Rehydration of food powders

In the vast majority of cases, powdered foods are intended for rehydration with water or, respectively, an aqueous liquid. An ideal powder should be wetted quickly and thoroughly, sink into the liquid rather than float on the surface and disperse/dissolve within a short period of time without lump formation. This ideal behaviour is difficult to achieve, since the, manufacturing processes usually yield particles of rather small size and/or unfavourable structure. The presence of fat or oil may further decrease the wettability. Food powder manufacturers are therefore struggling to improve the properties of such products, e.g., by agglomeration or by adding wetting agents to their recipes. Other materials may be so hydrophilic that they dissolve almost instantly, making them equally unwettable if applied in larger quantities. This work overviews current methods of characterising the wetting, sinking and dispersing behaviour of powdered foods used in our laboratory, the majority also having been developed there. Objective measuring methods, in conjunction with some basic knowledge of the mechanisms of wetting bulk materials, are indispensable tools for improving the properties of powdered products. The usefulness of these measuring methods and their limitations are discussed by means of measurement examples carried out on a variety of powdered foods. Simple measuring methods may typically be used for quality control purposes, provided that a more general understanding of a powder's behaviour has been gained before by use of more elaborate methods. It is thus generally possible to control the quality of consumer products, which a, re usually used in small quantities, using rather simple devices. The characterisation of powders intended for large scale application requires considerably more effort and, especially when the dispersion ability of a powder is to be described, the proper choice of measuring principle.