Use of mild-heat treatment following high-pressure processing to prevent recovery of pressure-injured Listeria monocytogenes in milk

Abstract

This study examined the inactivation of Listeria monocytogenes in milk by high-pressure processing (HPP) and bacterial recovery during storage after HPP. We developed a technique to inhibit the bacterial recovery during storage after HPP (550 MPa for 5 min) using a mild-heat treatment (30–50 °C). Various mild-heat treatments were conducted following HPP to investigate the condition on which the bacterial recovery was prevented. Immediately after HPP of 550 MPa at 25 °C for 5 min, no L. monocytogenes cells were detected in milk regardless of the inoculum levels (3, 5, and 7 log 10 CFU/ml). However, the number of L. monocytogenes cells increased by >8 log 10 CFU/ml regardless of the inoculum levels after 28 days of storage at 4 °C. Significant recovery was observed during storage at 25 °C; the bacterial number increased by >8 log 10 CFU/ml after 3 days of storage in the case of an initial inoculum level of 7 and 5 log 10 CFU/ml. Even in the case of an initial inoculum level of 3 log 10 CFU/ml, the bacterial number reached the level of 8 log 10 CFU/ml after 7 days of storage. No bacterial recovery was observed with storage at 37 °C for 28 days. Milk samples were treated by various mild-heat treatments (30–50 °C for 5–240 min) following HPP of 550 MPa at 25 °C for 5 min, and then stored at 25 °C for 70 days. The mild-heat treatment (e.g., 37 °C for 240 min or 50 °C for 10 min) inhibited the recovery of L. monocytogenes in milk after HPP. No recovery of L. monocytogenes in milk was observed during 70-day storage at 25 °C in samples that received mild-heat treatments such as mentioned above following HPP (550 MPa for 5 min). Moreover, the mild-heat treatment conditions (temperature and holding time) required to inhibit the recovery of L. monocytogenes in milk was modelled using a logistic regression procedure. The predicted interface of recovery/no recovery can be used to calculate the mild-heat treatment condition to control bacterial recovery during storage at 25 °C after HPP (550 MPa for 5 min). The results in this study would contribute to enhance the safety of high-pressure-processed milk.