The inactivation of the selected vegetative bacteria Escherichia coli, Listeria innocua, and Lactobacillus plantarum by high hydrostatic pressure (HHP) in physiological saline (PS) and in four fruit juices with pHs ranging from 3.4 to 6.3, with or without dissolved CO2, was investigated. The inactivation effect of HHP on the bacteria was greatly enhanced by dissolved CO2. Effective inactivation (>7 log) was achieved at 250 MPa for E. coli and 350 MPa for L. innocua and L. plantarum in the presence of 0.2 M CO2 at room temperature for 15 min in PS, with additional inactivation of more than 4 log for all three bacteria species compared with the results with HHP treatment alone. The combined inactivation by HHP and CO2 in tomato juice of pH 4.2 and carrot juice of pH 6.3 showed minor differences compared with that in PS. By comparison, the combined effect in orange juice of pH 3.8 was considerably promoted, while the HHP inactivation was enhanced only to a limited extent. In another orange juice with a pH of 3.4, all three strains lost their pressure resistance. HHP alone completely inactivated E. coli at relatively mild pressures of 200 MPa and L. innocua and L. plantarum at 300 MPa. Observations of the survival of the bacteria in treated juices also showed that the combined treatment caused more sublethal injury, which increased further inactivation at a relatively mild pH of 4.2 during storage. The results indicated that the combined treatment of HHP with dissolved CO2 may provide an effective method for the preservation of low- or medium-acid fruit and vegetable juices at relatively low pressures. HHP alone inactivated bacteria effectively in high-acid fruit juice.
Identification of novel genes involved in high hydrostatic pressure resistance of Escherichia coli