Inactivation of Microorganisms in Milk and Apple Cider Treated with Ultrasound

Abstract

Nonthermal technologies are emerging as promising alternatives to heat treatment for food processing. Ultrasound, defined as sound waves with a frequency greater than 20 kHz, has proven bactericidal effects, especially when combined with other microbial-reduction strategies such as mild heating. In this study, ultrasound treatment (sonifier probe at 20 kHz, 100% power level, 150 W acoustic power, 118 W/cm2 acoustic intensity) with or without the effect of mild heat (57°C) was effective at reducing microbial levels in raw milk, Listeria monocytogenes levels inoculated in ultrahigh-temperature milk, and Escherichia coli O157:H7 in apple cider. Continuous flow ultrasound treatment combined with mild heat (57°C) for 18 min resulted in a 5-log reduction of L. monocytogenes in ultrahigh-temperature milk, a 5-log reduction in total aerobic bacteria in raw milk, and a 6-log reduction in E. coli O157:H7 in pasteurized apple cider. Inactivation regressions were second-order polynomials, showing an initial period of rapid inactivation, eventually tailing off. Results indicate that ultrasound technology is a promising processing alternative for the reduction of microorganisms in liquid foods.