Inactivation of Escherichia coli O157:H7 in orange juice using a combination of high pressure and mild heat

The effect of high pressure on the survival of a pressure-resistant strain of Escherichia coli O157:H7 (NCTC 12079) in orange juice was investigated over the pH range 3.4 to 5.0. The pH of commercial, sterile orange juice was adjusted to 3.4, 3.6, 3.9, 4.5, or 5.0. The juice was then inoculated with 10(8) CFU ml(-1) of E. coli O157:H7. The inoculated orange juice was subjected to pressure treatments of 400, 500, or 550 MPa at 20 degrees C or 30 degrees C to determine the conditions that would give a 6-log(10) inactivation of E. coli O157:H7. A pressure treatment of 550 MPa for 5 min at 20 degrees C produced this level of kill at pH 3.4, 3.6, 3.9, and 4.5 but not at pH 5.0. Combining pressure treatment with mild heat (30 degrees C) did result in a 6-log(10) inactivation at pH 5.0. Thus, the processing conditions (temperature and time) must be considered when pressure-treating orange juice to ensure microbiological safety.