Gelation of pH-Aggregated Whey Protein Isolate Solution Induced by Heat, Protease, Calcium Salt, and Acidulant

Abstract

Effects of pH (3.0−7.0) on aggregation of whey protein solution (WPS, 18%) were investigated by examination of turbidity, aggregate size, and microstructure. As expected, maximum turbidity and aggregate sizes occurred at the isoelectric point (p I 5.2) of whey proteins. Lower or higher pH than the p I resulted in a steady decrease of the turbidity and aggregate size. Microstructure analysis revealed that the WPS at pH 5.7 contained loose and irregular aggregates with 200−400 nm sizes. From the pH (5.7)-aggregated WPS, gelation was induced by heating, hydrolyzing with a protease from Bacillus licheniformis ( BLP), increasing ionic strength with CaCl 2, and quiescently acidifying with glucono-δ-lactone (GDL), respectively. The hardness, color, and microstructure of the gels so formed were determined. Micrographs of BLP- and CaCl 2 -induced gels showed aggregates similar in size and shape to the parent aggregates in the WPS. Heat- and GDL-induced gels were structured with enlarged aggregates (500 nm and 1−2 μm). Separating the processes of formation of aggregates and gels may provide a means to manipulate the protein gel properties. Keywords: Whey proteins; aggregation; gelation; microstructure; pH