Ohmic heating (OH), also called Joule heating, electrical resistance heating, direct electrical resistance heating, electroheating, and electroconductive heating, is one of the earliest applications of electricity in food pasteurization and is defined as a pro- cess where electric currents are passed through foods to heat them. Heat is internally generated due to electrical resistance (De Alwis and Fryer 1990a). The OH technology is distinguished from other electrical heating methods by the presence of electrodes contacting the foods (in microwave and inductive heating, electrodes are absent); the
158 Engineering Aspects of Milk and Dairy Products
frequency applied (unrestricted, except for the specially assigned radio or microwave frequency range); and waveform (also unrestricted, although typically sinusoidal) (Vicente 2007). A successful application of electricity in food processing was devel- oped in the nineteenth century to pasteurize milk (Getchel 1935). This pasteurization method was called the Electropure Process, and by 1938 it was used in approximately 50 milk pasteurizers in five U.S. states and served about 50,000 consumers (Moses 1938). This application was abandoned apparently due to high processing costs (De Alwis and Fryer 1990a). Also, other applications were abandoned because of the short supply of inert materials needed for the electrodes, although electroconductive thaw- ing was an exception (Mizrahi, Kopelman, and Perlaman 1975). However, research on ohmic applications in food products, such as fruits, vegetables, meat products, and surimi has been undertaken by several authors, more recently Palaniappan and Sastry (1991a), Palaniappan and Sastry (1991b), Wang and Sastry (1997), and Castro, Teixeira, and Vicente (2003). In fact, OH technology has gained interest recently because the products are of a superior quality to those processed by conventional technologies (Castro, Teixeira, and Vicente 2003; Kim et al. 1996; Parrott 1992). The potential applications are very wide and include, for example, blanching, evaporation, dehydra- tion, and fermentation (Cho, Yousef, and Sastry 1996). Presently the focus of OH is being addressed to thermal processing operations, such as sterilization and pasteuriza- tion. This technology can be accomplished in a continuous in-line heater for cooking and sterilization of viscous and liquid food (Icier and Ilicali 2005). OH can be used for HTST pasteurization of liquid proteinaceous food products which tend to denature and coagulate when thermally processed conventional technologies are used. Due to its extremely rapid heating rates, OH technology enables higher pasteurization tempera- tures to be applied, with consequent increase in refrigerated shelf life, without induc- ing coagulation or excessive denaturation of the constituent proteins (Parrott 1992). The major benefits claimed for ohmic heating technology are as follows:
1. Temperature required for HTST processes can be achieved very quickly 2. Suitable for continuous processing without heat transfer surfaces 3. Uniform heating of liquids with faster heating rates
4. Reduced problems of surface fouling or overheating of the product com- pared to conventional heating
5. Fresher-tasting, higher-quality products than with alternate heat preser- vation techniques
6. No residual heat transfer after the current is shut off, and very low heat losses 7. Useful in preheating products before canning
8. Low maintenance costs (no moving parts) and high energy conversion efficiencies
9. Environmentally friendly system
For all these reasons, OH is now receiving increased attention by the dairy industry, once it is considered to be an alternative for the indirect heating methods of milk pasteuriza- tion, such as shell and PHE exchangers where heating of milk is achieved through direct contact with a hot surface. In OH, heat is generated directly within milk (volumetric heating) and, hence, the problems associated with heat transfer surfaces are eliminated
Novel Technologies for Milk Processing 159
(Bansal and Chen 2006). The electrical conductivity of foods together with the electri- cal field strength applied play a major role during OH processing. Furthermore, other properties related to the type of food, such as kind of phase (solid or liquid), size and shape of the particles, moisture content of the solids (if present), solids/liquids ratio, viscosity of the liquid component, possible occurrence of electrolysis, pH, and specific heat are also very important for the effectiveness of this technology (Fellows 2000).
Milk contains sufficient free water with dissolved ionic salts and therefore con- ducts sufficiently well for the ohmic effect to be applied (Palaniappan and Sastry 1991b), and because electrical conductivity increases with temperature, OH becomes more effective at higher temperatures. Furthermore, for materials of uniform elec- trical conductivity, such as milk, the energy generation is far more uniform than microwave heating (Sastry et al. 2002), where the limited penetration of the micro- wave radiation often promotes significant temperature gradients. Figure 7.1 shows a linear relation between electrical conductivity and temperature for the different field strengths applied during the heating of milk.
Overall, this technology provides a rapid and uniform heating and can be con- sidered a HTST process (Castro et al. 2004b; De Alwis and Fryer 1990b; Reznick 1996; Zareifard et al. 2003). Despite OH features, some disadvantages, namely those related to the high initial operational costs and the lack of generalized information or validation procedures, the absence of a hot wall should provide a considerable advantage for milk processing applications, by avoiding the degradation of thermo- sensitive compounds due to overheating and by reducing the fouling of the surfaces during processing (Ayadi et al. 2004a; Leizerson and Shimoni 2005).
95 20.6 V/cm 28.9 V/cm 37.5 V/cm 53.8 V/cm 85 75 65 55 45 35 25 15 0.00E+00 1.00E–03 2.00E–03 3.00E–03 4.00E–03 C on du ct iv ity (S · cm –1) 5.00E–03 6.00E–03 7.00E–03 8.00E–03 9.00E–03 1.00E–02 Temperature (°C)
FIGuRE 7.1 Relationship between electrical conductivity and temperature in milk, at dif- ferent field strength values.
160 Engineering Aspects of Milk and Dairy Products
7.3.1.1 Microbial Inactivation
The principal mechanisms of microbial inactivation in OH are thermal in nature. The destruction of microorganisms by nonthermal effects such as electricity is still not well understood and generates some controversy (Vicente 2007). Moreover, most of the pub- lished results do not refer to the sample temperature or cannot eliminate temperature as a variable parameter (Food Safety and Nutrition 2000; Palaniappan et al. 1990). However, studies such as that of Cho et al. (1996) provide evidence that OH may be useful in the dairy industry to shorten the time for processing yogurt and cheese production. Recently, the influence of OH on the heat resistance of Escherichia coli, which frequently contam- inates dairy products when their manufacture conditions are unsanitary, was studied in goat milk and compared to that of conventional heating. The results have shown that the microorganism’s inactivation was faster when the OH was applied, indicating that in addition to the thermal effect, the presence of an electric field provided a nonthermal killing effect over vegetative cells of E. coli (Pereira et al. 2007b). Sun and coworkers (2008) studied the effects of OH (internal heating by electric current) and conventional heating (external heating by hot water) on viable aerobes and Streptococcus thermophi- lus 2646 in milk under identical temperature history conditions. It was found that both the microbial counts and the calculated decimal reduction time (D value) resulting from OH were significantly lower than those resulting from conventional heating.
The main reason for the additional killing effect of ohmic treatment observed in different microorganisms seems to be linked with the electrical current and frequency applied during OH inactivation (Sastry et al. 2002; Sun et al. 2008). Several authors suggest that a mild electroporation mechanism may contribute to cell death, bring- ing a nonthermal effect to inactivation (Imai et al. 1995; Kulshrestha and Sastry 1999; Wang 1995). However, further research is needed to understand the inactiva- tion mechanisms of various microorganisms in different types of foodstuffs. Data on nonthermal effects are scarce (see Table 7.1), and more studies are needed to TAblE 7.1