a1 CSIRO Division of Food Processing, Dairy Research Laboratory, Highett, Victoria 3190, Australia
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 (– In 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.
(Received April 17 1990)
(Accepted September 13 1990)