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

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 23.5sec, respectively, for 16.5 and 10% milk-fat ice cream mix, 22.7sec for cream (40% milk fat), and 17.2sec for condensed skimmilk (40% solids) flowing in a tube sized for 26sec by salt injection; and 15.8 and 15.5sec, 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.