Keeping the quality of spray dried dairy ingredients: dependence on water activity and glass transition

The most widely used process for dehydration of dairy products is spray drying. This is an effective method for preserving biological products as it does not involve severe heat treatment and allows storage of powders at ambient temperature. Various authors have reported the physicochemical properties of free and bound water and the effects of water on physical state, transition temperatures, sticking temperature, reaction kinetics and stability of milk products. The emphasis is on the physical state of non-fatty solids and the effects of water and its physical state on chemical reaction rates, growth of micro-organisms and stability. Spray-drying, storage and quality of dairy powders are significantly dependent on both the physical state of lactose (one of the main components of dairy powders) and other carbohydrates, which themselves are dependent on the glass transition temperature (Tg) and water activity (aw). The maximum moisture content of a dairy powder (4% for skim milk powder) is defined in the product specification in relation to aw, and this must be close to 0.2 at 25 ◦C for optimal preservation. In these conditions of water content and aw, the Tg will be close to 50 ◦C. This paper is a short review of the different physical properties required to maintain the quality of dairy ingredient powders