Inactivation of Staphylococcus aureus by the Combined Treatments of Ultrasound and Nisin in Nutrient Broth and Milk

Abstract

In this study, ultrasound (US) and nisin, applied individually or in combination (US + nisin), were investigated to determine their inactivation effect on Staphylococcus aureus in nutrient broth (NB) and milk. The inactivation of S. aureus by nisin at 0, 25 and 37°C followed “fast to slow” two-stage kinetics, and there was no significant difference in S. aureus reduction under these temperatures. A maximum reduction of 1.40 log10 cycles was obtained by nisin at 37°C for 15 min. The inactivation of S. aureus by US also followed “fast to slow” two-stage kinetics at 0°C, but it followed “fast–slow–fast” there-stage kinetics at 25 and 37°C. A maximum reduction of 5.87 log10 cycles was obtained by US at 968.16 W/cm2 and 37°C for 15 min. The reduction of S. aureus in milk was similar as in NB when treated at 968.16 W/cm2 and 25°C for 5–15 min. However, the reduction of S. aureus by US + nisin was similar to that obtained by either nisin or US alone, indicating that US and nisin had no added or synergistic inactivation effect on S. aureus in both NB and milk under the tested conditions.