Water sorption and glass transition properties of spray dried lactose hydrolysed skim milk powder

The moisture sorption behaviour and glass transition temperature of spray dried skim milk powder with hydrolysed lactose (SMPHL) were determined. Spray drying of skim milk with hydrolysed lactose resulted in very low cyclone recovery of 25% and a large amount of powder remained stuck inside the spray dryer. The equilibrium moisture content of SMPHL was lower than that of lactose for each range of water activity when humidified for 21 days at 23 °C using saturated salt solutions. Unlike lactose, SMPHL did not lose water when the water activity exceeded 0.432 and no crystallization was noticed at water activity ⩾0.753. The sorption isotherm data for SMPHL fitted well with the BET and GAB models with monolayer moisture contents of 7.55 and 8.27 g/100 g, respectively. The glass transition temperature of anhydrous SMPHL was 49 °C. The critical water activity and moisture content for SMPHL were 0.15 and 2.4 g/100 g dry solid, respectively. The low critical values indicated hydrolysis of lactose necessities maintenance of very low moisture of powder for its long-term stability.