Flow Characteristics and Holding Time Calculations of Ice Cream Mixes in HTST Holding Tubes

In order to determine the potential for development of laminar flow and consequential underholding in the holding tubes of HTST pasteurizers, a study on the relationship between ice cream mix viscosity and shear rate at 80°C has been conducted. Typical shear rates at the wall were calculated for HTST holding tubes of standard industry sizes and flow rates. Shear rates in the holding tube were found to vary from 50 to 180 s^-1, depending on the conditions. Viscosity of ice cream mixes as a function of shear rate, stabilizer type, and stabilizer concentration were measured. Ice cream mix was found to be non-Newtonian and pseudoplastic. Viscosities ranged from 8.7 cP in an unstabilized mix at high shear rate (relative to the inside of the holding tube) to 103 cP for 0.25% carboxymethyl cellulose at low shear rate. Generalized Reynolds numbers inside the holding tubes varied from 100 to 1700, indicating a strong potential for the development of laminar flow. The apparent viscosities required to result in a minimum generalized Reynolds number of 2100 are very near to or less than the actual viscosities of stabilized ice cream mixes, and thus the potential for a laminar flow pattern within the holding tube needs to be addressed in determining holding tube lengths for a required holding time.