19 Ohmic heating R. Ruan, X. Ye and P. Chen, University of Minnesota; and C. Doona and I. Taub, US Army Natick Soldier Center 19.1 Introduction Preventing the loss of vitamins and nutrients in foods is a paramount concern at all stages of food processing involving heating. One example of the critical need for retaining vitamins is to nourish hospital patients who require vitamins to recover from the stress of illness or surgery. 1 This issue has invoked recent studies comparing cook/chill and cook/hot-hold foodservice practices in hospitals in an effort to minimise the loss of vitamins and nutrients that occurs when foods are heated. 2 Thermal processing is the most widely used method for destroying microorganisms and imparting foods with a lasting shelf-life. 3 Despite its many significant advantages, this mode of food preservation unavoidably degrades the vitamin and nutrient levels to some extent. As an alternative thermal method, ohmic heating ensures the benefits of conventional thermal processing (food safety and preservation) while offering the potential for improvements in the retention of vitamins and nutrients. This chapter starts with a brief introduction to ohmic heating followed by descriptions of its unique heating characteristics that can attenuate the thermal destruction of nutrients. The effects of ohmic heating on nutrients will be dis- cussed under three headings: (1) thermal destruction of nutrients and functional compounds, (2) nutrient loss through diffusion, and (3) electrolysis and con- tamination. Future trends and need for research are also discussed. 19.2 The principles of ohmic heating Ohmic heating is a thermal process in which heat is internally generated by the passage of alternating electrical current (AC) through a body such as a food system that serves as an electrical resistance. Ohmic heating is alternatively called resistance heating or direct resistance heating. The principles of ohmic heating are very simple, and a schematic diagram of an ohmic heating device is shown in Fig. 19.1. During ohmic heating, AC voltage is applied to the electrodes at both ends of the product body. The rate of heating is directly proportional to the square of the electric field strength, the electrical conductivity, and the type of food being heated. The electric field strength can be controlled by adjusting the electrode gap or the applied voltage, while the electrical conductivities of foods vary greatly, but can be adjusted by the addition of electrolytes. Sufficient heat is generated to pasteurise or sterilise foods. 3 Generally, pas- teurisation involves heating high-acid (pH < 4.5) foods to 90–95°C for 30–90 seconds to inactivate spoilage enzymes and microorganisms (vegetative bacteria, yeasts, molds, and lactobacillus organisms). Low-acid (pH > 4.5) foods can support Clostridum botulinum growth, and depending on the actual pH and other properties of the food, require heating to 121°C for a minimum of 3 minutes (lethality F o = 3 min) to achieve sterility (12D colony reduction). Within the past two decades, new and improved materials and designs for ohmic heating have become available. The Electricity Council of Great Britain has patented a continuous-flow ohmic heater and licensed the technology to APV Baker. 4 The particular interest in this technology stems from the food industry’s ongoing interest in aseptic processing of low-acid liquid-particulate foods. In the case of particulates suspended in viscous liquids, conventional heating transfers heat from the carrier medium to the particulates, and the time required to heat sufficiently the center of the largest particulate (the designated ‘cold-spot’) results in overprocessing. 5 In contrast, ohmic heating is volumetric and heats both phases simultaneously. Ohmic heating is a high-temperature short-time method (HTST) that can heat an 80% solids food product from room temperature to 129°C in about 90 seconds, 6 allowing the possibility to decrease the extent of high tem- perature overprocessing. A stark contrast between ohmic heating and conven- 408 The nutrition handbook for food processors S AC power supply Electrode Electrode Food Insulator tube Fig. 19.1 A schematic diagram of an ohmic heating device. tional heating is that ohmic can heat particulates faster than the carrier liquid, called the heating inversion, 7 which is not possible by traditional, conductive heating. 5 19.3 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 F o = 32 min would have produced 7 an F o 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 F o = 28.1–38.5 for the carrots and F o = 23.5–30.5 for the beef. 7 Additionally, the intra-particulate distribu- tion of F o values showed that the periphery and center had experienced similar temperature–time profiles (for carrots F o = 23.1–44.0 and the center F o = 30.8–40.2; for beef F o = 28.0–38.5 and the center F o = 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 demonstrated 6 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, microbiologi- cal 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 Ohmic heating 409 mimic equivalent electrical circuits and manipulating the relative electrical con- ductivity 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 partic- ulates 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 regula- tory 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 are 16 flow rate, temperature, heating rate, and holding time of the process. The factors influencing the heating in the food are the size (2.54 cm 3 ), shape (cubes, spheres, discs, rods, rectangles, twists), orientation, specific heat capacity, density (20–80%), and thermal and electrical conductivity for the par- ticle, and the viscosity, addition of electrolytes, thermal and electrical conduc- tivity, 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 differ- ences 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 conduc- tivity 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 conductiv- ity 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. 15,24,25 410 The nutrition handbook for food processors 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, vegeta- bles, 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 experi- enced by the coldest spot must experience sufficient heat treatment, and valida- tion 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 experi- ment 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 con- sisted 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 Ohmic heating 411 412 The nutrition handbook for food processors 40 40 20 100 50 0 0 60 30 70 60 60 70 50 50 40 40 30 30 20 20 20 10 10 0 70 60 50 40 30 20 10 0 70 60 50 40 30 20 10 0 0 0 60 60 80 70 50 40 40 30 20 20 10 0 2 4 8 ° ° ° Fig. 19.2 Temperature maps of whey gel during ohmic heating (2, 4, and 8 min). 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 flu- orescent 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. 19.4 The effect of ohmic heating on nutrient loss: thermal destruction Since systematic research on ohmic heating has a much shorter history than has conventional heating, food scientists and technologists might look to microwave heating for information on nutrient changes. In general, many improvements in nutritional quality were found using microwaves (cooking in a minimum of water retained more K, vitamin B 12 , and vitamin C, and the absence of surface brown- ing retained more amino acid availability, especially lysine), and microwave heating induces no significant effects different to those induced by conventional heating. 11 The benefit of attaining food safety with less nutrient degradation using HTST processes such as ohmic heating or microwave heating is based on differences in the kinetics parameters (k, z, Ea) for bacterial spores compared to those for bio- chemical reactions. 28 First, rate constants for microbial destruction are usually much larger than those for the chemical reactions responsible for nutrient degra- dation, and second, rate constants for microbial destruction are usually more sen- sitive to temperature increases (z(thiamin) = 48, z(peroxidase) = 36.1, and z(C. botulinum) = 10°C). 29 Methods for rapidly reaching the target temperatures there- fore tend to destroy microorganisms while giving less time to compromise the nutrient content and other quality attributes. 26,30 In fact, the slow heating rate asso- ciated with conventional retorting can activate protease to degrade myofibrillar proteins before the protease is eventually heat-inactivated. 31 Tests for conven- tional heating showed 9 that heating large (25 mm) particulates in a liquid medium at 135°C to achieve F o = 5 at the particulate center required extensive overpro- cessing of the liquid phase (F o = 150 for the liquid). For this reason, the common process conditions for scraped surface heat exchangers are maximum particulate sizes of 15 mm and sterilisation temperatures of 125–130°C (producing liquid F o = 25) while limiting particulates to 30–40% so that there is enough hot liquid available to heat particulates. For ohmic heating, direct heating sterilisation tem- peratures can reach 140°C (the temperature limit of plastics in the machinery) without grossly overheating the liquid phase and can support greater particulate loading suspended in highly viscous carrier liquids. For comparative purposes, conventional heating at 130°C to produce a lethality of F o = 8 produced a cook value Co (based on thiamin degradation) of Co = 8, whereas ohmic heating at 140°C produced F o = 24 and Co = 4. Vitamin losses in foods are determined by the temperature and the moisture of the applied heating method. Vitamin C is particularly temperature sensitive and destroyed at relatively low temperatures, 32 so heating foods must be for as short a time as possible to retain the vitamin C. Thiamin and riboflavin are un- stable at higher temperatures such as those used in rapid grilling. 3 Vitamin C is also water soluble and can be lost when cooking with moist heat or by autooxi- dation with dissolved oxygen in the food or cooking water. This reaction is catal- ysed by adventitious iron and copper ions. By comparison, thiamin is the most water soluble vitamin, and vitamin A and vitamin D are water insoluble. Unfor- Ohmic heating 413 tunately, studies on food nutrients affected by ohmic heating are sparse in the lit- erature. Ohmic heating is an effective method to pasteurise milk (220 V, 15 kW AC, C electrodes-70 C for 15 seconds) and has been used successfully to produce quality viscous products and to foods containing various combinations of par- ticulates such as meat, vegetables, pastas, or fruits in a viscous medium, 33 includ- ing a wide variety of high acid (ratatouille, pasta sauce and vegetables, vegetables Proven?ale, fruit compote, strawberries, apple sauce, sliced kiwi fruit) and low acid (tortelline in tomato sauce, cappaletti in basil sauce, tagliatelle a la crème, beef bourguignonne, Beijing lamb, beef and vegetable stew, lamb Wala Gosht, vegetable curry, minestrone soup concentrate) food products. Sensory evaluations of ohmically heated food dishes such as carbonara sauce, California Beijing beef, winter soup, mushroom à la Greque, ratatouille, and cappaletti in tomato sauce produced good to very good ratings. 34 Recent published data 35 compared the application of conventional and ohmic heating on the kinetics of ascorbic acid degradation in pasteurised orange juice exposed to identical temperature–time profiles in each case. The reaction fol- lowed pseudo-first-order kinetics and the kinetics parameters obtained from the Arrhenius plot in each case are similar. The data also indicate that ascorbic acid degrades as a result of thermal treatment and that the electric field contributed no additional influence on the degradation of the vitamin. Yongsawatdigul and co-workers 36,37 in their studies on gel functionality of Pacific whiting surimi found that ohmic heating can rapidly inactivate protease to avoid the enzymatic degradation of myofibrillar proteins, and hence increases the gel functionality of Pacific whiting surimi without the addition of enzyme inhibitors. Ohmic heating has been found to inactivate other enzymes. 38,39 Enzyme inactivation should help prevent or reduce enzymatic degradation of nutrients. However, more studies are warranted. There are several reports on relationships between ohmic heating and changes in properties of carbohydrates and fats. These studies did not directly address the nutrition issues of ohmically heated foods, although the physical changes that occur during ohmic heating affect the heating characteristics of the solids and liquids, which may have impact on thermal destruction of nutrients. Halden et al 40 suggested that changes in starch transition, melting of fats and cell struc- ture changes of the food material were responsible for changes in electrical con- ductivity that influenced the heating rate in foods such as potato during ohmic heating. In conventional thermal processes, starch gelatinisation was found to cause rheological and structural changes, and similar changes were observed for ohmic heating. Wang and Sastry 41 indicated that ohmic heating caused significant changes in physical properties including viscosity, heat capacity, thermal and electrical conductivity. They found that conductivity decreased with degree of gelatinisation. When we design ohmic heating processes, we must take changes in electrical conductivity caused by physical property changes of major com- pounds such as starch, fats and proteins into account so that no significant under- cooking of solids or over-cooking occurs. 414 The nutrition handbook for food processors 19.5 The effect of ohmic heating on nutrient loss: diffusion Studies 40,42 have shown that compared with conventional heating, ohmic heating enhanced diffusion of charged species between solid particles and the surround- ing liquid, which could have some impact on loss of nutrients from solid particles to carrier liquid. This becomes undesirable only if the carrier liquid is not to be consumed together with the solid particles. Figure 19.3 shows that the transfer of betanin dye between beetroot and the surrounding fluid increases linearly with applied electric fields. One explanation for the differences in this phenomenon between the ohmic heating and conven- tional heating is ‘electroosmosis,’ 43 which results in increased transport through the cell membrane. Another mechanism, ‘electroporation’, may also be responsible for enhanced diffusion between plant tissues and the surrounding liquid. When an electric field is applied across a membrane, it causes an induced membrane potential. When the induced membrane potential reaches a critical level, membrane ruptures occur, resulting in the formation of pores in the cell membrane, 44,45 and conse- quently increased permeability. On the other hand, ohmic heating is superior over conventional heating in the case of blanching of plant tissues such as vegetables. 46 The loss of soluble solids in water blanching of vegetables affects both the quality and nutrition of the products. In addition, blanching water containing a large amount of soluble solids Ohmic heating 415 0 2 4 6 8 10 12 0 400 800 1200 1600 2000 Electric field (V/cm) Beetroot dye efflux (x 10 4 kg/m 2 s) Fig. 19.3 Diffusion of betanin dye between solid beetroot and surrounding fluid as a function of applied electric field. 42 cannot be discharged without proper treatment. Mizrahi 46 compared hot water blanching and ohmic heating blanching. Hot water blanching was carried out by placing sliced or diced beet into boiling water and taking water samples every 30 seconds; blanching by ohmic heating was done by immersing whole, sliced or diced beets in an aqueous salt solution and passing an AC voltage through the medium. Betanine and betalamic acid concentration in the samples were deter- mined. Solute leaching with both methods followed a similar pattern, and was proportional to the surface to volume ratio and the square root of the process time. By removing the need for dicing and shortening the process time, ohmic heating blanching considerably reduced by one order of magnitude the loss of solutes during blanching of vegetables. 19.6 Electrolysis and contamination Another factor we must consider is electrolysis, particularly the dissolution of metallic (stainless steel) electrodes at 50–60 Hz, which could contaminate the finished products, and/or contribute to undesirable chemical reactions. Several measures have been taken to circumvent this problem. For example, commercial facilities using frequencies above 100 kHz showed no apparent indications of metal hydrolysis after 3 years (the industry safety standard). Low frequencies such as 50 or 60 Hz power can be used with inert carbon or coated electrodes without causing noticeable dissolution. Some new plastic materials with suitable electrical and mechanical properties can be used for housing the electrodes and for lining the stainless steel pipes through which food products flow. 19.7 Future trends We have demonstrated that ohmic heating is a very unique thermal process. Ohmic heating is considered a ‘minimal process’ besides the ‘HTST’ process. Potential uses of ohmic heating include: 15,47–50 1 Cooking. 2 Sterilisation and pasteurisation. 3 Blanching. 4 Thawing. 5 Baking. 6 Enhanced diffusion. However, as mentioned earlier, there has been only limited research quantifying the potential benefits of ohmic heating processes in terms of nutrition preserva- tion. More research is needed to realize the advantages of ohmic heating and to promote the commercialisation of the process. There are other major challenges hindering the commercialisation of the ohmic heating process. They are: (1) lack of temperature monitoring techniques for 416 The nutrition handbook for food processors locating cold/hot spots in continuous throughput systems, (2) differences in electrical conductivity between the liquid and solid phases, and their dynamic responses to temperature changes, which cause irregular heating patterns and complexity and difficulty in predicting or modeling heating characteristics of par- ticulates in carrier medium, and (3) a lack of data concerning the critical factors affecting heating (residence time distribution, particulate orientation, ratio of electrical conductivity, loading rates, etc.). These problems must be addressed before the process can be fully commer- cialised and gain approval from FDA. Below are listed some areas identified as research priorities for ohmic heating processing. 19.7.1 Quantification of effect of ohmic heating on major nutrients As mentioned earlier, there is serious lack of data demonstrating the changes in major nutrients in food products and quantifying the advantages of ohmic heating over conventional heating in terms of nutrition retention. Kinetic studies are desir- able to provide information that will be useful for process and product design. Occasionally, improvements in product throughput ‘accidentally’ result in better nutrient retention and sensory quality attributes, and directed studies on optimis- ing critical process factors to achieve food safety and improve nutrition retention with ohmic heating are highly recommended. 19.7.2 Reliable modeling and prediction of ohmic heating patterns Predicting the heating patterns of ohmic heating is a very difficult task because of its unique heating characteristics. The heating rate is critically dependent on parameters such as the electrical conductivity, temperature dependence of elec- trical conductivity, and volumetric specific heat. Furthermore, possible heat chan- neling, causing hot spots and cold spots, complex coupling between temperature and electrical field distributions, and sensitivity to process parameters, e.g. resi- dence time distribution, particle shape and orientation, etc., all contribute to the complexity of the process. To ensure sterilisation, the heating behavior of the food must be known. Without the information, process validation – an actual demon- stration of the accuracy reliability, and safety of the process – is impossible. Mathematical modeling allows insight into the heating behavior of the process. Spatial and temporal temperature distribution obtained from a reliable mathe- matical model which incorporates the critical factors can provide information for the calculation of lethality and cook value. It will also save time and money for validation experiments, process and product design. Modeling of a continuous ohmic heating process is extremely difficult due to a number of different physical phenomena occuring during the heating process. De Alwis, Fryer, Sastry, Palaniappan, and their co-workers are pioneers in modeling the ohmic heating process. Their published models have been used to predict the temperature within particles for very specific heating conditions. The models are of limited useful- ness in establishing the heating characteristics of a commercial product because Ohmic heating 417 of their inability to model a multicomponent system undergoing a continuous process. The verification of the models is also limited in selected regions within the system. Another limitation is the lack of understanding about some interac- tions within the system. For example, limited information is available for the temperature dependence of the electrical conductivity, and a reliable method does not exist to measure the convective heat transfer coefficient at the liquid– particle interface. These types of limitations require that actual physical mea- surements of the temperature of the product and its constituents be conducted when establishing a process. Some of these limitations can be compensated for by using appropriate conservative assumptions at the expense of the product quality. A more accurate and reliable model is needed. 19.7.3 Well-defined product specifications and process parameters Product specification includes information that defines the product and its physical/chemical aspects that play important roles in determining how much lethal treatment is delivered during the process. Critical factors may include par- ticle size and shape, liquid viscosity, pH, specific heat, thermal conductivity, solid liquid ratio, and electrical conductivity. It is also important to know how these factors interact and how they are influenced by the process, for which only limited information is available. Particulates are the centerpiece around which an ohmic heating formulation is built. Contrary to conventional heating where we would expect no difference due to the change in particle orientation, the heating pattern of an ohmically heated food system would be greatly affected by particle orientation. De Alwis and Fryer 51 showed the heating of identically-shaped potato particles parallel and perpendicular to the electrical field. The particle heating rate changed consider- ably as a result of the change in orientation. De Alwis and Fryer 51 explained that this uniqueness is due to the fact that the orientation of the particles changed the electrical field and thus the heating rate. Though there seems no limit to the particle size which can be processed in an electrically uniform mixture, cooling of particulates will always be controlled by thermal conduction and the cooling rates possible may impose an upper limit on the particle size. This is important in HTST processes, since rapid cooling is desired. The center of large particles may cool too slowly and thus become overprocessed during prolonged cooling. Unlike conventional heating where the outside may be overcooked, here the inside might be. Particulate size is typically limited to 2.54 cm 3 . Fundamental particulate considerations include size, shape, concentration, density, conductivity, and specific heat capacity. The fluid phase cannot be neglected. Liquid viscosity should be determined at various tempera- tures to assure adequate suspension of particulates. Moreover, the liquid viscos- ity may affect the liquid/particle interface heat transfer and thus the heating and cooling rates and process control. More research is needed to address and under- stand the many aspects of the product and process design and their effects on the product quality. 418 The nutrition handbook for food processors 19.7.4 Reliable real-time temperature monitoring techniques for locating cold/hot spots Pioneers of ohmic heating researches have documented that a particle does not heat uniformly during an ohmic heating process because of the non-uniform nature of the electric field and the food materials within the ohmic system. 52 As in other thermal processes, it is important to have information on the temperature–time history of the coldest point within the liquid–particulate system undergoing ohmic heating. It is assumed that the agitation of a continuous system minimises these vari- ations in temperature profiles. However, there is insufficient published evidence to indicate what the temperature is within a particle, let alone how the tempera- ture profile changes during a continuous process. It does appear that for a parti- cle with a homogeneous electrical conductivity, if the particle heats faster than the liquid phase, the particle’s coldest spot is at its surface. 53 There is little pub- lished information for particles with heterogeneous electrical conductivity (i.e., fatty meat). The location of the coldest spot is especially important because that is the place where the thermal lethality must be ensured and this is the key factor in determining the processing time. Conventional tools such as thermocouple and optic fiber are apparently invasive when used to measure the ohmic heated food system. A non-destructive and non-invasive technique which can be used to monitor the spatial distribution of temperature is important for understanding and control of ohmic heating technology. In addition, a non-destructive and non- invasive temperature mapping technique is essential for the model development and the validation of this novel process. MRI seems to be a valid approach to this problem. There is a need to improve the technique further, to collect more data under various product specifications and processing conditions with the technique, and to use this technique to validate mathematical models. 19.8 Sources of further information and advice Specific information can be found in the cited references provided in section 19.9. The following research institutes have major research programs on ohmic heating: (1) Department of Food, Agricultural, and Biological Engineering, The Ohio State University, expertise: ohmic heating in general, and mathematical modeling, contact: Professor S.K. Sastry (2) Department of Chemical Engineering, University of Birmingham, Birmingham, UK, expertise: general and mathematical modeling, contact: Professor P. Fryer (3) US Army Soldier Command, Natick RD&E Center, expertise: general and temperature mapping, contact: Dr. Irwin Taub, Dr. Christopher Doona (4) Department of Biosystems and Agricultural Engineering and Department of Food Science and Nutrition, University of Minnesota, expertise: MRI tem- perature mapping and mathematical modeling, contact: Professor R. Ruan Ohmic heating 419 19.9 References 1 donelan a and dobson d (2001), ‘Vitamin retention in prepared meal services’, Food Service Technol, 1, 123–4 2 williams p (1996), ‘Vitamin retention in cook/chill and cook/hot-hold hospital food- services’, Journal of the American Dietetic Assoc, 96, 490–8 3 ranesh m n (1999), ‘Food preservation by heat treatment’, in Handbook of Food Preservation, R S Rahman (ed), New York, Marcel Dekker, 95–172 4 skudder p j (1988), ‘Ohmic heating: new alternative for aseptic processing of viscous foods’, Food Engineering, 60(1), 99–101 5 nott k p and hall l d (1999), ‘Advances in temperature validation of foods’, Trends in Food Science and Technology, 10, 366–74 6 zuber f (1997), ‘Ohmic heating: a new technology for stabilising ready-made dishes’, Viandes et Produits Carnes, 18(2), 91–5 7 kim h-j, choi y-m, yang t c s, taub i a, tempest p, skudder p, tucker g and parrott d (1996), ‘Validation of ohmic heating for quality enhancement of food products’, Food Technology, 253–61 8 kim h j, choi y m, yang a p p, yang t c s, taub i a, giles j, ditusa c, chall s and zoltai p (1996), ‘Microbiological and chemical investigation of ohmic heating of particulate foods using a 5 kW ohmic system’, Journal of Food Processing and Preservation, 20(1), 41–58 9 tempest p (1992), Experience with Ohmic Heating and Aseptic Packaging of Partic- ulate Foods. 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