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.
Effect of electric current of ohmic heating on nonthermal injury to streptococcus thermophilus in milk
