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Residence Time of Milk Products in Holding Tubes of High-Temperature Short-Time Pasteurizers

National Center for Urban and Industrial Health U. S. Public Health Service, Cincinnati, Ohio

ABSTRACT

Holding tubes are sized by performing the salt conductivity test on water flowing at pasteurization temperature. Because dairy products have a higher viscosity than water, the fastest product particles could be held for shorter times than predicted by the salt test. A study was done to determine the error attributable to this practice. A high-temperature short-time pasteurizer unit was operated at 1,130 liters/hour at pasteurization temperature, and residence times were determined by measuring the time required for an injection of radio-active iodine to traverse the holding tube. Residence times of milk products were 21.9 and 23.5 sec, respectively, for 16.5 and 10% milk-fat ice cream mix, 22.7 sec for cream (40% milk fat), and 17.2 sec for condensed skimmilk (40% solids) flowing in a tube sized for 26 sec by salt injection; and 15.8 and 15.5 sec, respectively, for milk and chocolate milk flowing in a 16-sec tube. Sizing holding tubes by the salt test is satisfactory for milk and chocolate milk; however, to ensure proper holding times for ice cream mix, cream, and condensed skimmilk, tubes should be sized by calculating fastest particle velocity of product in laminar flow.