1. Study the role of ultrasonic transmission power levels and treatment times in inactivation of spores of aerobic spore formers in non-fat milk and non-fat milk concentrate. </P>
2. Study the induction of resistance in spores in non-fat milk. </P>
3. Measure sensory quality attributes of ultrasonicated milk.
Bacterial spores are a concern in dried dairy powders, such as nonfat dry milk, because they cause thermal instability issues in reconstituted milk. It is practically impossible to eliminate aerobic spore formers in raw milk due to their ubiquitous nature. Pasteurization inactivates vegetative cells but it is inadequate in killing their spores. The surviving spores of Geobacillus stearothermophilus, B. coagulans, B. sporothermodurans, and B. licheniformis germinate and quickly attain high numbers that spoil product. Additionally, sub-lethal heat treatments, acid or cold shock can also enhance the heat resistance of these spores. Researchers hypothesized that ultrasonication (US) would inactivate a majority of spores from these aerobic spore formers and lower their heat resistance so they are more susceptible to pasteurization. Applying US to fluid skim milk and skim milk concentrates would reduce the final load of aerobic sporeformers in skim milk powder. Researchers developed model studies by spiking samples of nonfat milk and nonfat milk concentrate with spores of Bacillus sporothermodurans, B. coagulans, B. licheniformis, and Geobacillus. stearothermophilus at cfu 108/mL levels. The inoculated milk samples were subjected to US transmission at about 400-500W input power for 15 sec, 30 sec, and 1.0 min, under regular and controlled temperature conditions. Bacterial counts and log reductions from pre- and post-treatment were compared to determine the effectiveness of US. The study found that US reduces vegetative cells of spore formers by 4 logs, but it also stimulates spore germination in skim milk and its concentrates. B. licheniformis was most resistant to US, but the process did not affect G. stearothermophilus.