Milk fouling at direct ohmic heating

Direct ohmic heating (Joule's heating) is a technology to warm up the food using an electric energy where electric current is passed through a material which gets heated by virtue of its electrical resistance. Advantages over conventional indirect heating methods are speed and uniformity of heating. On the other side, the direct ohmic heating of liquid foods has some problems, for example corrosion and deposits creation on electrodes (fouling). The present research on fouling concentrates upon the effects of material and surface properties of electrodes, and upon the effect of power frequency. This work evaluates influence of material (stainless steel, TiN and graphite electrodes), flow rate, electric current density (at constant frequency 50 Hz) and temperature (in a limited temperature range 65-75 degrees C), upon the fouling of skimmed milk. Results prove that the stainless steel electrodes are the worst while the graphite electrodes, where no fouling was observed, are the best, thus confirming the significant role of corrosion and electrochemical phenomena. The paper analyzes also another phenomenon typical for direct ohmic heating and it is an inner overheating of the formed fouling layer, representing a risk of overburning. Theoretical results show that the most intensive overheating occurs at relatively very thin deposit layers, depending upon the specific electrical conductivity of deposits. (C) 2010 Elsevier Ltd. All rights reserved.