Inactivation of thermoduric aerobic sporeformers in milk by ultrasonication

Thermoduric sporeformers can survive milk pasteurization and cause spoilages of dairy products. In the present study, ultrasonication was evaluated as a non-thermal processing technique to inactivate the thermally resistant vegetative cells of spore forming Bacillus spp. During the challenge studies, vegetative cells of Bacillus coagulans (ATCC^® 12245), Anoxybacillus flavithermus (DSM 2641), Bacillus sporothermodurans (DSM 10599), Bacillus licheniformis (ATCC^®6634), and Geobacillus stearothermophilus (ATCC^® 15952) were studied for their survivability to batch pasteurization (63 °C/30 min) in skim and whole milk samples. Experiments were conducted to study the effect of ultrasonication alone or in combination with pasteurization on inactivation of vegetative cells of some thermoduric Bacillus spp. Effect of ultrasonication on milk pH, color, and alkaline phosphatase activity was also investigated. Vegetative cells of B. coagulans and A. flavithermus survived pasteurization treatment in both skim and whole milk samples. Ultrasonication at 80% amplitude for 10 min however, inactivated the vegetative cells of B. coagulans and A. flavithermus in skim milk by 4.53, and 4.26 logs, respectively. A combined treatment of pasteurization (63 °C/30 min) followed by ultrasonication completely eliminated approximately log 6 cfu/mL of these cells in skim milk. As visualized under the scanning electron microscope, ultrasonication physically disintegrated vegetative cells of sporeformers. Ultrasonication treatment caused significant reduction (P < 0.05) in brightness and greenness of milk; whereas, blueness (b*) of milk was increased. However, pH and alkaline phosphatase activity (P > 0.05) of treated skim milk were not affected.