Growth of thermophilic spore forming bacilli in milk during the manufacture of low heat powders

The survival and growth of Bacillus stearothermophilus and Bacillus licheniformis, naturally present (30–300 colony forming units/ml) in late season skim milk, was monitored in a three effect evaporator during low heat skim milk powder manufacture. Substantial growth was shown to occur in the preheating stages prior to direct steam heating. A typical heat treatment (77°C, 15 s) used in the manufacture of low heat powder did not inactivate the bacteria, which continued to grow in the heater. The importance of preheaters in influencing thermophile growth in the evaporator is demonstrated by the finding that growth in the preheater stages was accompanied by growth in subsequent evaporator effects which significantly exceeded that observed when the final two preheaters were bypassed. A mid‐run mini‐clean procedure incorporating 0.2% hydrogen peroxide for decontaminating the evaporator was tested and may prove useful in extending evaporator run times