce WP gels. Observations also suggested that the extent to which the protein network became cross-linked during gelation was proportionally related to Cys concentration and presents new opportunities to modulate the properties of WP based delivery systems. (Abstract shortened by UMI.).