Gelation of pH-aggregated whey protein isolate solution induced by heat, protease, calcium salt, and acidulant

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 (pI 5.2) of whey proteins. Lower or higher pH than the pI 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 CaCl2, and quiescently acidifying with glucono-delta-lactone (GDL), respectively. The hardness, color, and microstructure of the gels so formed were determined. Micrographs of BLP- and CaCl2-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 nn and 1-2 mu m). Separating the processes of formation of aggregates and gels may provide a means to manipulate the protein gel properties.