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Flow Profiles in the Plates of a Milk Pasteurizer

U.S. Department of Health, Education, and Welfare, Public Health Service, Food and Drug Administration, Bureau of Foods, Cincinnati, Ohio 45226

ABSTRACT

Microbial lethality during the heating and cooling periods of ultra-high temperature pasteurization can be as much as 10 times that occurring in the holding tube. As a step in determining lethality in the heating and cooling sections of a pasteurizer, residence time profiles of milk in a single plate were determined at flow rates of from 500 to 2,000 liters/ h. The geometry of a typical flow passage was determined with a stainless steel plate and a duplicate clear plastic plate through which flow was observed. Dye was injected into the milk flow and photographed with a 16-mm camera. At the nominal flow rate of 1,100 liters/h, minimum residence time was .78 s, whereas the average residence time was 2.08 s. The ratios of minimum to average residence times for milk flowing at 500, 1,000, and 2,000 liters/h were .31, .38, and .46. Values were similar for both upward and downward flows. A high-velocity zone (short residence time) was observed along each side of the plate.