Effect of Whey Protein Denaturation on Structure of Casein Micelles and their Rennetability after Ultra-high Temperature Processing of Milk with or without Ultrafiltration

Whole milk and skim milk were ultrafiltered to 3 x concentration and heated to 72 degrees C, 89 degrees C, 106 degrees C, 123 degrees C, or 140 degrees C in an ultra-high temperature(UHT) plate heat exchanger. Whey protein denaturation increased with heating temperature and was higher in concentrated milk than in the corresponding unconcentrated milk. There was no difference in the patterns of whey protein denaturation and rennet coagulation times between whole milk and skim milk. Rennet coagulation time increased and gel strength decreased with milk processing temperature. Concentrating milk by ultrafiltration shortened coagulation time and increased gel firmness. UHT processed milk did not coagulate when rennet was added. Its 3X concentrated counterpart did coagulate although only a weak gel was formed. UHT heating caused the casein micelles to increase in size with additional protein material adhering to their surface, especially in the 3X skim milk heated to 140 degrees C. This diffuse layer of material around casein micelles was not observed in 3X whole milk. It is suggested that this denatured protein is adsorbed onto the fat-water interfaces during homogenization.