The aim of this study was to compare the inactivation effects of ohmic heating (internal heating by electric current) and conventional heating (external heating by hot water) on viable aerobes and Streptococcus thermophilus 2646 in milk under identical temperature history conditions. The effects of the two treatments on quality of milk were also compared by assessing degrees of protein denaturation in raw and sterilized milk (raw milk being sterilized by ohmic heating or conventional heating). It was found that microbial counts and calculated decimal reduction time (D value) resulting from ohmic heating were significantly lower than those resulting from conventional heating. There was no difference in degrees of protein denaturation during the two treatments. The results suggested that ohmic heating had not only a thermallethal effect, but also a nonthermal-lethal effect on microorganisms, due to the electric current. Based on the results, we propose that ohmic heating can be effectively used to pasteurize milk with no additional protein deterioration.

We previously reported that ohmic heating had not only a thermal lethal effect but also a nonthermal lethal effect on microorganisms because of the electric current. In order to find the reasons for these lethal effects, experiments on milk sterilization were conducted with Streptococcus thermophilus 21072. Sublethal‐ohmic heating and lethal‐hot‐water heating treatment (ohmic and hot‐water treatment), and sublethal‐hot‐water heating and lethal‐hot‐water heating treatment (hot‐water treatment as a control), were carried out under the condition of identical temperature histories. Microbial counts and calculated decimal reduction times indicated that the ohmic and hot‐water treatment had significantly higher lethality than the hot‐water treatment. However, no sterilizing effect of the electric current on microorganisms was observed during sublethal‐ohmic heating treatment. Compared to sublethal‐hot‐water heating treatment, larger amounts of adenosine triphosphate and lactate dehydrogenase were exuded when the cells were subjected to sublethal‐ohmic heating treatment. Therefore, the electric current of sublethal‐ohmic heating treatment increased the permeability of the cell membrane, resulting in nonthermal injury to S. thermophilus cell membrane. PRACTICAL APPLICATIONS This technology of ohmic heating will widely apply to milk and other liquid pasteurizations, based on the advantages of rapid and uniform heating, a significant sterilizing effect and a reduction in thermal damage to products compared with conventional heating. The technology of ohmic heating is promising as a more effective and cheaper method for the pasteurization of liquid type food products than conventional methods.

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