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 Log(10) (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 degrees C and 20 degrees C. Viable and injured bacterial counts were measured 2 h after UHPH treatment and after 3, 6 and 9 days of storage at 4 degrees 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 degrees 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 degrees C. No sublethal injuries were detected after treatments. The changes in cultivable cells during storage at 4 degrees 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.