Influence of Spray-Drying Conditions on Size and Size Distribution of Nonfat Dry Milk Particles

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Abstract
Particle size and size distribution of nonfat dry milk was determined by mi- croscopic, sieve, and Coulter Counter methods. Data obtained by using the microscopic method indicated the smallest mean particle size, while data from the sieve method indicated the largest mean size. Results of the Coulter Counter meth- od were changed slightly by the aperture size, but usually the curve was located be- tween the results of the microscope and sieve. The distribution of particle size in- creased with an increase of total solids (30 to 50%) or a decrease in pump pres- sure (245 to 106 kg/em~). Particle size was not significantly affected within the range of 121 to 177 C inlet air tempera- ture and 63 to 85 C preheat treatment. An accurate evMuation of the particle size distribution of nonfat dry milk requires measurements by at least two methods: Coulter Counter and sieve or microscope methods. The particular size and size distribution of dry milk are properties significantly influenced by the spray-drying conditions. These physical properties, in turn, have considerable influence on reconstitutability, packing density, and dustability of the product. Most previous investigations have used only the microscopic method of determining the particle size. In addition, methods used to calculate the mean particle size and deviation of size from the mean have not been uniform. Generally, the mean particle size will vary, depending on the method used. In our investigation three methods, micro- scopic, sieve, and the Coulter Counter, were used. Our objectives were to ascertain the particle size and size distribution of nonfat dry milk produced by different spray-drying conditions and to determine the relationship among the three methods on the same smnples. Hunziker (13) reported that dry milk par- ticles produced by commercial spray drying range from 10 to 100 /~. Washburn (26) indi-