Heat Treatment Improvement of Dairy Products via Ohmic Heating Processes : Thermal and Hyrodynamic Effect on Fouling

Fouling and consequently cleaning of heat exchangers in the dairy industry are nowadays a significant issue still not solved for the processing of quite a large variety of products. Ohmic heating processes for food products a priori are well known to minimize the fouling phenomenon due to a totally different way of heating food by admitting the current directly in the product. Such a technology could be a good alternative to counter both fouling and cleaning aspects when pasteurizing or sterilizing dairy desserts known to generate large amounts of soil on heated surfaces. The aim of this experimental study was to investigate the respective roles of both the hydrodynamic parameters and surface electrode temperatures on the fouling phenomenon when heating a simple dairy mix designed to mimic dairy product behaviors. INTRODUCTION The food industry in Europe, in particular the dairy industry, is faced with a very large environmental problem due to fouling of equipment during processing. In addition to the cost of effluent disposal, a large amount of money could be added for plant shut down for cleaning. As yet no solutions to such an issue can be given. In indirect heat transfer technologies (plate heat exchangers, tube heat exchangers) the presence of a fouling layer dramatically decreases the thermal performance and therefore the flow arrangement throughout the heat exchangers could deeply increase the fouling phenomenon. Fouling of plate heat exchanger treated milk has already been extensively investigated (Lalande et al., 1985; Leuliet, 1988; Rene et al., 1991; Belmar-Beiny et al., 1993; Delplace et al., 1995 and Changani et al., 1997). Recently Grijspeerdt et al., (2003) discussed the crucial effect of the hydrodynamic parameters on the fouling phenomena encountered in plate heat exchangers. All of these studies deal with the role of the plate design on the flow arrangement in the heated channels. Therefore any limitation of surface fouling is mainly due to the micro-mixing importance known to limit the attachment of the denatured whey proteins onto heated surfaces. Despite these studies, indirect heat technologies are still limited by fouling phenomena. Development of new technologies for continuous thermal food treatment are still of great industrial and scientific interests. Ohmic heating is one of these new technologies, which consist of the direct passage of electric current through the product. Permanent motion of electrical charges creates heat in the product in agreement with Joule’s law (Berthou and Aussudre, 2000). According to this principle ohmic technology could be considered as a purely bulk heating method. Such a principle is well known and was first proposed by Anderson & Finkelstein (1919), and Prescott (1927) for milk heating. Unfortunately, the technology did not succeed at that time because of technical limitations (electrode materials, process regulation...). During the past 10 years, new improved materials and equipment design for ohmic heating have been available (Amatore et al., 1998 and Roberts et al., 1998). However most of the studies have concerned the sterilization of food products containing particles (Fryer et al., 1989; De Alwis et al., 1989; Wadad et al., 1996; Sudhir et al., 1998; Benabderrahmane et al., 2000; and Eliot-Godereaux, 2001). A few scientific and technical studies (Ould Elmoktar, 1992 & Marcotte 1999) were dedicated to continuous food fluid treatment by ohmic heating. The aim of this experimental study was to investigate the respective roles of both the hydrodynamic parameters and surface electrode temperatures on the fouling phenomenon when heating a simple dairy mix designed to mimic dairy dessert behaviors. The heat exchanger consisted of plates and frames similar to conventional plate heat exchangers, electrodes being inserted between plastic insulating spacers. 1 Ayadi et al.: Heat Treatment Improvement of Dairy Products Published by ECI Digital Archives, 2003 The first part of this paper was focused on the improvement of the flow arrangement to minimize the fouling phenomenon as demonstrated in plate heat exchangers. In a second part the role of the electrode surface temperature on the deposit formation was discussed. MATERIALS AND METHODS Geometry and instrumentation of the ohmic heater The ohmic heater was made up of five ohmic cells, three of them ensuring heating and the two side cells ensuring electric insulation and the recovery of leakage currents. Each cell can be compared to a rectangular channel (L = 240 mm; l = 75 mm and thickness =15 mm), the electrodes constituting side surfaces. Figure 1 presents the schematic diagram of the ohmic heater. Figure 1: Ohmic heater apparatus. Two inlet designs for the cells were proposed, as shown in Figure 2, to minimize any negative effect on the flow arrangement inside the ohmic heating cells.