HEAT-STABILITY OF RECOMBINED CONCENTRATED MILK - CHANGES IN CALCIUM ACTIVITY AND PH ON STERILIZATION

Recombined concentrated milks (18 % SNF, 8 % fat) at their natural pH and at pH values in the range 6.28-6.68, made with powders subjected to different preheat conditions (high heat (85-degrees-C, 30 min), indirect UHT (120-degrees-C, 2 min) and direct UHT (120-degrees-C, 2 min), were sterilized at 120-degrees-C for 13 min. The heat stabilities of recombined concentrated milks were dependent on preheat treatment. Ca2+ activity and pH of sterilized recombined concentrated milks, measured 1 h after sterilization, were lower than those of corresponding unsterilized recombined concentrated milks. The magnitude of the decreases in Ca2+ activity and pH induced on sterilization were dependent on the pH of the unsterilized recombined concentrated milk but were not markedly influenced by the type of preheat treatment applied during powder manufacture. The results suggest that differences in heat stability of high heat, indirect UHT and direct UHT powders are unlikely to be due to Ca2+ activity.