Characterization of buttermilk serum fractions and their effect on rennet-induced coagulation of casein micelle dispersions

Upon centrifugation of milk and buttermilk (BM) the supernatant above the casein micelles pellet separates into three phases: a low-density opalescent fraction, a clear fraction, and a high-density opalescent (HDO) fraction. These fractions were collected and the concentrations of casein, whey and milk fat globule membrane (MFGM) proteins, fat and phospholipids determined. Reference casein micelles were pelleted from skim milk (SM) and re-suspended in the serum fractions. Rennet-induced aggregation, coagulation aptitude and gel syneresis were measured in these suspensions. Buttermilk serum fractions contained more MFGM components than SM fractions did. Overall, BM serum showed a negative impact on rennet-induced aggregation of casein micelles, gel formation and gel syneresis during cooking. This impact was greater for the HDO fraction, which contained much higher concentration of MFGM materials when isolated from buttermilk than from milk. The negative impact of buttermilk on rennet-induced coagulation of casein micelles was mainly attributed to the presence of tiny fat droplets and high density MFGM fragments.