DRIED MILK POWDER: I. METHODS OF ASSESSING QUALITY AND SOME EFFECTS OF HEAT TREATMENT

The suitability of a number of objective tests of milk powder quality was assessed against subjective scores of palatability. The objective tests investigated were: oxygen and water sorption of the powders; chlorophyll and peroxide oxygen values of the fat; 'browning' of the powder; fluorescence values; changes in peroxidase, trimethylamine, volatile sulphur compounds, and diacetyl content; solubility by centrifuging and a potassium chloride solution method; titratable acidity; pH; Congo rubin and iron numbers; foaming volume; coagulation by acid, alcohol, and rennet; dielectric constant; colour intensity and colour quality; refractive index; viscosity and surface tension. The subjective measurement of palatability was finally adopted as the most precise measure of milk powder quality.While measurement of peroxidase activity was unsatisfactory in the determination of quality, the activity of this enzyme was observed to decrease with increase in time and temperature.When palatability was used as a measure of quality, powders stored at 37.8 °C. for seven days were preferred to powders stored at 26.7°, 48.9°, or 60.0 °C. Interpretation of these results in terms of the temperature to which milk powder should be cooled indicated that 37.8° was the desirable temperature. Current commercial practice permits cooling to this temperature within a few minutes after the completion of drying.