CALCIUM-ION ACTIVITIES OF COOLED AND AGED RECONSTITUTED AND RECOMBINED MILKS

Milk powders were subjected during manufacture to the following treatments: low-, medium- or high-heat, indirect or direct UHT preheat, or no preheat. The Ca2+ activity-pH profiles of reconstituted milks (9% total solids, TS) in the pH range 6.06-6.98 and reconstituted concentrated milks (19.6% TS) and recombined concentrated milks (26% TS, 18% SNF, 8% fat) in the pH range 6.27-6.69 were determined. Statistical analysis of the results using a (- ln Ca2+ activity) transformation was used to quantify the effects of pH, preheat treatment and fat incorporation on Ca2+ activity. pH had a dominant effect on Ca2+ activity and the effects of preheat treatment and fat incorporation were small in comparison. Depending on the batch of milk used for preparation of powders, pH accounted for 98.8-98.9%, 96.4-96.7% and 93.7-97.3% of the total variation in Ca2+ activity in reconstituted milks, reconstituted concentrated milks and recombined concentrated milks. The corresponding contributions of preheat treatment to variation in Ca2+ activity were 0.26-0.47%, 0.34-1.67% and 0.24-0.40%. The addition of fat to reconstituted concentrated milk to yield recombined concentrated milk with the same SNF: water ratio resulted in 0.03-3.71% variation in Ca2+ activity. The results suggested that differences in heat stability among powders subject to different preheat treatments are not likely to be linked to Ca2+ activity.