Control of heat-induced aggregation of whey proteins using casein

The ability of alpha(s1)/beta-casein and micellar casein to protect whey proteins from heat-induced aggregation/precipitation reactions and therefore control their functional behavior was examined. Complete suppression (> 99%) of heat-induced aggregation of 0.5% (w/w) whey protein isolate ( pH 6.0, 85 degrees C, 10 min) was achieved at a ratio of 1: 0.1 ( w/ w) of whey protein isolate (WPI) to alpha(s1)/beta-casein, giving an effective molar ratio of 1: 0.15, at 50% whey protein denaturation. However, in the presence of 100 mM NaCl, heating of the WPI/alpha(s1)/beta-casein dispersions to 85 degrees C for 10 min resulted in precipitation between pH 6 and 5.35. WPI heated with micellar casein in simulated milk ultrafiltrate was stable to precipitation at pH > 5.4. Protein particle size and turbidity significantly (P <= 0.05) increased from an initial diameter of 165.5 nm in the unheated mixture to 272 nm following heating at 85 <= C for 10 min at pH 6. Whey protein denaturation was significantly (P <= 0.05) promoted when heated in the presence of micellar casein, but whey protein aggregation was controlled down to pH 5.4. The protective behavior of alpha(s1)/beta-casein and micellar casein differed in that the former inhibited denatured whey protein aggregation, whereas the latter system promoted denaturation but controlled aggregation.