Role of colloidal calcium phosphate in the acid gelation properties of heated skim milk

Heated (80 °C/30 min) skim milk samples were acidified at different temperatures and the properties of the acid gelation process were followed by rheology. At pH just below the point of gelation, a maximum in tan δ (tan δmax) was observed, and this became more pronounced as the acidification temperature was increased. At an acidification temperature of 30 °C and at pH below gelation, the elastic modulus (G′) increased uniformly with decreasing pH; however, at higher acidification temperatures, the G  ′ versus pH curves did not increase uniformly and a shoulder (Gshoulder′) appeared in the curves. When the colloidal calcium phosphate (CCP) was progressively removed from the milk prior to heating and acidification, the Gshoulder′, when present, was less pronounced, whereas the tan δmax was more pronounced. These results indicate that the CCP was involved in the early stages of milk gelation during acidification. The results are discussed in relation to the changes in casein micelles during acidification.