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

• Pre-heat temperature, pre-heat holding time and high-heat temperature influenced fouling.
• A low pre-heat temperature of 65 °C caused rapid fouling.
• A high-heat temperature of 150 °C caused rapid fouling.
• A longer pre-heat holding time of 25 s caused less fouling than the 5 s holding.
• Preheating at 85 °C or 95 °C for 25 s with high-heating at 142 or 145 °C gave the longest run-times.

The effects of pre-heat temperature (65–95 °C) and holding time (5 and 25 s) 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 (25 s) in the pre-heating section significantly increased the run-time of the plant compared with 5 s 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 25 s, combined with a high-heat temperature of 145 °C, and pre-heating at 95 °C for 25 s combined with high-heat temperatures of 142 °C, produced the longest run-times of the bench-top UHT plant.