Inactivation of Escherichia coli and Listeria innocua in apple and carrot juices using high pressure homogenization and nisin

High pressure homogenization has been of growing interest as a nonthermal technology for the inactivation of microorganisms in fruit and vegetable juices. Cells of Escherichia coli and Listeria innocua, used as surrogates for foodborne pathogens, were inoculated into apple or carrot juice (~ 7 log₁₀ CFU/ml) containing 0 or 10 IU/ml nisin and subjected to 350 to 0 MPa high pressure homogenization. At 50 MPa homogenization pressure intervals, juice samples were collected, immediately cooled to < 10 °C, and then serially diluted and plated on nonselective recovery media. Following incubation, survivors were enumerated. As processing pressure increased, inactivation of E. coli increased, and a > 5 log reduction of cells was achieved following exposure to pressures in excess > 250 MPa. In contrast, little inactivation was observed for L. innocua with pressure < 250 MPa and up to 350 MPa processing pressure was required to achieve an equivalent 5 log inactivation. The addition of 10 IU nisin, together with high pressure homogenization, did not exhibit significant additional E. coli inactivation, but interactions were observed with L. innocua. Results indicate that high pressure homogenization processing is a promising technology to achieve pathogen decontamination in fruit and vegetable juices.