Gelation Profiles of Yogurt as Affected by Heat Treatment of Milk

Milk was heated at 140°C for 6s (UHT), 98°C for 1.87min (HTST) and 85°C for 10min (vat-pasteurized), homogenized, inoculated with yogurt culture, and incubated at 45°C. During incubation, pH, ionic Ca, and viscosity changes were continuously monitored by interfacing to a microcomputer. Samples for transmission electron microscopy were taken at selected pH intervals. Acid fermentation of milk was a multistage process comprising 1) an initial lag period of low viscosity; 2) a period of rapid viscosity change, and 3) a stage of high constant viscosity. The shapes of the viscosity versus incubation time curves were the same for all heat treatments differing only in the extent of viscosity change in stage 2. A microstructural study of UHT milk indicated that at pH 5.1, casein micelles appeared to dissociate into subparticles of approximately 30 to 40nm diameter. By pH 4.8, subparticles reassociated into large casein aggregates of nonspecific shape and dimensions. Micellar dissociation was thought to be influenced by conversion of colloidal Ca to ionic Ca.