The advantages of ohmic heating

Ohmic heating has unique characteristics with associated advantages, which will certainly have significant impact on the nutritional values of ohmically heated products. Briefly, these characteristics and advantages are:4,8

1 Heating food materials volumetrically by internal heat generation without the limitations of conventional heat transfer or the non-uniformities commonly associated with microwave heating due to dielectric penetration limit.

2 Particulate temperatures similar to or higher than liquid temperatures can be achieved, which is impossible for conventional heating.

3 Reducing risks of fouling on heat transfer surface and burning of the food product, resulting in minimal mechanical damage and better nutrients and vitamin retention.

4 High energy efficiency because 90% of the electrical energy is converted into heat.

5 Optimisation of capital investment and product safety as a result of high solids loading capacity.

6 Ease of process control with instant switch-on and shut-down.

Microbiological and chemical tests demonstrated the characteristics and benefits of ohmic heating as an HTST thermal processing method for particulate-liquid mixtures.7,9 Conventionally heating a mixture of carrot and beef cubes in a viscous liquid to attain a lethality in the liquid phase of Fo = 32min would have produced7 an Fo value at the particulate center of 0.2 min. Ohmically heating the same mixture containing alginate analogs of beef and carrot cubes inoculated with spores of Bacillus stearothermophilus produced Fo = 28.1-38.5 for the carrots and Fo = 23.5-30.5 for the beef.7 Additionally, the intra-particulate distribution of Fo values showed that the periphery and center had experienced similar temperature-time profiles (for carrots Fo = 23.1-44.0 and the center Fo = 30.8-40.2; for beef Fo = 28.0-38.5 and the center Fo = 34.0-36.5). Particulates were heated by electrical resistance and not simply by conductive heat transfer from the carrier liquid.

Other tests with a commercial facility demonstrated6 that after ohmic heating the particulates transferred sufficient heat to the liquid to increase the liquid temperature eight degrees in the third holding tube. Accordingly, microbiological measurements and intrinsic chemical analysis verified that the particulate center experienced a higher temperature-time profile than the particulate surface.7 In a bench-top ohmic heating set-up, configuring whey protein gels samples to mimic equivalent electrical circuits and manipulating the relative electrical conductivity of each phase by the addition of electrolytes also demonstrated the capacity to heat the food solids faster than the liquid phase.10 Heating the particulates faster than the liquid can ensure greater lethality in the solids, which means that the carrier liquid can serve as a convenient monitor of sterility for regulatory purposes, although it is recommended that validation be carried out with each type of food product in order to establish the correct temperature-time profile and ensure a safe, stable product.

19.3.1 Effect of electrical conductivity on heating rate

Ohmic heating is considered very suitable for thermal processing of particulates-in-liquid foods because the particulates heated simultaneously at similar or faster rates than the liquid.11-15 However, a number of critical factors affect the heating of mixtures of particulates and liquids. For commercial ohmic heating facilities, the control factors are16 flow rate, temperature, heating rate, and holding time of the process. The factors influencing the heating in the food are the size (2.54 cm3), shape (cubes, spheres, discs, rods, rectangles, twists), orientation, specific heat capacity, density (20-80%), and thermal and electrical conductivity for the particle, and the viscosity, addition of electrolytes, thermal and electrical conductivity, and specific heat capacity of the carrier medium. The electrical conductivity and its temperature dependence are very significant factors in ohmic heating for determining the heating rate of the product.

Generally, samples with higher conductivities show higher heating rates, with variations in heating rates in different materials most probably caused by differences in specific heat.16 When the product has more than one phase, such as in the case of a mixture of particulates and liquid, the respective electrical conductivity of all the phases must be considered. The solid particulates usually have smaller electrical conductivities than the carrier liquid. Interestingly, the heating patterns are not a simple function of the relative electrical conductivities of the particulates and liquids. When a single particulate with an electrical conductivity much lower than the carrying liquid is undergoing ohmic heating, the liquid is heated faster than the particulate. However, when the density of the particu-lates in the mixture is increased, the heating rate for the particulates will increase, and even exceed that for the liquid.17

The electrical conductivity of particulates or liquids increases linearly with temperature.18,19 Differences in the electrical resistance (and its temperature dependence) between the two phases can make the heating characteristics of the system even more complicated. Furthermore, the orientation of particulates in the carrier liquid has a very strong effect on the heating rates of the particulate phase and liquid phase.17,20-23 Since electrical conductivity is influenced by ionic content, it is possible to adjust the electrical conductivity of the product (both phases) with ion (e.g., salts) levels to achieve balanced ohmic heating and avoid overprocessing.15,24,25

It should be noted that although the conductivity of each component plays a role in how the total product heats, knowing the total electrical conductivity of a food product is insufficient to characterise how individual particulates heat. For instance, fats and syrups are electrical insulators, and strong brines, pickles, and acidic solutions have high conductivities. Heating might not be uniform because the conductivity of the individual types of particulates may vary (meats, vegetables, pastas, fruits) or because a particulate might be heterogeneous (meat interspersed with fat). Non-uniform heating patterns could potentially create cold spots that promote the growth of vegetative pathogenic microorganisms such as Salmonella, Listeria, Clostridia, and Campylobacter. Since microbial destruction occurs in response to heating irrespective of its mode of generation (thermal, ohmic, or microwave),26 generating an average temperature of a food product that surpasses minimal lethal requirements does not ensure the complete sterility of that product. The temperature-time profile for all regions of the food product undergoing thermal treatment must surpass sterility to ensure sterilisation of the entire food product.27 In particular, the actual temperature-time history experienced by the coldest spot must experience sufficient heat treatment, and validation with each type of food product to establish correct temperature-time conditions to ensure a safe, stable product is therefore recommended.

19.3.2 Temperature distribution in ohmically heated foods

A heating method as complex as ohmic heating requires the development of more innovative techniques to validate its efficacy, and noninvasive MRI methods are suitable for mapping temperature distributions in samples containing water or fat. To demonstrate the unique heating patterns of the ohmic process, Fig. 19.2 shows several magnetic resonance images of a whey gel-salt solution model. These temperature maps, showing the levels and distribution of temperature were obtained using a special magnetic resonance imaging (MRI) technique called 'proton resonance frequency shift (PRF)'. The sample preparation and experiment procedures are as follows: whey gels composed of 20% Alacen whey protein powder (New Zealand Milk Products) and 80% distilled deionised water, and NaCl solution were used as models of particulate-liquid mixtures. Two samples of the model system were prepared. The sample consisted of a 305 mm long hollow cylinder of whey gel containing 1.5% NaCl and a 0.01% NaCl solution. A PVC thermal/electrical barrier was inserted into the hollow whey gel to form an isolated passage in the centre of the gel cylinder. The configuration of the model system resembled a parallel electrical circuit, which was ohmically heated by the application of an AC power supply with a constant voltage of 143 V and frequency of 50 Hz.

An experimental ohmic heating device was constructed of Plexiglas. It consisted of a Plexiglas vessel with a 43 mm inner diameter and a nylon stopper at each end. A 35 mm diameter stainless steel electrode was fixed to each of the stoppers and connected to the power supply. The distance between the two

Fig. 19.2 Temperature maps of whey gel during ohmic heating (2, 4, and 8min).

electrodes was 305 mm. A small hole was drilled in one of the stoppers of the Plexiglas vessel to allow the release of pressure build-up during heating. Two fluorescent fiber-optic temperature sensors were inserted through the holes into the whey gel and the solution at the same cross-sectional location that would be scanned to monitor the temperature for calibration. The absolute accuracy of the fiber-optic measurements was ±0.2°C. The use of these non-metal temperature sensors eliminated MR susceptibility artifacts.

The temperature maps shown in Fig. 19.2 were obtained at 2, 4 and 8 minutes during heating. The spatial resolution and temporal resolution were 0.94 mm and 0.64 sec respectively. PRF shift was linearly and reversibly proportional to the temperature change. The temperature uncertainties determined were about ±1°C for the whey gel and about ±2°C for the NaCl solution. The temperature maps show that there existed a gradient in the radial direction. The existence of this gradient is due to the internal heat generation of the ohmic heating process and the radiation heat transfer from particle surface through the vessel wall to the ambient. Therefore, the cold spots of the particle should be the surfaces and corners.

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