ABSTRACT
The design of a continuous Ohmic heating processing line is principally determined by the selected configuration of the system, the heating rate, the flow rate, and the desired temperature rise of the food product (Reznik, 1996). The Ohmic heating rate is calculated from the electric field, electrical conductivity, density, and specific heat of the food product. Therefore, the heating rate depends largely on the physical properties of the food and the electrical conductivity. Ohmic heating works effectively for foods because most pumpable foodstuffs contain dissolved ionic salts and acids, and water in excess of 30%, which render the material electrically conductive (de Alwis and Fryer, 1992). Pure fats, oils, alcohols, and sugars are not suitable materials for Ohmic heating. These substances are not sufficiently electrically conductive. Though, many researchers have worked on the context of Ohmic heating, its application in food processing is not yet fully developed due to the many unknowns in foods undergoing such a heating process. A considerable amount of reliable data on electrical conductivities has been published; however, discrepancies do exist among the available data or the estimates from the proposed empirical models. In some cases, the electrical conductivity of the food materials have been measured at room temperature, which is not the temperature experienced by the food during the Ohmic heating. For proper design of Ohmic heating system, it is essential to measure and collect the electrical conductivity data of foods as well as how they influence parameters such as frequency of the AC voltage applied, the voltage gradient, and the composition of food matrix. Some data has been published for high-frequency processes (e.g., microwave) but they are not applicable to low-frequency Ohmic heating processes. Only a few publications (Fryer et al., 1993; Zareifard et al., 2003; Salengke and Sastry, 2007; Tulsiyan, 2008) have reported electrical conductivities of real food matrices as a mixture of both liquid and solid components. It is generally observed that the electrical conductivity of liquids is higher than that of solids (Sastry and Palaniappan, 1992). In liquid foods, the effect of the proportion of solids is of less importance but still significant. Usually, as the percentage of solids increases (except for salt) the electrical conductivity decreases.
