CHANGES IN THE GELATION MECHANISM OF WHEY-PROTEIN CONCENTRATE WITH PH AND TEMPERATURE

The percolation model of Steventon et eel. (1991) successfully detected changes in the mechanism of whey protein concentrate gelation with pH and temperature by comparing simulated with experimental gelation times. The results demonstrated that at 71 degrees C and pH 5.2 the formation of an aggregate from two denatured protein molecules (initiation) was the rate-determining step in the gelation process, while at pH 7.0 the addition of denatured protein molecules to an aggregate (propagation) was rate-determining. At pH 5.9, the gelation process was also initiation limited, with the rate being slower than for pH 5.2 solutions, probably owing to electrostatic effects. Analysis of the temperature dependence of the gelation time: and percolation analysis both showed that there was a change in the rate-controlling reaction at 73 degrees C for gelation at pH 5.2 and 7.0. In the case of pH 7.0 gelation, this change in rate-controlling reaction was not due to a change from denaturation- to aggregation-controlled gelation, but was probably due to a change in the relative rates of interactions between protein molecules. Gelation at pH 5.2 was aggregation-controlled at temperatures below 73 degrees C, and denaturation-controlled at higher temperatures; there appeared to be another change in rate-limiting reaction at 80-85 degrees C without a change in mechanism (i.e. it remained denaturation-limited). The activation energies of the rate-limiting reactions determined from analysis of the temperature dependence of gelation time and from percolation analysis were in agreement. This is evidence that the changes in the rate-controlling reaction of whey protein concentrate gelation with temperature and pH were real.