Inactivation and injury of pressure-resistant strains of Escherichia coli O157 and Listeria monocytogenes in fruit juices

Aims: To investigate methods for inactivating a pressure-resistant strain of Escherichia coli O157 in fruit juices. Methods and Results: Cells of a pressure-resistant strain of E. coli O157 (C9490) were exposed to pressures of between, 0.1 and 500 MPa for 5 min in orange, apple or tomato juice. Treatment at 500 MPa achieved an immediate reduction of 5 log units in apple juice (pH 3.5) and tomato juice (pH 4.1), but only about a 1-2 log(10) reduction in orange juice (pH 3.8). The greater level of inactivation in tomato juice than in orange juice of lower pH was due to the presence of low levels (0.7%) of salt in the tomato juice. With the type-strain of E. coli (ATCC 11775) and Listeria monocytogenes NCTC 11994, similar levels of inactivation were achieved at pressures 200 MPa lower. Following storage of pressure-treated orange juice at 4 degreesC for 24 h or 25 degreesC for 3 h, the level of inactivation of E. coli O157 strain C9490 increased to 4.4 or > 7 log(10) units, respectively. Conclusions: Treatment at 500 MPa may be insufficient to achieve a '5D' reduction in counts of pressure-resistant strains of E. coli, but subsequent death during storage substantially increases process lethality. Significance and Impact of the Study: Commercially-practicable pressure processes can be used to inactivate even the most pressure-and acid-resistant strains of E. coli O157, provided that processing and subsequent storage conditions are carefully optimized.