The effect of preheat temperature and urea addition on the seasonal variation in the heat stability of skim-milk powder

Abstract

SUMMARYThe manufacture of skim-milk powder with heat stable characteristics was investigated commercially during the course of 8 trials carried out over a 12-month period. Skim-milk was preheated to temperatures ranging from 110 to 130 °C with a 2-min holding time prior to evaporation and drying. The effect of added urea was also examined during each trial run. Heat coagulation times at 120 °C were determined upon reconstitution of the powders to 20% total solids. From February to April the heat stability of the skim-milk powders increased, with the more heat stable samples being obtained at the higher preheating temperatures. Addition of urea increased the heat stability, particularly so in those milks which had been preheated to 130 °C. The presence of added urea during preheating was not found to be necessary as an equal effect on heat stability was observed when urea was introduced at the concentrate stage before drying. Later in the season optimum heat coagulation times were obtained by maintaining preheating temperatures at 118–120 °C.