Effects of high pressure and mild heat on endogenous microflora and on the inactivation and sublethal injury of Escherichia coli inoculated into fruit juices and vegetable soup

The objective of this study was to determine the effects of high-pressure treatments and mild temperatures on endogenous microflora and Escherichia coli CECT 515 artificially inoculated into orange and apple juices and vegetable soup. In general, the viability of aerobic bacteria was significantly reduced as pressure and temperature increased. Although the greatest reduction in the concentration of aerobic mesophilic vegetative cells was reached at 350 MPa and 60 degrees C, the same reduction occurred in fruit juices at 350 MPa and 20 degrees C. Yeasts and molds were below the level of detection (1 log CFU/ml) for the fruit juices and did not exceed 2 log CFU/ml for vegetable soup. Foods inoculated with E. coli were subjected to several treatments as indicated by the mathematical model applied in response surface methodology to obtain the maximum information with the minimum number of experiments. The number of tests for a range of pressures (150 to 350 MPa) and temperatures (20 to 60 degrees C) was limited to 11. The models were considered adequate because of satisfactory R2 values. The optimum process parameters (pressure and temperature) for a 6-log reduction of E. coli were obtained at 248.25 MPa and 59.91 degrees C in orange juice, 203.50 MPa and 57.18 degrees C in apple juice, and 269.8 MPa and 59.9 degrees C in vegetable soup. Sublethal injury of E. coli occurred as pressure and temperature increased. Nearly all of the E. coli cells were injured at 350 MPa and 20 degrees C in fruit juices and after all treatments in vegetable soup.