PREPARATION AND PROPERTIES OF ACID-INDUCED GEL OF WHEY-PROTEIN

Preparation and theological properties of acid-induced gel of whey protein were investigated, using whey protein isolate (WPI) and glucono-delta-lactone (GDL) as an acidulant. A 6% WPI solution, pH 7.0, was heated at 65-90 degrees C in the absence of acidulant, then cooled to room temperature for obtaining partially heat-denatured WPI solution. The solution was incubated at 20-50 degrees C in the presence of GDL. During incubation, the WPI solution denatured by heating to a certain extent was found to form gel below pH 5.8, whereas undenatured WPI solution did not. The pH of the resultant finally reached 4.3 after incubation. Rheological properties of the acid-induced gel were dependent on temperature and time of the heat treatment of WPI solution, GDL concentration, and incubation temperature. HPLC analysis indicated the formation of aggregates in heated WPI solution, and scanning electron microscopy showed the presence of finer network structure of whey protein aggregates in acid-induced gel with higher incubation temperature. It was concluded that both the formation of whey protein soluble aggregates by heating and a decrease of negative charge on the surface of the aggregates arising from a decrease of pH during incubation caused gelation.