Inactivation of two strains of Escherichia coli inoculated into whole and skim milk by ultrahigh-pressure homogenisation

The inactivation by ultrahigh-pressure homogenisation (UHPH) of Escherichia coli ATCC 10536 and Escherichia coli O157:H7 CCUG 44857 inoculated into whole and skim milk was investigated. Samples of UHT whole and skim milk inoculated at a concentration of approximately 7.0 Log10 (cfu.mL-1) were pressurised in a two-valve system UHPH machine at 300 MPa at the primary homogenising valve and at 30 MPa at the secondary valve. Inlet temperatures of milk were 6 °C and 20 °C. Viable and injured bacterial counts were measured 2 h after UHPH treatment and after 3, 6 and 9 days of storage at 4 °C. The type of milk significantly influenced (P < 0.05) the degree of inactivation reached in both strains of E. coli, being higher at 20 °C in whole milk. The level of inactivation was similar for E. coli ATCC 10536 and E. coli O157:H7 CCUG 44857, reaching lethality values of 4.30 and 3.94 Log 10 cfu.mL-1, respectively, at an inlet temperature of 20 °C. No sublethal injuries were detected after treatments. The changes in cultivable cells during storage at 4 °C were similar in whole and skim milk although the E. coli O157:H7 CCUG 44857 strain showed significant differences, with a decreasing tendency of approximately 0.3 logarithmic units between 0 and 9 days of storage. © INRA, EDP Sciences, 2006.