Effect of ohmic heating for inactivation of food-borne pathogens in fruit juice

Abstract

The effect of electric field-induced ohmic heating for inactivation of Escherichia coli O157:H7, Salmonella Typhimurium and Listeria monocytogenes in buffered peptone water (BPW

pH 7.2) and apple juice (pH 3.5, 11.8 °Brix) was investigated in this study. BPW and apple juice were treated at temperatures (55, 58, and 60°C) and times (0, 10, 20, 25, and 30s) by ohmic heating and compared with conventional heating. The electric field strength was fixed at 30 V/cm and 60 V/cm in BPW and apple juice, respectively. Bacterial reduction resulting from ohmic heating was significantly different (P < 0.05) from that of conventional heating at 58 and 60°C in BPW and at 55, 58 and 60°C in apple juice for intervals of 0, 10, 20, 25, and 30s. These results show that electric field-induced ohmic heating led to additional bacterial inactivation at sublethal temperatures. Transmission electron microscopy (TEM) observations and the propidium iodide (PI) uptake test were conducted after treatment at 60°C for 0, 10, 20, 25 and 30s in BPW to observe the effects on cell permeability due to electroporation caused cell damage. PI values when ohmic and conventional heating were compared were significantly different (P < 0.05) and these differences increased with increasing levels of inactivation of three food-borne pathogens. These results demonstrate that ohmic heating can more effectively reduce bacterial populations at reduced temperatures and shorter time intervals, especially in acidic fruit juices such as apple juice. Therefore, loss of quality can be minimized in a pasteurization process incorporating ohmic heating. Additionally, the optimization of ohmic heating conditions for inactivation of E. coli O157:H7, S. Typhimurium, and L. monocytogenes in 72, 48, 36, 24, and 18 °Brix apple juices was examined in this study. Voltage gradients of 30, 40, 50, and 60V/cm were used in all samples for recording temperatures. In all experiments, the heating rate was most rapid in 36 °Brix apple juice at 30, 40, 50, and 60V/cm. System performance coefficients (SPC) were obtained in 72, 48, 36, 24, and 18 °Brix apple juice at 30, 40, 50, and 60V/cm. For 24 °Brix apple juice, system performance at 30V/cm was the most efficient of all voltage gradients. Also, SPC values between 24 and 36 °Brix apple juice at 30V/cm were not significantly (P > 0.05) different, and 36 °Brix apple juice was more effective with respect to product yield. At 60V/cm, peak system efficiency occurred in 48 °Brix apple juice. Although system efficiency of 36 °Brix apple juice at 30V/cm and 48 °Brix apple juice at 60V/cm were significantly (P < 0.05) different, the disparity between those values was only 8%. In addition, 5-log reduction was accomplished in 36 °Brix apple juice at 30V/cm for 60s while it was achieved in 48 °Brix apple juice at 60V/cm within 20s. These results demonstrate that treatment of 48 °Brix apple juice at 60V/cm was the most effective combination with regard to economic and bactericidal considerations.