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 ntroduction 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. 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. Thermal processing is widely used method for destroying microorganisms and imparting foods with a lasting shelf-life. 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 tention of vitamins and nutrients This chapter starts with a brief introduction to ohmic heating followed by escriptions 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
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 discussed under three headings: (1) thermal destruction of nutrients and functional compounds, (2) nutrient loss through diffusion, and (3) electrolysis and contamination. 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
408 The nutrition handbook for food processors AC power supply Food Electrode Insulator tube Electrode Fig. 19.1 A schematic diagram of an ohmic heating device. 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. Generally, pas- teurisation involves heating high-acid (pH 4.5)foods to 90-95oC 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 121C for a minimum of 3 minutes (lethality Fo=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. 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. 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 129C in about 90 seconds, allowing the possibility to decrease the extent of high tem- perature overprocessing. A stark contrast between ohmic heating and conven-
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, pasteurisation 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 Fo = 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 temperature 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
Ohmic heating 409 tional heating is that ohmic can heat particulates faster than the carrier liquid called the heating inversion, which is not possible by traditional, conductive heating 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 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 chieved, 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%o of the electrical energy is converted into 5 Optimisation of capital investment and product safety as a result of high solids 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. 79 Conventionally heating a mixture of carrot and beef cubes in a viscous liquid to attain a lethality in the liquid phase of Fo= 32 min would have produced 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 nd Fo=235-30.5 for the beef. Additionally, the intra-particulate distribu tion of Fo values showed that the periphery and center had experienced similar emperature-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 demonstrated that after ohmic heating e particulates transferred sufficient heat to the liquid to increase the liquid temperature eight degrees in the third holding tube. Accordingly, microbiolog cal measurements and intrinsic chemical analysis verified that the particulate center experienced a higher temperature-time profile than the particulate surface. In a bench-top ohmic heating set-up, configuring whey protein gels samples to
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 particulateliquid 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 = 32 min 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 Ohmic heating 409
410 The nutrition handbook for food processors 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. 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 liquid foods because the particulates heated simultaneously at similar or faster ates than the liq However. a number of critical factors affect the heatin of mixtures of particulates and liquids. For commercial ohmic heating facilities, he control factors are 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). 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. 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 heatin 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 and even exceed that for the liquid, b ating ate for the particulates will increase, lates in the mixture is increased. the h The electrical conductivity of particulates or liquids increases linearly with temperature.. 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. 20-2 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
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 particulatesin-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 particulates 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
Ohmic heating 411 It should be noted that although the conductivity of each component plays 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), 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. 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 193.2 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 method 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 reso lages 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%o Alacen whey protein powder(New Zealand Milk Products) and 80%o 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 thermallelectrical 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 143V and frequency of 50Hz 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
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 Ohmic heating 411