The influence of high intensity ultrasound coupled with thermoprocessing on the inactivation of Geobacillus
stearothermophilus vegetative cells and spores in skim milk powder was explored using response
surface methodology and two polynomial models were developed. Optimization of cell reduction (4.8
log) was found to be at 19.75% total solids (TS), 45 C, and 30 s, while optimization of spore reduction
(0.45 log) was found to be at 31.5% TS, 67.5 C, and 17.5 s. Model verification experiments were performed
using common milk powder processing conditions. Results showed the inactivation of cells and spores to
be most effective before (9.2% TS, 75 C, and 10 s) and after (50% TS, 60 C, and 10 s) the evaporator
during milk powder processing and may produce an additive effect in microbial reduction when the two
locations are combined, resulting in a 5.8 log reduction for vegetative cells and 0.51 log reduction for
spores.