Pasteurized concentrated skim milk containing 35 to 40% total solids was inoculated with cultures of Bacillus subtilis, Micrococcus flavus, or Escherichia coli to contain 1 × 106 organisms per g and spray-dried to determine the effect of exit air temperatures of 93.3, 82.2, and 71.1 C on survival of the organisms and moisture content of the finished product. The numbers of survivors increased as the drying temperature decreased. The percent survivors varied from 27.57 in the product made from milk inoculated with M. flavus and dried at 71.1 C to 0.02 in the product made from milk inoculated with E. coli and dried at 93.3 C. The organism most resistant to drying and most persistent during storage was B. subtilis, followed by M. flavus and E. coli, with the latter showing low survival during drying and abrupt die-off during the first 4 weeks of storage. The moisture content of the dry milks varied from 2.75 to 4.80% with low moisture associated with high drying temperature.
1Michigan Agricultural Experiment Station Journal Article NO. 7931.