Physical changes in heat-treated whole milk powder

Abstract

The heat treatment of milk powders during food manufacturing induces physical and chemical modifications that are governed by material phase transitions. The main aim of this work was to describe the heat-induced physical changes occurring in whole milk powder (WMP). The study was based on increasing the water activity (aw) of the powder in order to relate the glass transition temperature (Tg) to the parameters resulting in changes in physical properties. Colour changes in milk powder due to non-enzymatic browning (NEB) were monitored by spectrophotometry based on the CIE L*a*b* function. Changes in particle structure were investigated by scanning electron microscopy (SEM). WMPs at low and high water activities (0.28 and 0.35) were heated in closed tubes and their physical properties (aw, lactose crystallisation and free fat release) monitored. The results showed that the kinetics of lactose crystallisation and the resulting increase in aw were clearly influenced by the Tg, modulated here by the water content of WMP at the time of heat treatment. The kinetics of free fat release was related to the trend of lactose crystallisation. SEM showed that the free fat was released onto the particle surfaces and formed a fat layer covering the lactose crystals. Porosity then resulted inside the powder matrix due to empty cavities formerly occupied by fat globules. In addition, the chemical reactivity of WMP expressed by colour changes showed a delay in triggering the NEB reaction in milk powder at low water content only. The results of this study showed that the heat treatment of WMP under specific process parameters (aw, temperature and time) enables not only modulation of free fat release and browning but also changes in the microstructure of the powder particles.