Compositional and analytical factors affecting the stickiness of dairy powders

Abstract

Spray dying is a dehydration technique used in the dairy industry for the preservation and creation of a wide range of valuable dairy products. However, challenges associated with stickiness development are often encountered during spray drying, particularly with spray dryer feed streams containing high levels of lactose, which can lead to lower yields, reduced powder quality and shorter runs. Stickiness in lactose-containing powders is related to the glass transition phenomenon, in which a phase change occurs in the amorphous form of lactose, causing a decrease in the viscosity of the powder particle surface, leading to liquid bridging and ultimately stickiness between particles and/or to equipment surfaces. There is a wide variety of compositional and environmental factors that affect the rate and extent of stickiness development in dairy powders, such as the temperature and relative humidity of the air or the protein content of the powder. The first objective of this study was to investigate the influence of particle size on the physicochemical properties and stickiness behaviour of a selection of lactose-containing dairy powders. Using a fluidisation technique, this work demonstrated that stickiness increased with decreasing particle size for lactose-containing dairy powders. Stickiness may be characterised using a number of different instrumental approaches, which can be categorised as direct/indirect or static/dynamic techniques. However, most methods provide a binary definition of stickiness (i.e., sticky or non-sticky), which while pragmatic, does not provide information regarding the mechanical relaxations which contribute to stickiness. Therefore, the second objective of this study was to examine the use of dynamic mechanical analysis (DMA) to characterise temperature- and humidity-induced relaxation behaviour of whey protein concentrate (WPC) powders; results were also compared to two other established techniques, differential scanning calorimetry (DSC) and a fluidisation method. The results demonstrated that while DMA may not be an accurate method for stickiness determination, it could prove useful as a complementary method when combined with stickiness techniques (e.g., fluidisation) to provide more detailed information on the physical changes occurring during stickiness. Overall, the findings of this research will prove useful to dairy processors at minimising issues with stickiness during drying and may also potentially provide powder technologists with a new method for tracking the physical transitions that occur during stickiness development of dairy powders.