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Influence of Drying Methods on Density and Porosity of Milk Powder Granules

Dairy Products Laboratory, Eastern Utilization Research and Development Division, U. S. Department of Agriculture, Washington, D. C.

ABSTRACT

Gas and liquid displacement techniques were used to determine the true and apparent densities as well as the relative porosities of milk powder granules produced by various drying techniques. Using helium as the displacing gas, rapid penetration of conventional spray-dried, spray-dried foam and vacuum-dried foam particles was noted; all whole milk powders had true densities averaging 1.30 g/cm³. By this method, the measured true density of a commercial instantized skimmilk powder was 1.48 g/cm³. When hydrogen, nitrogen, decalin, or mercury was used as a displacing medium, data were obtained showing a wide variation in the porosity of particles produced by different drying techniques. It was concluded that conventional spray-dried material contained small amounts of occluded gas exchanged only with great difflculty, if at all. Spray-dried foam particles contain relatively large volumes of gas which can undergo exchange slowly and completely. The small quantities of trapped gas in vacuum foam-dried particles can undergo rapid and complete exchange.