246 M.J. Lewis compared to centrifugal forces, drag forces or electrical, electrostatic or magnetic forces Also, the flow characteristics and behaviour of food powders are markedly different to those of fluid Some of the physical properties of food powders will now be considered in more detail, especially those which will influence the effectiveness, quality and nature of the eparation process. 9. 2.1 Classification of powders Powders can be characterised in a large number of ways; Peleg (1983) gives some by usage: e. g flours, beverages, spices, sweeteners; by major component: e. g starchy, proteinaceous, fatty y process: e. g. ground powders, freeze-dried, agglomerated y size: e.g. fine, coarse y moisture sorption characteristics: e. g hygroscopic; by flowability: free flowing, sticky, very cohesive Further classification could be by hardness, by explosion potential or by microbial hazards. Hayes (1987)summarises a detailed system used for characterising a wide range of food powders based on density, size, flowability, abrasiveness, a range of miscellane- ous properties and hazards such as flammability, explosiveness and corrosive nature Some important physical, chemical and functional properties of powders are given in Table 9. 4. For products such as beverages, the palatability and sensory characteristics of the reconstituted products are important and may be variables considered when grading these products. Care should also be taken to ensure that the microbial count is within cceptable limits for the products Determination of some of these properties for milk powders is described in publica tions by the Society of Dairy Technology(SDT, 1980), and Schubert (1987a) Table 9. 4. Factors contributing to the quality of powd ppearance Size and shape stabilit Bulk density and particle density Nutrient content Microbiological quality246 M. J. Lewis compared to centrifugal forces, drag forces or electrical, electrostatic or magnetic forces. Also, the flow characteristics and behaviour of food powders are markedly different to those of fluids. Some of the physical properties of food powders will now be considered in more detail, especially those which will influence the effectiveness, quality and nature of the separation process. 9.2.1 Classification of powders Powders can be characterised in a large number of ways; Peleg (1983) gives some examples: by usage: e.g. flours, beverages, spices, sweeteners; by major component: e.g. starchy, proteinaceous, fatty; by process: e.g. ground powders, freeze-dried, agglomerated; by size: e.g. fine, coarse; by moisture sorption characteristics: e.g. hygroscopic; by flowability : free flowing, sticky, very cohesive. Further classification could be by hardness, by explosion potential or by microbial hazards. Hayes (1987) summarises a detailed system used for characterising a wide range of food powders based on density, size, flowability, abrasiveness, a range of miscellane￾ous properties and hazards such as flammability, explosiveness and corrosive nature. Some important physical, chemical and functional properties of powders are given in Table 9.4. For products such as beverages, the palatability and sensory characteristics of the reconstituted products are important and may be variables considered when grading these products. Care should also be taken to ensure that the microbial count is within acceptable limits for the products. Determination of some of these properties for milk powders is described in publica￾tions by the Society of Dairy Technology (SDT, 1980), and Schubert (1987a). Table 9.4. Factors contributing to the quality of powders Appearance Size and shape Wettability Sinkability Solubility Dispersibility Bulk density and particle density Palatability Nutrient content Microbiological quality