Whey Proteins Cross-linked by Transglutaminase or Glycated with Maltodextrin: Physicochemical Bases of the Improved Heat Stability

Whey protein, as a byproduct in cheese manufacturing, is an ideal source for producing ready-to-drink protein beverages for different market demands, attributed to its high content of essential amino acids and versatile functionalities, bland flavor and good digestibility. Whey protein is a mixture of proteins, which can be categorized into whey protein concentrate (WPC) with a protein content of 50~80% and whey protein isolate (WPI) with a protein content higher than 90%, depending on different separation techniques. Thermal processing is required to ensure microbiological safety and quality of dairy products, leading to denaturation of whey proteins, especially at pH around its isoelectric point. Denaturation facilitates the aggregation of whey proteins that involves a number of colloidal interactions such as hydrophobic, electrostatic, hydrogen bonding interactions and covalent disulfide bonds, which are significantly influenced by pH, concentration, temperature, ionic strength, and the presence of co-solutes. Based on the understanding of physic forces, the thermal stability of protein can be enhanced via two approaches, which are restricting the denaturation and creating repulsive forces. A combination of preheating and microbial transglutaminase cross-linking can improve the thermal stability of whey proteins at neutral pH after second heating at 80 °C or 138 °C in the presence of different concentration of NaCl. Whey proteins glycated with saccharides creates a structure with internal whey protein core and an external saccharide shell. Further aggregation of protein molecules is suppressed because the approaching of protein molecules has to overcome the saccharide shell. Glycation with saccharides is markedly affected by reaction conditions such as pH (powder acidity for dry base reaction), temperature, time, protein:saccharide ratio and types of protein and saccharide, etc. Addition of monosaccharide such as D-glucose and disaccharide such as sucrose in the whey protein aqueous system also improve the thermal stability of protein. When combined with optimized glycation, whey proteins can be stabilized at pH ranging from 4 to 7.