Effects of limited proteolysis on the microstructure of heat-induced whey protein gels at varying pH

Whey protein solutions, reconstituted 12% commercial whey protein isolate, were hydrolyzed at pH 7.0 by one of three proteases: trypsin, Neutrase(R) (from Bacillus subtilis), and a Bacillus licheniformis protease. Heat-induced gels were made from the hydrolysates after pH adjustment, and their microstructure was examined by transmission electron microscopy and compared with the structure of control gels made from unhydrolyzed whey protein solutions. The gels formed from the unhydrolyzed whey protein solutions, as expected, were fine-stranded when set at pH 3-0 and 7.0 and particulate when set at pH 5.2. The enzymatic treatments caused alterations in the microstructure of the heat-induced gels. The changes were small at pH 3.0, marked at pH 5.2, and strongest at pH 7.0. Of special interest was the dramatic change in the microstructure of the gels set after treatment with Bacillus licheniformis protease. The Bacillus licheniformis protease gels consisted of small aggregates (similar to 0.1 mu m) arranged in open clusters (pH 5.2) or tightly packed in a regular pattern (pH 7.0), corresponding to a high gel strength. Limited hydrolysis is thus a way to change the microstructure and properties of gels at weakly acidic and neutral pH, a range that is suitable for the production of many foods.