Influence of pre-heat temperature, pre-heat holding time and high-heat temperature on fouling of reconstituted skim milk during UHT processing

Abstract

The effects of pre-heat temperature (65–95°C) and holding time (5 and 25s) on fouling of reconstituted skim milk at high-heat temperatures of 135–150°C using a bench-top UHT plant were investigated. Fouling was monitored by observing the variation in overall heat transfer coefficient (OHTC) with time and consisted of an induction phase (non-fouling) and a decline phase (fouling). A significantly shorter run-time was obtained with pre-heating at 65°C than with pre-heating at 75, 85 and 95°C. A longer holding time (25s) in the pre-heating section significantly increased the run-time of the plant compared with 5s holding. The high-heat temperature also affected the OHTC and run-time; similar run-times were observed at 135, 140, 142 and 145°C but at 150°C there was no induction phase and fouling rapidly occurred. Overall, pre-heating at 85°C or 95°C for 25s, combined with a high-heat temperature of 145°C, and pre-heating at 95°C for 25s combined with high-heat temperatures of 142°C, produced the longest run-times of the bench-top UHT plant.