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

The survival and growth of Bacillus stearothelmophilus and Bacillus licheni formis, naturally present (30-300 colony forming units/ml) in late season s kim milk, was monitored in a three effect evaporator during low, heat skim milk powder manufacture. Substantial growth was shown to occur in the prehe ating stages prior to direct steam heating. A typical heat treatment (77 de grees C, 15 s) used in the manufacture of low hear powder did not inactivat e the bacteria, which continued to gr ow in the heater. The importance of p reheaters in influencing thermophile growth in the evaporator is demonstrat ed by the finding that growth in the preheater stages was accompanied by gr owth in subsequent evaporator effects which significantly exceeded that obs erved when the final two preheaters were bypassed. A mid-run mini-clean pro cedure incorporating 0.2% hydrogen peroxide for decontaminating the evapora tor was tested and may prove useful in extending evaporator run times.