EFFECT OF HEAT-TREATMENT, STABILIZING SALTS AND SEASONAL-VARIATION ON HEAT-STABILITY OF RECONSTITUTED CONCENTRATED SKIM MILK

Samples of milk powder were manufactured at five different dates in one year (February, April, June, September and December). Heat treatment of the milk before evaporation and spray drying to milk powder was at 85-120-degrees-C for 1-3 minutes. Two methods were used to determine heat stability in terms of heat-coagulation time (HCT) of the reconstituted concentrated milk: one that made use of a new apparatus (the Klaro-graph) and the method of the Australian Standard for determination of heat stability of skim-milk powder. The Klaro-graph is based on the principle of a falling-ball viscometer; it operates at temperatures up to 140-degrees-C, repeatedly measuring a falling time for assessment of heat stability (HCT) and for calculation of apparent viscosity. There was a good correlation between the HCT values of reconstituted concentrated skim milk as determined by the Klaro-graph and by the Australian Standard. Results of the assessment of HCT show that the season of manufacture is an important factor for heat stability, which probably depends on the non-protein nitrogen content of reconstituted milk. Improvement of HCT can best be achieved by heat treatment at high temperature. The Australian Standard requires that disodium hydrogen phosphate and sodium dihydrogen phosphate are added to the reconstituted milk at five levels, resulting in a series of pH adjustments of the milk before determining heat stability. Addition of disodium hydrogen phosphate causes a slight increase of HCT, whereas added sodium dihydrogen phosphate shows the opposite effect.