Effect of Preheating on Fouling of a Pilot Scale UHT Sterilizing Plant by Recombined, Reconstituted and Fresh Whole Milks

The principal aim of this study was to confirm and extend previous findings on the effects of milk preheating on fouling of UHT (ultra-high temperature) sterilizing plants by recombined whole milks. A comprehensive replicated experiment was carried out to examine the effects of preheat treatment on fouling by milk recombined from skim milk powder (SMP) and milk fat, milk reconstituted from whole milk powder (WMP), and fresh milk. Within each replicate the SMP, the WMP and the fresh milk were all derived from the same batch of milk. Fouling rates were measured in the high-temperature section of a pilot-scale indirect UHT plant. The effects of preheating conditions were compared as follows: UHT preheating conditions (recombined and fresh milk): 75°C × 11 s, 85°C × 147 s, and 95°C × 147 s; evaporator preheating conditions for skim milk powder manufacture: 75°C × 2 s, 85°C × 155 s, and 95°C × 155 s. The evaporator preheating conditions in whole milk powder manufacture (95°C × 33 s) were not varied. In all cases it was found that the more severe was a preheat treatment, whether evaporator preheating or UHT preheating, the higher was the fouling rate. The results, which are discussed in relation to previous findings for both recombined milks and fresh milk, suggest that the preheating of previously homogenized whole milks exacerbates fouling.