Importance of glass transition and water activity to spray drying and stability of dairy powders

Spray-drying is a rapid dehydration method allowing production of high quality dairy powders. In dehydration and subsequent powder handling and storage, however, both chemical and physical changes, such as caking, lactose crystallisation, and nonenzymatic browning, may impair powder characteristics and result in loss of powder quality. Many of these changes are related to the physical state of lactose, as rapid removal of water in spray drying results in the formation of low-moisture, amorphous, noncrystalline structures of lactose and other milk components. The amorphous components may exist as solid-like glasses or highly supercooled, viscous liquids. The formation of amorphous, glassy lactose during spray drying allows production of a free-flowing powder. High temperatures or residual water contents at the later stages of the drying process, however, may cause stickiness, caking, browning, and adhesion of the powder particles to the processing equipment. The glass transition of amorphous lactose occurs in the vicinity of room temperature at a water content of about 6.8 g (g x 100)(-1) of lactose corresponding to an equilibrium relative humidity of 37% and 0.37 a(w) (water activity). At higher water contents, as the glass transition of amorphous lactose is well below storage temperature, dairy powders become sticky and the amorphous lactose may exhibit time- dependent crystallisation. Crystallisation of amorphous lactose may also release sorbed water from the amorphous material, which enhances other deteriorative changes, such as the nonenzymatic browning reaction. Amorphous lactose in dairy powders encapsulates milk fat, which, as a result of lactose crystallisation, is released and becomes susceptible for rapid oxidation. The glass transition and water activity are, therefore, important factors controlling processability, handling properties and stability of dairy powders.