An overview of ohmic heating utilization in the processing of food

Ohmic heating is a variation of conventional heating which generates heat by passing an electric current through a material. It is known as Joule heating and involves passing electricity flowing through a material that can withstand the passage in electrical to produce heat. Numerous scientific studies contend that ohmic heating is an evolution of traditional heating in the food industry owing to its capacity to warm products swiftly and uniformly. Other benefits of ohmic heating include its capacity to inactivate microorganisms, low maintenance costs, high energy efficiency, and the ability to preserve food’s nutritional value. Due to these benefits, ohmic heating is commonly employed in food processing. There is considerable research on the use of ohmic heating in various food processing techniques, such as sterilization, pasteurization, blanching, fermentation, and microbial inactivation. This paper provides a summary of several research on the utilization regarding ohmic heating methods in food processing.


Introduction
Food preservation technology is one of the fields that are currently quite a lot being studied and developed.This technology and the increasing need for food ingredients with a long storage life are created.Elevated temperatures to destroy or inactivate microorganisms is a conventional preservation method currently undergoing substantial development [1].
The use of heat, in addition to inactivating microorganisms, can also result in several adverse effects on food ingredients, such as degradation of nutritional content [2], sensory character [3], and the range of physicochemical compounds [4].Heating technology is a technology that is quite widely applied.However, the effects can reduce the quality of food ingredients.This prompted the development of heating methods with the least possible adverse effects.Several ways that are currently being studied quite frequently include the use of microwave and radiowave [5,6], radiation [7], infrared [8] and ohmic heating [9].These methods have advantages over conventional heating because they result in moderate temperatures, and the time required is relatively short, minimizing food damage due to heating.
In principle, ohmic heating converts electrical energy into heat by utilizing the heat resistance character of food [10].Ohmic heating has several advantages, such as being able to inactivate pathogenic microbes and preventing the decrease in functional character, nutritional content, and sensory value of food ingredients, extending their shelf life [11].Ohmic heating technology is widely applied to food processing, particularly liquid ingredients such as fruit juice, eggs, milk, and others [12].Ohmic heating technology is currently used to assist the fermentation process of Galván-D'Alessandro & Carciochi [13] because ohmic can produce fast and even heating [14].This paper summarises multiple research 1230 (2023) 012182 IOP Publishing doi:10.1088/1755-1315/1230/1/012182 2 studies on the utilization of ohmic heating in food processing, beginning with the ohmic heating principle, its use in food, and its advantages and disadvantages.

Ohmic principle
Ohmic heating is the application of an electrical current to a material to heat it.Ohmic heating does not require heat transfer from the solid to the liquid surface [15].Food materials have resistance to electric current, and this characteristic is utilized in ohmic heating.When an alternating current flows to food with resistance characteristics, heat is generated, which can be used in the processing and preservation of food [16].The ohmic heating system comprises an ohmic chamber with two electrodes at the chamber's extremities.A current is supplied from one electrode to another by an electric generator.A thermocouple inserted in the sample's centre is utilized to measure temperature.Variables such as current, voltage, and temperature are captured through continuous data recording.All documented information is subsequently accessible for analysis and display on a computer [17].
Most food contains water and dissolved ions, which can conduct electricity via electrolytic conduction.Electrical conductivity is a measure that states the ability of a substance to conduct electricity.This characteristic is an essential parameter in ohmic heating techniques, where the efficiency depends on the material's conductivity [18].One of the components that also plays a critical role in ohmic heating is the electrode that shoots an electric current.When the electrolyte is placed in an electric field, the ions move toward the electrode with the opposite charge.The movement of ions in this electrolyte will generate heat [19].
When the material contains enough water and electrolyte to allow the electric current to move through, ohmic heating can produce heat within the product [20].Ohmic heating is passing a (usually alternating) electric current through a material for the express purpose of heating it.Internal energy transformation (electricity to heat) within the material causes heating [21].
The movement of ions in food causes collisions, which causes resistance to ion movement to form, thereby increasing kinetic energy.The heat generated due to the movement of ions in the product is generated instantaneously and volumetrically [9].According to Joshi; Silva et al. [22,23] several factors must be considered in ohmic heating because they can interfere with the efficiency of this process.These factors are electrical conductivity, frequency and waveform, current intensity, voltage, joule effect (wasted force), moisture content, electrolytic concentration field strength, and the properties of the system (particle size, concentration, heat capacity, viscosity).
The effectiveness of ohmic heating depends on the rate of heat generation in the system, the electrical conductivity of the food, the intensity of the electric field, the length of time the food remains in the system, and how the food flows through the system.The food functions as an electrical resistor in ohmic heating.The dissipation of electrical energy heats the material.Heating with electrical resistance causes particulate and liquid to heat simultaneously [24,25].
OH (also known as Joule heating, electro-heating, and electroconductive heating) is the process of passing an electric current (typically alternating) through a material to heat it [26].Ohmic Heating (OH) can generate heat within a product when it contains enough water and electrolyte to enable electric current to flow [20].Heating occurs through internal energy transformation (from electricity to heat) within the material [21].
Using the HTST (High-Temperature Short-Time) method, the thermal degradation of food product quality can be minimized.However, there may be a maximum temperature to prevent overcooking the food's exterior.When food products contain enough water and electrolytes to conduct an electric current, ohmic heating can generate heat by conducting an alternating current through the food product.This technique permits the solid phase to heat up at the same rate as the liquid phase, allowing the HTST technique to be applied to solid foods.The degree of ohmic heating highly depends on the food product's electrical conductivity [20].Proper electrical conductance management is critical to successfully implementing ohmic heating [27].
The electrical conductivity of a material has the most significant impact on the ohmic heating method.It depends on the food ingredient's temperature, ionic bond dissociation, and electric field intensity [28].Ohmic heating is contingent upon the base material to be treated, its moisture content, and its ion concentration.The electrical conductivity of solid and liquid food materials increases linearly with conductivity, which is affected by temperature, voltage, and concentration.The correlation between temperature and the concentration of food constituents is inverse.Electrical conductivity increases linearly with water content and temperature, and solid foods resist electrical conductivity more effectively [29].
Ionic compounds such as acids and bases can also enhance electrical conductivity but to a lesser extent than non-polar compounds like lipids.Therefore, the electrical conductivity value of the material is not constant [11].Generally, ohmic heating is utilized to transfer electric energy to various dietary components.Because heat is generated volumetrically, it is possible to heat solids and liquids at the same rate [30].
The majority of foods contain significant concentrations of water and dissolved salts; electrolytic conduction allows these solutions to conduct electricity.When an electrolyte is positioned in an electric field, the ions move toward the oppositely charged electrode.The migration of ions in the electrolyte generates heat.In addition, the ions collide, which creates resistance for their movement and increases their kinetic energy, thereby heating the product [31].
The use of electrodes directly affecting the heated medium is an essential distinction between ohmic heating and other methods.Ohmic heating has a lower frequency than microwaves and radio waves [23].The ohmic heating electrode can be considered an electric current connecting with the heated medium, with the electric current also uniformly distributing the electrodes across the medium.[32].The ohmic method can heat edibles with conductivities ranging from 0.1 to 10 S/m.The application of ohmic technology can be made in segments or continuously.The electric field's intensity, the electrodes' configuration, and the heating material's conductivity cause the current flow.The recent flow results in a high power density and a rapid thermal energy input.The voltage is 400 to 4000 V, the field strength is 20 to 400 V/cm, and the distance between the electrodes is 10 to 50 cm.The energy source's efficiency determines the heating rate, the design of the apparatus, and the conductivity, viscosity, and specific heat capacity of the thermal medium [33].

Ohmic heating for sterilization and pasteurization
Regarding product safety and quality management, microbial inactivation is a crucial factor in food production that must be considered.Certain unwanted microorganisms or their presence in large quantities can cause product injury.(e.g., degradation of substances), decreased quality (e.g., changes in appearance, unpleasant odor, bad taste, and decreased color), and health problems (e.g., disease) [15].Because of that, we need a way to inactivate these microbes using sterilization and pasteurization.
Ohmic heating technology has been around since the 19th century and was applied to the pasteurization of milk.Due to process control issues, high electrical costs, and a paucity of suitable electrode materials, this technique has not yet been developed for commercial applications [34].The application of ohmic heating has grown significantly; where it was initially used only for pasteurizing milk, it can currently be applied for pasteurizing solid food materials.Much still needs to be improved in pasteurizing solid materials using ohmic heating technology, particularly in maintaining stability and uniformity of material heat.For this reason, experiments were carried out using models to predict and understand this phenomenon, thus can help reduce differences in product temperature to produce a more even pasteurization temperature [35].Another modeling was carried out by Marra [36], where from the results obtained, it was concluded that the product's outermost area experienced the slowest temperature increase.Therefore, it must be used as a target for observing the death of microorganisms.
Ohmic heating is considered a reliable method for aseptic food processing.This method employs electrical resistance heating, which occurs when an alternating current is passed through electrically conductive foods.Since most foods requiring thermal processing contain ionic components such as acids and salts, they can be transmitted by an electric current.Due to the food's electrical resistance, it is heated internally without the heating medium or heat transfer surfaces being involved [37].
In inhomogeneous materials, such as soups containing solid food slices, the electrical conductivity of the particles and their relationship to the conductivity of the fluid is shown to be a critical parameter for understanding the heating rate of the particles under ohmic heating.Increasing the electrolyte content of food to increase its electrical conductivity can be achieved by marinating or blanching the solids in a saline solution before infusing them with salt.It can be used as a pretreatment for ohmic heating of particulate foods to achieve uniform heat treatment if the food's composition and other properties are not adversely affected [37].
Continuity of flow the four primary steps of ohmic heating processing are as follows: heating the product to the target temperature; maintaining that temperature for a specified time to inactivate the target microorganism; immediate cooling; and aseptic packaging.During the heating segment, an ohmic heating system replaces the heat exchanger.All ohmic-treated foods can be packaged in aseptic techniques such as cartons, cans, bottles, sacks, and cups [17].
The material must have electrical conductivity for ohmic heating.Non-conductive materials can be made conductive by adding electrolytes such as NaCl (sodium chloride) or dissolved organic salts (tetraalkylammonium) that do not interfere with the reaction or by using naturally conductive ionic solvents [23].The product is heated to transform the food into electrical resistance, and the internal heat is generated instantly and in volume by the movement of ions.Heat is proportional to the food's current, voltage, electric field, and electrical conductivity [28].
The electrical conductivity of simple heating is one of the most critical parameters in the ohmic heating process because it depends on temperature, frequency, electrolyte concentration, and applied voltage gradient [38].The presence of ionic substances such as acids and ions increases conductivity, while non-polar constituents such as fats and lipids decrease it [39].Under ohmic heating, the electrical, thermal conductivity of various materials, including fresh fruits such as apples, pineapples, pears, strawberries, and peaches, ranges from (0.05 to 1.2) S/m [18].The electrical conductivity of pure water is approximately 0.05 S/cm.[39] Ohmic heating technology is also applied to the fruit drink pasteurization process.Kumar [28], experimented with pasteurizing grape juice with several combinations of voltage, temperature, and processing time.The results indicated that the overall amount of microorganisms decreased as voltage, temperature, and time gradients increased.
Castro et al. [40] investigated the radiation of vitamin C in pasteurized strawberry products via ohmic and traditional pasteurization.They concluded that an electric field did not affect ascorbic acid degradation.Leizerson& Shimoni [41] investigated the effect of ohmic heat treatment on the quality of orange juice by pasteurizing it at 90, 120, and 150C for 1.13, 0.85, and 0.68 seconds on an ohmic heater.Consequently, ohmically heated orange juice has a comparable flavor to conventionally heatpasteurized orange juice.Funcia et al. [42] reported that the electric field could substantially affect enzyme inactivation (pectin esterase inactivation) in orange juice by ohmic heating compared to conventional heating.
Several studies have investigated ohmic heating for eggs because eggs can cause infection with Salmonella bacteria.According to research conducted with ohmic heating, as the temperature rises, the egg yolk progressively transforms from its typical orange to a brilliant yellow, while the egg white transforms from clear to cloudy [43].Eggs are a source of protein and are sensitive to high temperatures; therefore, care must be taken during pasteurization to prevent protein denaturation and coagulation.Compared to conventional pasteurization, ohmic heating can increase the hardness and foaming capacity of the whole egg, has a minimal effect on protein denaturation, and causes minor color changes in eggs [44].Joeres et al. [45] demonstrated that the egg white protein was not wholly denatured during ohmic heating.

Ohmic heating for extraction
The use of ohmic technology today is not only limited to thermal processes for product sterilization but is also used for other purposes, such as extraction.Ohmic technology is considered an alternative technology that is greener and friendlier than other conventional extraction processes.This is because the product heating process does not involve high temperatures, and the heat received by the product is more evenly distributed.Numerous investigations have been conducted to examine the effects of ohmic processes for extraction purposes and research for the assembly of ohmic apparatus.The results of research by Markhali et al. [46], showed that the use of ohmic technology is very effective in terms of increasing extraction yields, total phenol content, and antioxidant activity of olive leaves compared to conventional processes.
Extraction methods can isolate active components in natural materials.However, these compounds are generally less stable and can be damaged by high temperatures in conventional extraction processes.Ohmic technology, which uses electric voltage, is considered more effective in maintaining active compounds' effectiveness and more capable of saving energy and processing time [47].Other studies have demonstrated that low-energy electric fields and thermal processes can be combined and optimized to extract anthocyanins and phenolic compounds from plant tissues in a single phase.With less refining time, less energy consumption, and no use of organic solvents, the yield of the extracted material is increased (green extraction) [48].
In addition, the role of ohmic heating in the extraction of β-carotene and lycopene was compared to conventional techniques [49].In their investigation of Gac aryl oil, ohmic heat treatment was found to enhance the extraction yield of lycopene and -carotene.In the wine industry, ohmic heat treatment has been used to extract Vine Pruning (VPR) residue from lignocellulosic materials.In addition, ohmic heat treatment increased the extraction of bioactive compounds from VPR compared to conventional heating procedures.Due to increased phenolic release by ohmic heating, the extract's polyphenols profile, total phenolic content (TPC), antimicrobial activity, antioxidant activity, and anticancer activity increased [50].If not considered waste, tomato industry byproducts (skins and seeds) are an essential source of bioactive that can be used as pigments in commercial aquaculture.In addition, this tomato residue contains naringenin, kaempferol, and rutin, for which ohmic heating increases extraction [51].

Ohmic heating for drying
The current problems faced by the food industry are high energy consumption and the negative impact of chemical compounds on the environment.Therefore, various processing processes have been developed to minimize this negative impact.In conventional drying, using rather large energy becomes a problem that significantly affects production efficiency.Moreover, high temperatures during drying are feared to reduce the product's nutritional content.A study on the use of ohmic technology has been carried out by Stojceska et al. [52], which concluded that thermal drying consumes 3.5 to 5 times more energy than ohmic drying.The same is true for costs and greenhouse gas emissions.Research by Anjaly & Shaju [53], confirmed that treatment with ohmic can significantly reduce the time required for vacuum drying and positively influence product quality and economic benefits.Zhong [54] suggests that ohmically heated samples require less drying time than fresh samples in a vacuum dryer.The utmost time reduction is 24%.

Ohmic heating for thawing
Thawing frozen products is an activity that should be conducted carefully because it will significantly affect product quality, particularly texture and the risk of microbial contamination.Several studies on the use of ohmic technology for the thawing process have been carried out, and the results demonstrated that increasing the frequency (50 Hz -20 kHz) causes a decrease in resistance and an increase in the heating rate, thereby reducing the thawing time of frozen tuna muscle [55].Another experiment was conducted using meat to see the thawing time and the rate of weight loss.The conclusion obtained by Duygu & Ümit [56], is that ohmic heating technology with a voltage of 50 V can shorten thawing time and lower weight loss.

Ohmic heating for fermentation
Fermentation using conventional thermal processes is one of the old methods to extend shelf life and provide good organoleptic properties.However, this process can cause changes, especially in compounds that cannot withstand heat.Various new thermal technologies (radio frequency, microwave heating, and ohmic heating) were developed to replace conventional thermal treatment in fermentation [57].
The ohmic technology that continues to develop makes various studies of this technology, one of which is for fermentation purposes.Gavahian & Tiwari [58], stated that ohmic technology is a prospective technology that can be applied to the fermentation process because it provides various advantages, where the use of this technology based on research results can effectively affect the fermentation of Lactobacillus, Streptococcus, and Saccharomyces which are involved in the production of bakery products, milk, and alcohol.Furthermore, ohmic heating can accelerate fermentation by providing the optimum temperature for faster fermentation.Gally et al. [59], stated that ohmic heating can speed up the fermentation process since ohmic can provide the right, fast and uniform fermentation temperature in batch fermentation.
Ohmic heating techniques can minimize the lag phase of fermenting bacteria, thereby shortening the fermentation time of yoghurt, cheese, beer, and wine [60].Research suggests that ohmic heating is valuable for laboratory-scale lactic acid and alcoholic fermentation.The benefits of this technology are associated with thermal and non-thermal effects on microorganisms and fermentation media.For instance, ohmic heating can rapidly achieve optimal fermentation temperatures and shorten the fermentation period.Due to the microorganisms' stress response, it is possible to obtain a shorter latency phase and greater concentrations of the desired fermented product [58].
Research Cho et al. [60] evaluated the effect of conventional and ohmic heating on electric field intensities of 1.1 and 2.9 V/cm and a frequency of 60 Hz in cultures of L. acidophilus incubated at 30, 35, or 45 °C.The lag period of L. acidophilus is affected by fermentation temperature, heating method, and the interaction between the two parameters.Compared to the conventional heating method, the lag period for fermentation at 30 °C was reduced 18-fold by the low-intensity ohmic process, E = 1.1 V/cm.This observation can be explained by the increased transport of nutrients to the cell's interior due to the electroporation of the membrane.These findings establish ohmic heating as a valuable tool for industrial fermentation processes, particularly during the initial phases of fermentation [61].
Research by Sagita et al. [14] designed a fermentor using an ohmic heating system.The electrodes are placed between the fermenter reactor tubes and transmit electric current.Materials designed to carry out the fermentation process will be placed in the fermentor reactor.The results of this ohmic-based fermentor are proven to maintain, control and produce temperature uniformity in the reactor for liquid and non-liquid products.The resulting energy efficiency reaches 81.96% -86.29%.
Regulation of fermentation conditions using ohmic technology has also been utilized in the fermentation process of Arabica coffee beans.This is done to enhance the coffee's quality.Fermentation was carried out at 30, 35, and 40C and fermentation time was 2, 6, 12, and 18 hours.Fermentation uses an ohmic reactor made from PVC pipes, and an electric current is passed into the pipe using electrodes [62].
Ohmic technology can also be applied to cocoa fermentation [63].Risqan et al. [64], researched cocoa fermentation using ohmic technology, where fermentation is carried out by utilizing the heat generated by controlled electricity to maintain the fermentation temperature at 40C, 45C, and 50C for five days.The results are the percentage level of fermented cocoa beans reaching 96%.Table 1 summarizes several studies studying the use of ohmic in various food processing processes.

The advantages and disadvantages of ohmic heating
The heating rate in an ohmic process depends on the material's electrical conductivity and field strength.This method produces products with better quality since it results in minimal structural damage, hence it can retain product nutrition.Ohmic heating can be utilized in several food processes, such as blanching, evaporation, dehydration, fermentation, extraction, sterilization, pasteurization, and others.The results showed no protein denaturation due to high temperatures in the ohmic heating process [81].Joshi [22], concluded that ohmic heating technology is a new technique that can provide fast and constant heating and cause little thermal damage to the product.Moreover, this technology also has high energy efficiency, is environmentally friendly, has low investment costs, and is technically considered relatively simple.
Sakr & Liu [39], summarize the advantages of ohmic heating, including the shorter time needed to reach the desired temperature, low maintenance costs and high energy conversion efficiency, reduce maintenance costs due to lack of moving parts, a relatively environmentally friendly system, reduce fouling due to heat transfer from the surface, and has no residual heat transfer after the system is turned off.Meanwhile, the drawbacks of this technology are, it requires adjustments based on product conductivity, it is difficult to monitor and control, and the frequency band is narrow.
Several studies have shown that the ohmic process produces products with better structure and aroma, fewer colour variations, increased bioactive compounds, and better sensory properties when compared to conventional heating.Nonetheless, the use of this technology is considered less suitable for products with a high-fat content because of the low electrical conductivity of fat [34] Some of the advantages of ohmic technology, as described by Silva et al. [23], are high heating speed and a significant reduction in reaction time, more effortless process scalability for pilots and industry than microwave technology, better and more accessible process control hence reducing maintenance costs, more efficient energy conversion, and environmentally friendly processes.However, some limitations may be encountered are that the product must have good conductivity; hence it is challenging to apply to non-conductive products.In addition, the lack of knowledge and information regarding this process also needs to be a concern, one of which is how to quantify heat/energy flow.Therefore, further research is required to examine its performance and safety aspects when used in food processing.

Conclusion
Ohmic technology is the latest technology that can be utilized for various food processing processes such as pasteurization, sterilization, extraction, drying, fermentation, and others.The principle of this technology is to pass an electric current through the material using electrodes and then convert it into heat energy.This technology can be used in various products, especially those that have good electrical conductivity.The advantages of ohmic heating compared to other technologies are saving time and energy, low maintenance costs, and minimizing thermal damage to materials hence the resulting product has better characteristics.Nonetheless, besides these various advantages, this technology also has disadvantages.It is difficult to apply to products with low electrical conductivity, thus additional treatment is required to use this technology.

Table 1 .
Utilization of technology for various food processing processes Process objectives