The atomization stage in spray drying of a liquid requires a spray of droplets with a high surface-to-mass ratio. Dried products that result from moisture evaporation of atomized spray can be prepared to reach the desired particle size distribution through control of atomization variables. To meet these requirements, many atomization techniques such as pressure nozzles, two-fluid nozzles and rotary atomizers have been used in spray dryers. Many authors have proposed various relationships between droplet size, physical properties (e.g. viscosity of concentrate) and technological parameters (e.g. nozzle pressure). To optimize the moisture content in milk powder, it is necessary to control the thermodynamic parameters of the drying air, the conditions of the atomization and the properties of the concentrate. Droplet size varies in direct relationship with viscosity to the power of 0.17-0.25. There are, in fact, two possible solutions for measuring online viscosity: either an online viscometer (direct measurement) or measurement of the amperage of the concentrate extraction pump (indirect measurement). The aim of this study was therefore to develop an indirect method to estimate the viscosity of a concentrate after concentration by filtration or vacuum evaporation and before spray drying. For example, while varying the viscosity of different dairy products (concentrated whey, and whole and skim milk) by changing the parameters of concentration by vacuum evaporation, we obtained a very good correlation between concentrate viscosity and the extraction pump amperage, whatever the product. Concentration parameters can be modified during evaporation to obtain a concentrate of the same viscosity, whatever its temperature or total solid content, just by following the extraction pump amperage.
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Schuck et al. (2005) studied this question.
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