Abstract
The influence of an alternating current (AC) frequency from 50 to 20,000 Hz at 30 V/cm using ohmic heating on the inactivation of E. coli O157:H7 in buffer peptone water (BPW) and pomelo juice was investigated. Frequency affected bacterial reduction in both BPW and pomelo juice with the same trend, and the most efficient reductions were obtained at 60 and = 500 Hz. Besides, the resistance of E.coli O157:H7 was investigated. BPW and pomelo juice were treated at different temperatures (62-71 C) for various times (0-30 s) by ohmic heating (OH) and compared with conventional heating (CH). The technical parameters of OH were an electric field strength of 30 V/cm and frequencies of 60 and 500 Hz. At each temperature, the decimal reduction time (D) and the decimal reduction temperature (z) were determined. Microorganism reduction was significantly higher (P < 0.05) with OH than with CH, and there was no significant difference between the juice samples treated by OH at 60 and 500 Hz. For OH at 60 Hz, the D values of E. coli O157:H7 in pomelo juice were 59.0, 27.3, 10.3, and 3.6 s at temperatures of 60, 62, 65, and 68 C, with a z value of 6.7 C. For CH, the D values were 35.4, 14.3, 8.2, and 2.9 s, with a z value of 8.6 C. From the calculated D and z values, a processing schedule for the pasteurization of pomelo juice can be estimated. Observing the bacterial cells by transmission electron microscopy (TEM) showed considerable changes in the morphology of the bacterial cells with OH, which might be the cause of cell death. These results demonstrated that the electric field obtained with OH yields additional microbial destruction. As a result, the time and temperature required for bacterial inactivation can be reduced, diminishing the negative heat effects of pasteurization on the food products.
