The temperature prediction of some botanical beverages, concentrated juices and purees of orange and pineapple during ohmic heating Titaporn Tumpanuvatr, Weerachet Jittanit ⇑ Department of Food Science and Technology, Faculty of Agro-Industry, Kasetsart University, 50 Phaholyothin Road, Chatuchak, Bangkok 10900, Thailand Center for Advanced Studies for Agriculture and Food, KU Institute for Advanced Studies, Kasetsart University, Bangkok 10900, Thailand article info Article history: Received 10 November 2011 Received in revised form 1 May 2012 Accepted 31 May 2012 Available online 13 June 2012 Keywords: Botanical beverages Concentrated juice Concentrated puree Electrical conductivity Mathematical model Ohmic heating abstract In this research, 10 kinds of botanical beverages, concentrated juices and purees of orange and pineapple were heated applying a static ohmic heating system. The objectives were (1) to develop the mathematical models for predicting the temperature changes of these samples during ohmic heating and (2) to com- pare between the qualities of ohmically-heated and conventionally-heated specimens. The results indicated that the mathematical models can precisely predict the temperature variations of the concen- trated purees during ohmic heating although the heat losses to the surroundings and evaporated mois- ture are neglected. However, the temperature estimation of botanical beverages and concentrated juices were more accurate if the heat losses to the surroundings and evaporated moisture were included in the mathematical models. Furthermore, at the same heating rate the ohmic and conventional heating meth- ods resulted in the insignificant difference in the sensorial qualities and remaining vitamin C contents of orange and pineapple juices. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Beverages, concentrated juices and purees are vital food prod- ucts due to the massive demand of the global market. Nowadays, the consumers have an increasing need about the quality of these products while the industry desires to enhance their current pro- duction efficiency. Ohmic heating is a thermal processing tech- nique that has potential to respond these requirements. It has some advantages over the conventional system because the heat is instantly generated within the food when passing the electrical current through food (Icier et al., 2006) whereas the conventional method must rely on the heat transfer mechanisms especially heat conduction and convection. These heat transfer mechanisms are usually limited due to the thermo-physical properties of food and the fouling on the heat contact surface (Lalande et al., 1985; Bansal and Chen, 2006). Furthermore, the conventional heating process generally requires the costly boiler to produce the heating med- ium; on the other hand, the ohmic heating system does not. Although ohmic heating has a number of benefits, this tech- nique also has some limitations. It is because the amount of gener- ated heat during ohmic heating is directly related to the electrical conductivity of food (Icier and Ilicali, 2005a). Thus, the foods which have lower electrical conductivities will be heated slower than those of higher electrical conductivities if the same electrical field strength is applied. In other words, to obtain the same heating rate more intense electrical field strength is needed for the lower con- ductive food. The electrical conductivities of food products nor- mally depend on their temperatures and constituents especially salt, acid and fat (Shirsat et al., 2004; Sarang et al., 2008). A number of researchers pointed out that the food products with high salt or acid contents would have high electrical conductivity and ohmic heating rate; in contrast, an increase in fat content results in the re- verse effect (Shirsat et al., 2004; Engchuan and Jittanit, accepted for publication). The salts and acids are normally dissipated to be ions in the liquid and act as the electrical current carriers during ohmic heating; on the other hand, the fat itself has poor conductivity and act as the barrier for the passage of electrical current (Shirsat et al., 2004; Sarang et al., 2008). Furthermore, many published works proved that the electrical conductivities of foods would rise if their temperatures were raised (Shirsat et al., 2004; Icier and Ilicali, 2005a; Sarang et al., 2008; Icier, 2009). However, it appeared that the electrical conductivities of liquid food systems instantaneously dropped after the occurrence of bubbling (Icier and Ilicali, 2005a). The bubbles are characterized as the electrical insulators; there- fore, they will interfere the flow of electrical current and lessen the electrical conductivity of food system as a whole. Zhao et al. (1999) also reported that the gas bubbles observed during ohmic heating were the result of either various oxidation and reduction reactions or the water boiling. 0260-8774/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jfoodeng.2012.05.044 ⇑ Corresponding author at: Department of Food Science and Technology, Faculty of Agro-Industry, Kasetsart University, 50 Phaholyothin Road, Chatuchak, Bangkok 10900, Thailand. Tel.: +66 2 562 5026; fax: +66 2 562 5021. E-mail address: fagiwcj@ku.ac.th (W. Jittanit). Journal of Food Engineering 113 (2012) 226–233 Contents lists available at SciVerse ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng