PRELIMINARY RESULTS USING ANEW METHOD TO OPTIMIZE A SPRAY DRYER PROCESS FOR PRODUCING HIGH QUALITY MILK POWDER FROM COW, GOAT AND SHE-ASS MILK CONCENTRATES

As quality is a very important factor in milk powder produced by drying, the optimal process must protect both nutritional and sensorial properties. Although heat damage indices (namely the insolubility index (IINS), thermal damage (IDT), protein denaturation) could be used to evaluate the correct processing of milk, they are very time-consuming. Hence a chemical marker, like vitamin C, is proposed for rapid assessment of the overall damage to the quality of the produced milk powder. Trials were carried out on milk concentrates from cow, goat and she-ass so as to optimize the process performance of the spray dryer, for each kind of milk, at three inlet temperatures (120, 150, 185 degrees C); the feed flow rate was set at 0.5 dm(3)/h with outlet air RH% in the range 10-40%; raw milk was concentrated using a low pressure evaporator until an average level of 23% dry matter was reached. As expected, the thermal damage of the milk powder increased as the inlet air temperature increased; the outlet powder RH% was 96-98% poorly correlated with the mass flow rate of the concentrate inlet. Moreover, the destruction kinetic of vitamin C was found highly correlated with the thermal damage to the milk powder. At 175 degrees C inlet air temperature the overall thermal treatment on the she-ass milk concentrate, which is very heat-sensitive due to its high lactose content, was "weak" (IDT<80) and the milk powder of "premium or extra" quality (IINS<1.25ml and lactic acid = 0.07% < 0.15% ADMI). The titratable acidity values are uncorrelated with the process air temperature but depend uniquely from the raw milk freshness.