ABSTRACT
3.1 Water activity is related to water vapor pressure in a material, and it is a useful parameter relating to glass transition, water content and water plasticization. The glass transitions of dairy solids, particularly that of lactose, determine the dehydration characteristics and successful spray-drying conditions of dairy liquids. Water and glass transition relationships assist in the selection of spray-drying parameters as well as required storage conditions. A critical water activity and glass transition-related strength of dairy solids are useful in the formulation of dairy food with desired physicochemical stability in processing and storage.
3.2 Caking is a common root of many problems encountered when handling powders and can lead to unwanted shutdowns, client claims and product recycling/rejection due to the clogging of conveying lines, feeding devices and storage silos. Caking can be defined as a formation of material bonds at the points of contact between particles that leads to undesired and uncontrolled agglomeration of powders. It is a very complex process bringing into play a multitude of interactions and mechanisms resulting from variations in environmental conditions, in particular those of temperature and relative humidity. The objective of this chapter is to describe the fundamental mechanisms of powder caking and especially those involved in the caking of dairy powders.
3.3 This chapter presents the structural changes at protein/aggregate scale and the changes of functionalities (water-holding capacity, viscosity, foaming, emulsifying and gelling properties) resulting from the pre-texturizing of whey proteins by heating in dry state. The first part of this chapter describes the main chemical/structural modifications occurring during protein dry-heating (limited protein unfolding, Maillard reaction, dehydration reactions) and the specificities of the aggregation reactions that are controlled by molecular mobility (or a w ) and the presence of heat-sensitive compounds (type and amount of sugars). Then, the second part of the chapter describes the functionalities of the pre-texturized whey protein by dry-heating. The change of functionalities can be huge, as shown for the modification of viscosity of protein solutions after whey protein dry-heating at alkaline pH (protein particles are able to trap up to 30g of water per g of protein). The strength of the whey protein gels is increased two-fold by the dry-heating treatment, favoring the formation of soluble aggregates. The change in the foaming properties after whey protein dry-heating is much more limited.
3.4 The perceived quality of dairy powders obtained by spray-drying depends on their functional properties, i.e., their ability to flow easily and rehydrate quickly without lump formation. These requirements can be achieved provided that the physicochemical properties of the powders are mastered by an adequate choice of formulation and spray-drying conditions. On one hand, this chapter presents the impact of the spray-drying conditions on the physicochemical characteristics of the resulting dairy powders and highlights the tremendous role of the evaporation rate in the particle structure and surface composition. On the other hand, the influence of the main powder physical properties (particle size, shape, microstructure and surface mechanical properties) on flowing properties and rehydration capacity is discussed through examples of dairy powders produced by spray drying. In order to complete this discussion of the link between spray-drying conditions, physicochemical properties and functionalities, the main analytical methods used to assess spray-dried dairy powder characteristics are briefly reviewed throughout the chapter.
3.5 Initial milk contamination may occur due to intramammary infections, microorganisms on the external udder surface, poorly sanitized handler hands and milking equipment. During the refrigeration, storage and transport stages, lapses in hygienic-sanitary conditions can lead to milk contamination from pathogenic and spoilage microorganisms.
Once raw milk reaches the processing plant for dehydration, it undergoes processes including cooling, heat treatments, evaporative procedures and membrane filtration in order to inhibit microbial growth and destroy microorganisms. Following thermal treatments and membrane filtration, caution should be taken to ensure the hygienic-sanitary conditions of equipment and utensils to avoid product recontamination. All the stages of dried dairy products (from milking to packaging) should be monitored to guarantee the quality and microbiological safety of these foods.
