The inactivation of Shiga toxin-producing Escherichia coli (STEC) and Listeria monocytogenes using isochoric freezing in raw milk and carrot juice

Abstract

Isochoric freezing is a novel food preservation method that may maintain the quality and safety of products by simultaneously applying low temperatures and high pressures. This work aims to determine whether isochoric freezing can improve food safety by inactivating Shiga toxin-producing Escherichia coli (STEC) and Listeria monocytogenes in raw milk and carrot juices. Raw milk and carrot juice samples were inoculated with cocktails of either STEC or L. monocytogenes and then subjected to isochoric freezing conditions in pressurized chambers set at −5, −10, and −15 °C and treated for 1, 5, 7, or 10 days. Following treatment, the product was sampled for surviving pathogens. The results demonstrated that a 5-log reduction of both STEC and L. monocytogenes can be achieved for raw milk and carrot juice without adversely affecting quality. For STEC inoculated in raw milk, both the linear and Weibull models suggest that 10 days is required to achieve 5-log reduction at −10 °C. Decreasing the temperature to −15 °C led to accelerated log reduction but induced phase separation in the milk. Listeria in raw milk demonstrated a biphasic reduction, indicating 1.3 days is required for a 5-log reduction at −10 °C. In comparison, pathogens demonstrated faster log reduction in carrot juice due to its intrinsic properties. The survival curves for STEC-inoculated carrot juice demonstrated that the 5-log reduction times were 6.9 days at −10 °C and 3.8 days at −15 °C. L. monocytogenes in carrot juice required 1.5 days at −10 °C for a 5-log reduction and was eliminated (7 log cfu/ml) within 24 h at −15 °C. Isochoric freezing could be an option for milk and carrot juice processors wishing to improve food safety without applying heat. This method is not a one-size-fits-all solution, and the time and temperature will need further optimization depending on the target pathogen and intended commodity.