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

Abstract

To investigate methods for inactivating a pressure-resistant strain of Escherichia coli O157 in fruit juices. 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 log10 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 degrees C for 24 h or 25 degrees C for 3 h, the level of inactivation of E. coli O157 strain C9490 increased to 4.4 or > 7 log10 units, respectively. 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. 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.