Temperature Variations in the Holding Tube During Ultra High-Temperature Pasteurization by Steam Injection

Abstract

Temperature variations after steam injection were monitored at 10 positions along a 61-cm holding tube with highspeed thermocouples (thermometric lag of approximately 40 milliseconds). Three steam injectors were tested with water (tap and deaerated) and milk at from 880 to 1,330 liters per hour, temperature increases by steam injection from 22 to 33C, holding temperatures from 88 to 110C, and holding pressures from 1.2 to 3.0atm (abs.). Skimmilk (9% and 42% solids), chocolate milk, cream, and ice cream mix were also studied under selected processing conditions. Temperatures varied greatly (±15C) with water, milk, and skimmilk (42% solids) under some conditions; however, temperature variations at the injector exits (5cm downstream) were eliminated when the following were imposed: a) all injector ports were orificed to isolate the product and steam flows from pressure fluctuations in the injection chamber, b) the process was as free as possible of noncondensable gases, and c) the holding pressure was at least 0.2atm above the product saturation vapor pressure. Accordingly, temperature variations in the holding tube can be eliminated by adhering to these operating conditions.