Preparation and Characterization of Acid-induced Gels at pH 1.0-4.0 and Thermal-stable Dispersions at pH 5.5-6.0 using Preheated Whey Protein

Whey protein (WP) is a good source for producing protein-rich products, including satiety foods and beverages. Thermal aggregation of WP during sterilization or pasteurization impacts food quality important to shelf-stable beverages. Preheating WP improves the thermal stability at neutral acidity, and forming complexes with other molecules to provide charge and/or steric hindrance can be used to improve thermal stability of WP at acidic conditions. Conversely, aggregation properties of preheated WP upon acidification can be used to create unique functional properties. Therefore, studying properties of preheated WP can broaden its application in the food industry.

Gelation of preheated WP and pectin mixtures at gastric acidity (pH 1.0-4.0) was studied in the first part of this thesis because of the potential to control satiety. The fast-acidified gels had less homogenous microstructures and lower strengths than gels slowly-acidified by glucono-delta-lactone. Preheated WP gelled at pH 3.5-4.0, while the WP-pectin mixture at mass ratios of 20:1, 10:1 and 5:1 formed gels at pH 2.0-3.5, 1.5-3.0 and 1.5-2.5, respectively. WP-pectin gels were weakened by NaCl but strengthened by CaCl₂, indicating the significance of electrostatic attraction on gelation. Without hydrophobic attraction and hydrogen bonds, gelation was not observed for preheated WP but was evident for the WP-pectin mixtures at pH 2.5 and 3.0. The findings suggest the potential application of preheated WP-pectin mixtures to increase the viscosity in the stomach.

The second part of this thesis research was focused on combining sodium stearoyl lactylate (SSL) and preheating treatment to improve thermal stability of WP at pH 5.5-6.0. The binding between SSL and WP was promoted by heating based on particle size and zeta-potential results. Circular dichroism spectroscopy indicated the formation of a more ordered secondary structure of WP after binding with SSL. SSL increased the zeta-potential magnitude, reduced the extent of denaturation, the exposure of sulfhydryl groups, and surface hydrophobicity of WP. These properties in turn improved the thermal stability of WP to make it possible to produce transparent fluids containing 4.5% WP and 0.2% SSL at pH 5.5-6.0.