Influence of protein concentration on surface composition and physico-chemical properties of spray-dried milk protein concentrate powders

Surface composition, moisture sorption behaviour and glass–rubber transition temperature (Tgr) were determined for spray-dried milk protein concentrate (MPC) powders over a range of protein contents (35–86 g 100 g−1). Surface characterisation of MPC powders indicated that fat and protein were preferentially located on the surface of the powder particles, whereas lactose was located predominantly in the bulk. Moisture sorption analysis at 25 °C showed that MPC35 exhibited lactose crystallisation, whereas powders with higher protein contents did not and continually absorbed moisture upon humidification up to 90% RH. The GAB equation, fitted to sorption isotherms of MPCs, gave increases in monolayer moisture value (mm) with protein content. Tgr, measured with a rheometer, decreased significantly (P < 0.05) with increasing water content and increased with increasing protein content (P < 0.05). In conclusion, increasing protein concentration of MPCs resulted in altered surface composition and increased mm value and Tgr values.