R. Ruan

13 Ohmic heating

Over the past two decades, there has been an increasing shift from batch thermal operation towards continuous High Temperature Short Time (HTST) processing of foods. In HTST processes, food is processed continuously through plate or scraped surface heat exchangers at temperatures as high as 140oC. At this temperature, only a few seconds are needed for sterilization, during which the products suffer only a slight quality deterioration. HTST processes rely on rapid convection heat transfer and are thus well suited to liquid foods. They are, however, limited in application to particulates since, for particles more than a couple of millimeters thick, the processing time is insufficient for heat to transfer to the center to give sterility. In ohmic heating processes, foods are made part of an electric circuit through which alternating current flows, causing heat to be generated within the foods due to the electrical resistance of the foods. Therefore, in a liquid-particulate food mixture, if the electrical conductivity of the two phases are comparable, heat could be generated at the same or comparable rate in both phases in ohmic heating. In other conditions, heat can also be generated faster in the particulate than in the liquid. Ohmic methods thus offer a way of processing particulate food at the rate of HTST processes, but without the limitation of conventional HTST on heat transfer to particulates.