Ohmic heating is a food processing method in which an alternating electrical current is passed through a food sample. This results in internal energy generation in foods. This produces an inside-out heating pattern, which is much faster than conventional outside-in heating. Ohmic heating is somewhat similar to microwave heating but with very different frequencies. The advantage of ohmic heating is that it uniformly heats foods with different densities, such as chicken noodle soup, for example.
Ohmic heating technology has been around since the early 1900s, but it was not until the late 1980s that food pro-cessing researchers began investigating the potential of ohmic heating for food quality, and cost and energy savings in food processing.
Most initial research on ohmic heating has been conducted on heat transfer and thermal processing of food mixtures. Recently, issues involving mass transfer of components in foods during ohmic heating have been studied. Potential applications for ohmic heating include blanching, evaporation, dehydration, fermentation and extraction. LSU AgCenter researchers are investigating ways to use ohmic heating in food processing.
Freeze-drying sweet potatoes
Sweet potato samples were ohmically heated and freeze-dried, and their freeze drying rate was measured and compared with a control (no heating). Ohmic heating increased the rate of freeze-drying up to 25 percent, a significant time and energy savings for processing. Freeze-drying is a time- and energy-intensive process, thus any method that cuts drying time significantly is important. Additionally, freeze-drying is one step in the supercritical fluid extraction process, which shows promise as a fast, environmentally friendly way to extract beta-carotene and other high value components from sweet potato tissue.
Extracting oil from rice bran
Rice bran oil can be extracted from rice bran and used as cooking oil. Although rice bran oil has outstanding nutritive, sensory and cooking characteristics, it is relatively expensive to produce. Ohmic heating could enhance the extraction of rice bran oil from rice bran, with the ultimate goal of making the production of rice bran oil economically feasible. Ohmic heating is a fast process (on the order of seconds) that enhances the extraction of apple juice from apples, sucrose from sugar beets and soy milk from soybeans. LSU AgCenter researchers conducted studies to determine if ohmic heating could enhance the extraction yield of rice bran oil from rice bran.
Rice bran was ohmically heated and the oil subsequently extracted. Ohmically heated samples yielded more total lipids from rice bran (Figure 1). Researchers also determined that lowering the frequency of alternating current during ohmic heating resulted in enhanced extraction yields.
Ohmic heating saves time
Ohmic heating saves significant time and energy in hot air and freeze drying of foods and enhances extraction yields in some processing operations. The parameters used during ohmic heating, such as frequency of alternating current, applied voltage and the temperature to which the sample is heated, have a significant effect on its success. The electrical conductivity of the food or food mixture is a significant factor, too. Ohmic heating is a useful tool for value-added processing, and it has great potential for use in a wide variety of food processing operations involving heat and mass transfer.
Acknowledgments Vicki Lancaster and Terry Walker contributed to this research; James Finney, Tom Bride and the late Malcolm Gaspard provided technical assistance.
Marybeth Lima, Associate Professor; and Tuoxiu Zhong and N. Rao Lakkakula, former Graduate Students, Department of Biological & Agricultural Engineering, LSU AgCenter, Baton Rouge, La.
(This article appeared in the fall 2002 issue of Louisiana Agriculture.)