Standardized reaction times used to describe the mechanism of enzyme-induced gelation in whey protein systems

Whey protein isolate (WPI), either untreated or pretreated at 80 degrees C for 30 min, was incubated with a proteinase from Bacillus licheniformis unt il a gel was for med. Standardized reaction times, directly linked to the d egree of hydrolysis, were obtained from plots of the relative amount of pep tides released upsilon. reaction time obtained under different conditions ( enzyme concentration, temperature, pH, NaCl addition). This provided a conn ection between the gelation profile and the degree of hydrolysis. In the ca se of untreated WPI gelation occurred at lon-er degrees of proteolysis when the enzyme concentration was decreased, demonstrating that a rate-limiting aggregation process occurred at the same time as the proteolysis in a mann er similar to the renneting of milli. This was not the case for preheated W PI, when gelation was found to take place at a constant degree of proteolys is, independent of the enzyme concentration. In this case, the mechanism co uld be described by assuming the thermally induced aggregates present in th is substrate had progressively more stabilizing peptide segments shaved off ; resulting in increased attraction between individual aggregates that ulti mately led to gelation. Results obtained at 40-60 degrees C supported, this , as we found no effect of temperature on the degree of proteolysis at gela tion for the untreated WPI, whereas the degree of proteolysis decreased wit h increasing temperature when heated WPI was hydrolysed. The effect of pH a nd NaCl addition on the process was to reduce repulsion between the aggrega ting species so that gelation was induced at a decreased degree of proteoly sis.