Introduction

Many processes in the food industry are based on thermal treatments, including microbial inactivation, cooking, baking, blanching, evaporation, dehydration, and thawing [1, 2]. Conventional heating (CH) systems used by manufacturers to apply these treatments to solid, particulate, or viscous products involve heat transfer from the thermal source to the cold center of the food. In these kinds of products, heat transfer occurs by thermal conduction, which is very slow. On the other hand, to ensure that heat treatment is correctly applied, which is essential in pasteurization and sterilization processes, the coldest point must receive the required treatment to guarantee food safety and stability. Thus, CH of solid food products is not only very time-consuming but also results in the overtreatment of the food surface, potentially affecting its quality and, therefore, leading to products that are far away from fresh food [3]. All this makes CH systems unable to meet today’s demand for minimally processed products that maintain the freshness of food.

For this reason, in recent years, alternative thermal technologies have been investigated to achieve rapid and volumetric heating while trying to minimize the impact of heat on food quality [4,5,6]. These novel technologies include systems based on electromagnetic heating (radiofrequency and microwave) and ohmic heating (OH). This review will focus on the latter with a special emphasis on the different ways developed in the last years to apply it.

OH is based on the electrical resistance that conductive materials (such as food) offer to current flow. Due to the Joule effect, when an electric current flows through them, the electrical energy is directly converted into thermal energy by heating the food itself [7]. Thus, the heating is considered volumetric being more uniform than CH. In addition, heating is faster, energy consumption is lower, and product quality is improved compared to traditional systems [8, 9]. Although studies on the environmental and energy impact of the application of OH are still scarce, Ghnimi et al. [10] observed energy savings of 65% in pasteurization treatments; the energy efficiency of the ohmic system was 77% while that of the conventional system was 25%. Furthermore, in the same study, a 41% reduction in CO2 emissions was determined when using the OH system.

By contrast, the major disadvantages of OH are the electrochemical reactions and the erosion of the electrodes happening as a consequence of its application, especially when it is applied directly to foods. To address this problem, alternating current is applied, different electrode materials have been evaluated (best results were obtained with titanium), and the process has been optimized by applying higher frequencies of the order of kilohertz (due to the inhibiting of Faraday reactions) [11,12,13]. In addition, food products are often complex matrices composed of different components, in different physical states, and with different thermal and electrical properties, which means that OH is usually not as uniform as expected, and the process has to be optimized. It should also be noted that process parameters such as the electric field strength, frequency, electrode material, and the geometry of the treatment chamber also influence the uniformity of heating [6].

The application of OH for food preservation has been researched since the nineteenth century [14]. However, in recent years, new ways of application have started to raise interest. These new systems involve the application of higher electric field strengths with the aim of achieving faster heating rates and improving heating uniformity. Moderate electric fields (MEF), which in general apply electric fields higher than 100 V/cm, have been the most widely investigated strategy. It has been demonstrated that the application of MEF could promote the electroporation of eukaryotic cells which favors the outflow of intracellular material that would make the electrical conductivity of the whole food more uniform, improving the uniformity of heating and increasing the speed of heating [15]. Thus, the application of MEF has been studied for osmotic dehydration, cooking, spore inactivation, pasteurization, enzyme inactivation, and fermentation. More recently, the application of pulsed electric fields (PEF)—which work at electric fields higher than 1000 V/cm—as an OH technology for microbial inactivation [16, 17] and for blanching of vegetables [18] has also been proposed since it can result in an even faster heating and higher electroporation.

Some very good reviews on the application of OH in the food industry can be found in the literature [2, 6, 19,20,21], but to the best of our knowledge, none has been focused on comparing and discussing the major benefits and drawbacks of each one: OH, MEF, and ohmic-PEF. Therefore, the aim of this work is to present an update on OH technology, with a special focus on these two new ways of applying it: MEF and PEF technologies.

Basic Aspects of Ohmic Heating

OH consists of the application of electric fields of 20–100 V/cm, frequencies of 50 Hz to 30 kHz, current intensities of up to 5000 A/m2, and maximum power of up to 480 kW [22]. As long as an electric field is applied, a certain amount of current flows through the food, which causes it to heat up, regardless of the intensity of the electric field and the way in which the heating is applied (continuous or pulsed).

OH or direct resistance heating is based on Joule’s law (Eq. 1) in which the heat (Q, joules) generated is linearly proportional to the resistance (R, ohms) of the product but proportional to the square of the current (I, amperes) over a time (t, seconds).

$${\text{Q}}={{\text{I}}}^{2}\cdot {\text{R}}\cdot {\text{t}}$$
(1)

In the case of OH for food processing, Joule’s law can also be expressed as

$${\text{W}}=\underset{0}{\overset{{\text{t}}}{\int }}\upsigma {{\text{E}}}^{2}{\text{dt}}$$
(2)

where σ is the electrical conductivity of the treated medium or product (S/m), E is the electric field strength (V/m), and dt is the time (s) during the electric field strength is applied [23]. The reason for using this simplified equation is to get a better understanding of the effect of OH on food as the electric field is directly related to cell electroporation and electrical conductivity which is a food characteristic. Nevertheless, these points will be discussed in more detail below.

In addition to the electric field applied and the electrical conductivity of the food, frequency is also another parameter to consider when applying OH. OH can be applied by both direct and alternative current, although the latter is the most commonly used since direct current promotes corrosion of the electrodes [6]. Thus, frequency is defined as the number of cycles per second of a wave [20].

Considering Eq. 2, it is clear that the applied electric field (E) has a high impact on the heat generated by increasing quadratically the heating rate. That is one of the major benefits of the new ohmic technologies (MEF and ohmic-PEF) since they are based on the application of higher electric fields than OH heating. However, the electric field strength thresholds differentiating each technology are not clearly defined in the literature. Indeed, in some cases, OH and MEF are used indistinctly [24,25,26]. Thus, in order to establish clear and meaningful comparisons, the following definitions/limits will be used in this work: OH (E < 100 V/cm), MEF (E = 100–1000 V/cm), and ohmic-PEF (E > 1000 V/cm).

Electrical Conductivity

Food heated by OH must have a certain electrical conductivity to allow current to flow through them. The conductivity depends on the temperature, frequency, and food composition. Solid foods composed of different components do not have a uniform electrical conductivity throughout the product and thus, the application of pre-treatments (blanching, soaking, marinating, and vacuum infusion in salt solutions) to the different components that alter its conductivity has been investigated in order to reach an overall uniform conductivity [27, 28].

Due to the difference of conductivities within a product, heterogeneous temperatures occur, resulting in the presence of hot spots and cold spots. The current will go primarily through the areas with the least resistance, i.e., the areas with the highest electrical conductivity, so the hot spots will be found there. On the contrary, in those areas with a lower conductivity, there will be cold spots [6]. These areas with lower temperature will be the limiting zones when (and where), for example, determining the microbial inactivation by ohmic treatment. Moreover, in fatty products (non-conductive materials), OH is not effective; then the food safety cannot be guaranteed since regions coated with fat will not be heated and will not receive the heat treatment required for microbial inactivation [29,30,31]. A similar situation occurs with the extraction processes. Moongngarm et al. [32] used dehydrated stevia leaves that were hydrated with distilled water at 20%, 30%, and 40% (wet weight) and applied OH at 50 Hz and electric fields of 75–200 V/cm. At 40% humidity, the maximum temperature was 114 °C while at 20% humidity, the heating could not be applied due to the low conductivity of the product.

Another aspect to consider is that the electrical conductivity is temperature-dependent and will increase during the heating process. In liquid food, the conductivity increase is linear with temperature [33, 34]. However, in solid materials, this does not happen because during heating, cell rupture occurs, and intracellular ions are released, causing a sharp increase in conductivity and temperature [35, 36], as shown in Fig. 1 [37]. When potato samples were heated, two kinetics were observed: from 20 to 60 °C, the increase in conductivity was slower, but from 60 °C onwards, there was a faster increase. This correlates with the cell disintegration index shown in the right axis of the figure, increasing rapidly when 60 °C was reached, increasing up to 0.5 with final values of 0.85–0.90 (value = 1 means maximum cell rupture).

Fig. 1
figure 1

Evolution of electrical conductivity (white-shaded box (□)) and cell disintegration index (black-shaded box (■)) in potato samples when ohmic heating was applied (Figure adapted from [37])

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In this scenario, the application of ohmic-PEF on vegetables at electric fields higher than 1000 V/cm could result in early electroporation of the tissue prior to the actual rupture caused by heat at temperatures above 60 °C. This could make the conductivity of the material more homogeneous in the early stages of heating, thus improving the uniformity of heating and the speed. One of the methods used to evaluate electroporation in plants is the cell disintegration index by impedance analysis, which consists of measuring the proportion of permeabilized cells based on the frequency dependence of the electrical conductivity of intact as well as permeabilized plant tissues [38]. Figure 2 shows the cell disintegration index obtained by applying OH from 20 to 80 °C to carrot samples by two different systems: OH (54.2 V/cm, 12 kHz) and ohmic-PEF (1330 V/cm, 100 Hz).

Fig. 2
figure 2

Cell disintegration index of carrot samples when applying OH from 20 to 80 °C by ohmic system (54.2 V/cm, 12 kHz) and by ohmic-PEF (1330 V/cm, 100 Hz)

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The result for the cell disintegration index for ohmic-PEF was 0.87, whereas for OH was 0.51. This means that the electroporation effect when applying the ohmic-PEF would have been larger than with OH and, therefore, the release of ionic compounds would have been higher and then the electrical conductivities of the food treated by ohmic-PEF.

Electric Field Strength

The applied electric field in OH has a direct effect on two aspects. The first one, as ruled out from Joule’s equation (Eq. 2), is the generation of heat whose increase is quadratically proportional to the applied electric field. That is, increasing the electric field will lead to huge temperature increases [16, 39]. Rascón et al. [40] applied OH to Agave salmiana samples at different electric fields (20, 30, and 40 V/cm), and the processing time to reach 90 °C was 250, 96, and 70 s, respectively. Therefore, the heating rate increased 1.37 and 3.57-fold when applying 30 and 40 V/cm compared to 20 V/cm.

The second aspect related to the electric field is the phenomenon of electroporation. Its occurrence is widely accepted when applying PEF as a non-thermal technology (1–40 kV/cm). However, due to the low electric fields applied (20–400 V/cm) in OH, there is no clear statement on this because it is very difficult to discern between the thermal and the purely electrical effect. Due to the interest of this point, it will be discussed in more detail in Sect. 4.

As discussed earlier, MEF and ohmic-PEF systems apply electric fields higher than OH. The application of MEF together with temperature (30–45 °C) has been demonstrated to favor the permeabilization of plant membranes [41]. This has been studied in extraction and concentration processes with positive results in both process optimization and product quality improvement [32, 42]. Moongngarm et al. [32] applied MEF (50 Hz) at 75–200 V/cm electric fields on stevia leaves to favor the extraction of steviol glycosides and phytochemicals. The application of 75 V/cm allowed to reach 75 °C after 100 s, while the application of 200 V/cm reached 110 °C after the same period of time. Moreover, stevioside extraction increased 3.15-fold and 3.80-fold when applying 75 V/cm and 200 V/cm, respectively, compared to the control. In the case of ohmic-PEF, electroporation of plant tissues also takes place as discussed above and shown in Fig. 2.

The distribution of the electric field inside the treated product is also important because it determines the heating rates and the extent of electroporation. However, for complex products, it is quite challenging, and therefore numerical simulation tools are needed. Shim et al. [36] simulated the OH (5 V/cm) of three samples with different conductivities (carrot, meat, and potato) immersed in a NaCl solution (3%) in the same treatment chamber. The conductivity of the solution was between 1.8 and 15-fold higher than that of the solid samples. The maximum electric field was located inside the carrot and potato samples (higher electrical conductivity than meat), and the areas of lower electric field were located in the areas between the solid samples. The same was observed by Casaburi et al. [43]. Nevertheless, more complex studies are needed in which not only simple solids (composed of a single component) but also mixtures of solids with components of different electrical and thermal properties are analyzed.

Frequency

As it has been indicated, the frequency of OH technology varies from 50 to 60 Hz. However, recent works have been focused on the application of frequencies in the kilohertz range. It has been observed that at higher frequencies, the electrochemical reactions of the electrodes that occur when applying low frequencies, or direct current are reduced [6]. For this reason, studies have recently been carried out using frequencies from 12 to 300 kHz [25, 37, 37, 44,45,46,47,48,49].

One advantage of using higher frequencies is the improvement of heating uniformity, a new approach that has recently been investigated. Gratz et al. [37] observed that by applying 300 kHz frequencies to potato tubers immersed in a salt solution with a conductivity of 5 mS/cm, the temperature gradient between the core and the edges was 1.5 °C, while at lower frequencies of 12 kHz, the value was 7.1 °C. The improvement in heating uniformity by increasing the frequency was also observed when numerically simulating the ohmic-PEF of agar cylinders at 100 and 200 Hz, in which the increase in frequency resulted in a reduction of the temperature gradient of the cylinder from 7 to 4.5 °C [50].

Treatment Chamber

OH can be applied in different ways: in solid foods, it can be applied in direct contact with the product with the electrodes or by immersing the food in a saline solution in a static process, while in liquids, it is applied in direct contact both in batch or continuous flow. In general, in batch systems, which lead to a more homogeneous distribution of the electric field, the chambers used are made of insulating materials with two parallel electrodes, which can be of different materials stainless steel, platinum, titanium, etc., at the ends. Some of the chambers can be pressurized in order to be able to apply sterilization treatments [51]. At an industrial level, tubular heaters are commercially available for continuous treatment of liquids or liquid-particulate mixed products with different chamber configurations (for more information see Ghnimi and Fillaudeau [52]).

Although OH is considered a volumetric heating, cold spots appear in the areas close to the electrodes [20]. This phenomenon has been extensively studied in the literature [16, 53,54,55,56,57]. Moya et al. [50] conducted a numerical simulation of heating at 2.5 kV/cm and 50 Hz to a technical agar sample and observed temperature gradients inside the sample of up to 30 °C when the treatment was applied at room temperature (Fig. 3).

Fig. 3
figure 3

Temperature evolution along a treatment chamber (electrodes plus agar sample) when ohmic heating is applied (2.5 kV/cm, 50 Hz) at room temperature (adapted from [50])

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For this reason, attempts have been made to minimize this problem by developing treatment chambers in which systems have been introduced to minimize heat loss through the electrodes (Fig. 4).

Fig. 4
figure 4

Treatment chamber configurations. A Chamber covered on the sides with saline water (adapted from [58]). B Chamber with the electrodes heated by the circulation of hot oil inside the chamber (obtained from [17]). C Chamber with a rotating electrode in the center of the treatment zone to apply shear stress during ohmic heating (adapted from [59])

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In the study conducted by Zell et al. [57], they evaluated the heating (230 V, 50 Hz) of mashed potatoes using different coatings for the chamber: with a thermal insulation material (mineral wool pipe insulation with an aluminum facing), with a hot water jacket, with a heating tape, and with a silicon heating panel. The latter panel design yielded the best results, with a minimum determined temperature gradient of 11.9 °C. From a similar approach, Gratz et al. [58] developed a treatment chamber with an inner part where the sample was located and a jacket coating it with salt water at the same conductivity as the sample (Fig. 4A). This design ensures that the heating rates of the sample (vegetable puree) and the saline solution are very similar, as the electrical conductivity is one of the parameters that determines the heating rate by OH (as it was explained in Eq. 2). Instead of covering the area where the sample was located, Astráin-Redín et al. [17] used two tempered electrodes by circulating hot oil (25, 32, and 39 °C) inside them (Fig. 4B). They were able to keep the electrodes at higher temperatures and minimize heat loss through them. When they applied 2.5 kV/cm and 50 Hz on agar cylinders contaminated with Listeria monocytogenes and tempering the electrodes at 39 °C (non-lethal temperature), an inactivation of 5 Log10 cycles was achieved after 23 and 26 s of treatment in the central and electrode contact zone, respectively. In this case, gradients of 5 °C would be estimated based on numerical simulation between the coldest and hottest points [50].

Moreover, due to the increasing interest in combined technologies to achieve a synergistic effect, Mok et al. [60] developed a cylindrical treatment chamber with a static electrode on the outside and a rotating electrode in the center so that the sample was located in the gap between the two electrodes (called Taylo-Couette flow) (Fig. 4C). In this way, they were able to apply shear stress during OH for the inactivation of Escherichia coli K12 in apple juice. The results showed a synergistic effect of both technologies achieving a 5 Log10 reductions after 6.2 min of treatment at 50 °C, 120 V/cm, and 2879 s−1. However, information about the uniformity of temperatures inside the chamber is missing, so it is not possible to identify the cold spots or, if none exists, to verify the homogeneity of temperatures achieved. Using the same technologies, they evaluated this synergistic effect of MEF and shear stress with the subsequent addition of nisin to assess whether it improved their efficacy in gram-negative bacteria where antimicrobials are not effective because of the outer membrane. Inactivation of 5 Log10 cycles of E.coli K12 was achieved after 5 min of MEF application under shear stress (2879 s−1) with a duty cycle of 0.99 and 100 IU/ml nisin [59]. Therefore, authors indicated that this MEF and shear stress combined treatment chamber would have a great potential to promote microbial inactivation and thus reduce heat application times even further, thus enhancing the sensory properties of the product.

Despite the good results obtained with these treatment chambers, all of them are designed at laboratory level and therefore, further studies are needed to scale them up to pilot plant and/or industrial scales.

OH, MEF, and Ohmic-PEF

The first industrial application of OH appeared in 1920, and it was designed to heat milk in a continuous flow using an alternating current of 220 V at a frequency of 60 Hz [61]. Advances in technology have enabled the application of higher electric fields leading to the development of novel systems such as MEF and ohmic-PEF. However, although PEF technology is presented in this review as an OH system, for more than 70 years of research on this technology, it has been focused on its application as a non-thermal food processing technology based on the phenomenon of electroporation. Thus, PEF, as a non-thermal technology, has shown great results for improving mass transfer (improving juice extraction yields, polyphenol extraction during red wine maceration, accelerating vegetable drying, among others) and preservation processes (pasteurization of liquid food such as juice, milk, or liquid egg) [62,63,64].

Depending on the intensity of the electric field and the energy input applied, the treatment effect will be either more thermal or mostly cell electroporation (Fig. 5). The best-known effects are found in OH (low electric fields and thermal effect) and conventional PEF (high electric fields and electroporation effect at non-lethal temperatures). In the middle are MEF and ohmic-PEF where thermal effects and electroporation occur together, and it is very complex to analyze each of them separately since there are many parameters to consider (cell type, frequency, electric field, and treatment temperature, among others).

Fig. 5
figure 5

Correlation between the ohmic technologies applied and the degree of occurrence of thermal and electrical effects

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Regarding MEF (E < 1000 V/cm), research has been carried out since the beginning of the twenty-first century [65] in order to achieve non-thermal (electrical) effects. Several studies have been conducted on its application for salmon salting [26, 66], osmotic dehydration [67], cooking [68, 69], spore inactivation [70], pasteurization [71], and enzyme inactivation [24, 72]. Recently, its great potential for fermentation processes has been identified, as MEF induces a stress-response that reduces the lag phase and increases the concentration of desirable fermentation products, as well as reaching the fermentation temperature quickly and homogeneously [73].

The application of PEF technology as a non-thermal system or as a thermal system depends on the processing parameters (electric field, specific energy, and treatment time). In conventional PEF (non-thermal process), electric fields are applied in order to minimize temperature rise and promote electroporation. In order to do so, food with low electrical conductivity is preferred and low-energy treatments in the form of high electric fields (1–40 kV/cm), short pulse widths (1–20 µs), and micro- and millisecond treatment times are applied [74]. Conversely, ohmic-PEF is designed to induce a rapid, volumetric heating of the food over longer treatment times (in the order of seconds or minutes) by applying higher energies. In general, Ohmic-PEF allows a higher electric field strength (> 1000 V/cm) to be applied than that used in MEF heating (< 1000 V/cm). The purpose of applying electric fields even higher than those applied in MEF is to favor cell electroporation during the first seconds of the treatment when temperatures are not yet high (< 50 °C). Thus, favoring the release of intracellular ionic compounds and improving the uniformity of the electrical conductivity of the food. Figure 6 shows the cell disintegration index (measured by impedance analysis) of carrots blanched in water at 85 °C and by ohmic-PEF at 1330 V/cm and 100 Hz [18]. When the samples reached 40 °C, the conventionally blanched carrots had a cellular disintegration index of 0.13, whereas those that had received ohmic-PEF showed a value of 0.77. Tissue disintegration would be mainly due to electroporation generated in the first seconds of treatment at low temperatures as the electric field is enough for plant cell electroporation (< 5 kV/cm) [75].

Fig. 6
figure 6

Adapted from Astráin-Redín et al. [18]

Cell disintegration index at different stages of blanching of carrots blanched in water at 85 °C (unfilled bars) and by ohmic-PEF heating at 1330 V/cm and 100 Hz (grey bars).

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In addition, it has also been reported that the application of ohmic-PEF at electric fields of 3750 V/cm to agar cylinders improved the heating uniformity by reducing the temperature gradient between the cold and hot spots of the sample from 8 to 4 °C [50]. Nevertheless, this way of applying PEF for ohmic heating purposes is very recent, and there are hardly any studies about it. Due to the fact that the application of ohmic-PEF is very recent, the equipment is currently not optimized for it, and it is not powerful enough to apply ohmic large-scale treatments.

Mechanisms of Action

The thermal effect is, by nature, the main mechanism behind the application of OH. However, some studies have reported the presence of a non-thermal effect (due to the electric current) that would increase cell permeabilization and the degree of microbial and enzyme inactivation by the generation of sublethal or additional damages [76, 77]. In this section, the effect on cells (eukaryotic and prokaryotic cells) and the effect on enzymes will be discussed.

Effects on Cells

It is well known that the application of enough intense electric fields leads to the disruption of the membranes of cells by the formation of pores. This phenomenon is known as electroporation and occurs when a minimum electric field is reached, which depends on the size of the cells (see Eq. 3) [78].

$$ITV=\frac{3}{2} \left|E\right| r\,\left({\text{cos }}\theta \right)$$
(3)

where ITV is the induced transmembrane voltage, E is the electric field strength, r is the cell radius, and θ is the angle of the position of the cell in relation to the direction of the electric field. Therefore, for microbial cells (size of 1–10 µm), a minimum electric field strength of 10,000 V/cm [79] would be needed whereas for eukaryotic cells (size of 40–200 µm), the minimum value is 100–1000 V/cm [80]. During OH, in which the applied electric field is less than 100 V/cm, electroporation would not theoretically occur, but in MEF and ohmic-PEF treatments, where higher fields (> 100 V/cm) are applied, eukaryotic cells could be permeabilized.

Eukaryotic Cells

Non-thermal effects derived from the application of MEF and PEF result in the permeabilization of eukaryotic cells when sufficiently high electric fields are applied for electroporation to take place. The application of a MEF treatment at 200 V/cm and 10 pulses or 133 V/cm and 100 pulses caused the permeabilization of the tonoplast membrane of onion cells [81]. In other words, considering Eq. 3, a smaller number of pulses is needed to reach the induced transmembrane voltage when applying higher electric fields. Therefore, the application of ohmic-PEF would allow an earlier cell electroporation than would be achieved with MEF.

In plant tissues, non-thermal effects are most clearly observed at temperatures below 50–60 °C since thermal breakdown occurs above these temperatures [82]. For example, applying MEF treatment at 100 V/cm to sugar beet effectively damaged the tissue at moderate heating temperatures of 50–60 °C and treatment below 100 s. At such treatment conditions, the thermally induced damage effects were inessential, and the observed disintegration was mostly of an electroporation nature [83]. For the ohmic-PEF, a tissue disintegration of 0.77 at temperatures of 40 °C has also been observed in carrots [18]. Based on this effect, the cellular breakage caused by the application of MEF and PEF-ohmic would enhance extraction processes by improving mass transfer. Therefore, many studies on MEF technology have focused on the extraction of components [32, 82].

Microbial Inactivation

Microbial inactivation by heat is due to the alteration of several cellular components such as the outer and inner membrane, the peptidoglycan cell wall, the nucleoid, the cell’s RNA, the ribosomes, and diverse enzymes, which lead to the loss of the cells’ potential to multiply, and they ultimately die [84]. On the other hand, the application of high electric fields causes the loss of membrane selective permeability, which produces an imbalance in cell homeostasis and can lead to cell death [85].

Most studies on microbial inactivation by OH focus only on the inactivation kinetics [86,87,88], while very few deal with its mechanisms of action and are mainly related to spore inactivation [51, 70, 89]. The inactivation of vegetative cells by OH treatments is considered primarily thermal, although studies have reported additional inactivation related to the effect of electroporation when an electric field is applied (Table 1). However, although the application of PEF (electric fields > 20,000 V/cm) together with mild temperatures (< 50 °C) has been shown to favor the electroporation of bacterial membranes [90, 91], the electric fields applied in OH, MEF, and even ohmic-PEF, are significantly lower. Nevertheless, Pereira et al. [88] studied Listeria monocytogenes inactivation by OH when a treatment of 6 V/cm, 60 Hz, and 60 °C was applied to whey dairy beverage. In order to compare with the heat treatment only, samples were treated using a water bath. The results showed that for the inactivation of 4 Log10 cycles, 2.73 min and 4.35 min were necessary when applying OH and CH, respectively. These results have to be taken with care, and more detailed research is necessary since in this last study, no time–temperature profiles were shown, assuming that the heating phase until the sample reached the holding temperature did not affect the inactivation.

Table 1 Evaluation of the presence of additive effects in the inactivation of microbial vegetative cells by ohmic heating
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The additive effect on microbial inactivation when applying OH is a controversial subject.

In order to compare both heat and ohmic technologies, the same temperature–time profile should be applied [92] or mathematical models based on heat resistance data have to be used [16, 17]. Schottroff et al. [92] made a great effort to achieve a reliable working method in which the heating kinetics would be the same for both technologies and in which no hot or cold spots would occur. After testing several systems, the use of a capillary batch system allowed to applied the same temperature ramps for OH and conventional heating [51, 77]. In these conditions, no additional inactivation was observed for Microbacterium lacticum, Listeria innocua, Staphylococcus carnosus, Escherichia coli, Saccharomyces cerevisae, or Rhodotorula glutinis at two different frequencies 12 kHz (55.5 V/cm) and 300 kHz (41.7 V/cm). Likewise, Astráin-Redín et al. [17] conducted inactivation of Listeria monocytogenes at 50 Hz by employing a batch cylindrical chamber, but instead of using liquid samples, they used a solid technical agar cylinder. This allowed them to select the sample area as the central zone, and the zone close to the electrodes whose heating rates were different (1.10 °C/s and 0.88 °C/s, respectively). Based on the time–temperature values of ohmic heating and the thermoresistance values of L. monocytogenes from conventional heating (D58 = 2.277 min; z-value = 5.37 °C), they modeled inactivation and observed no difference between experimental and simulated inactivation based on the microbial thermoresistance.

A second point which could explain the additional inactivation by the non-thermal effect of OH would be the presence of compounds generated/released due to the electrochemical reactions from the electrodes. This phenomenon is enhanced when OH is applied by direct currents or at low frequencies with long pulse widths (> 20 µs) [93]. Nevertheless, it should be noted that for non-thermal PEF treatments at high voltages, and if the PEF system is well designed, the outflow of metal ions into the food is minimal [94]. Therefore, the application of ohmic-PEF could also help to minimize these electrochemical reactions by applying shorter pulse widths (3 µs), medium electric fields (1000–5000 V/cm), and reduced heating times. In addition, the use of platinum electrodes and the application of frequency in the kilo-hertz range have also been found to minimize electrochemical reactions of the electrodes [11, 12].

Everything discussed earlier is related to the inactivation of vegetative cells. However, it seems that OH could be a promising alternative for bacterial spores inactivation, which are very resistant to preservation technologies [95]. Spore stress resistance is believed to be due to different factors including its complex multilayered envelopes, the low water content of the core, and the presence within the later of small acid-soluble proteins (SASP) that protect the chromosome and high amounts (up to 10% of dry weight) of dipicolinic acid (DPA) and divalent metal ions, together with the low water content of the core [95,96,97,98]. In some cases, the application of OH treatment has resulted in higher inactivation of Geobacillus stearothermophilus [77], Bacillus licheniformis [99], and Bacillus subtilis [51, 70, 100, 101]. In a study conducted by Schottroff et al. [51], an OH treatment of 60 V/cm, 84 µs of pulse width with a temperature increase of 1.1–1.3 °C/s up to 108 °C was applied for the inactivation of B. subtilis PS533. Conventional heating and OH followed the same heating kinetics but for F121°C = 4.4 s, the ohmic inactivation was 2.2 Log10 higher. Thus, in order to determine which cellular component of the spore structure was being affected by OH, several trials were carried out on mutants that lacked the genes involved in spore resistance. The results indicated that these differences were not related to any/major effect of OH on the envelopes, DPA release was comparable to that generated by conventional heating.

Therefore, further research is needed to assess which cellular components are affected by the application of OH, and how the processing parameters (electric field) influence both vegetative cell and spore microbial inactivation.

Enzyme Inactivation

The application of food preservation treatments also involves the inactivation of enzymes. As described for microbial inactivation, the thermal effect would be the major cause of enzyme inactivation, but an electrical (non-thermal) inactivation has also been reported in the literature [102,103,104]. In this case, this non-thermal effect would be related to changes in enzyme structure leading to increased structural dynamics and reduced cohesion of the protein [105]. The application of rigid body dynamic models has shown that the exposure of enzymes to an electric field induces electrophoretic translational motion which make enzymes behave as if they were at a higher temperature than their surroundings [106, 107]. It has also been observed that the magnitude of these non-thermal effects is very field-strength and treatment-time dependent. Consequently, this can be used as a tool to improve the efficacy of OH treatments for enzyme inactivation [108].

In a recent study, Samaranayake et al. [104] evaluated the application of ohmic inactivation of PPO and POD enzymes in grape juice when 82 and 87 V/cm OH treatments were applied at 65 and 75 °C. The results were statistically significant only for a temperature of 75 °C where the application of electric fields increased the inactivation rate by 33.15% for PPO and 55% for POD. Additionally, they simulated the molecular motion in response to the electric field and the temperature and reported that the operating temperature could be reduced by increasing the electric field strength. Moreover, the PPO inactivation in pineapple juice was evaluated by heating at 80 °C and different electric fields (16–36 V/cm). The results showed that for the same treatment temperature, enzyme inactivation increased with increasing electric field, i.e., an increase from 16 to 36 V/cm resulted in a reduction of the residual enzyme activity from 0.67 to 0.10. In this sense, it seems that ohmic-PEF could still lead to better results as higher electric fields are applied.

Applications in the Food Industry

OH has been extensively investigated in various thermal food processes focusing on pasteurization and enzymatic inactivation of liquid products, extraction of compound from vegetables and fruits, peeling, cooking of meats, concentration of fruit juices, and thawing of fish. There are several publications related to OH [20, 21, 65, 93, 109] using the majority of them electric field strengths lower than 100 V/cm. Table 2 summarizes the recent studies (last 5 years).

Table 2 Ohmic heating applications in the food industry
Full size table

Food Preservation Based on Microbial and Enzymatic Inactivation

In order to guarantee the safety, nutritional, and sensory quality of food, it is necessary to achieve a certain level of microbial and enzymatic inactivation. However, the required inactivation must be achieved in the whole food. When heat treatments are applied using conventional systems, some areas result over-treated to ensure that the cold points reach the required level of inactivation. Ohmic pasteurization is successfully applied to liquid or viscous foods where heating is more uniform than in solid foods due to the homogeneity of electrical conductivity. Pasteurization by OH has been widely studied for fruit and vegetable juices and purees as well as for milk [71, 116, 118, 119, 121].

The application of ohmic pasteurization allowed to reduce processing times and energy consumption by applying rapid and volumetric heating. Several studies have been conducted by applying ohmic technology. Giuliangeli et al. [117] applied a treatment of 0.21 V/cm at 60 Hz to guava pulp at 60, 70, and 80 °C for 110 min. A reduction in energy consumption of 93% and a constant heating rate of 0.16 °C/s were observed, which was 2.6-fold higher than conventional heating. Similarly, Balthazar et al. [121] reported 73% energy savings when pasteurizing sheep’s milk at 8.33 V/cm and 60 Hz; Gratz et al. [58] reduced sterilization time of carrot purée by 51–60% by applying 62.5 V/cm at 12 kHz; Kuriya et al. [145] reduced the pasteurization time of dairy dessert by 43% by applying 9.1 V/cm and 60 Hz. This improvement would be amplified by applying higher electric fields such as those used in MEF and ohmic-PEF. Astráin-Redín et al. [17] applied ohmic-PEF (2.5 kV/cm and 50 Hz) for the pasteurization of agar cylinders contaminated with L. monocytogenes and observed a reduction of the treatment time of 83.3–99.3% compared to conventional heating.

Furthermore, this improvement in processing times also results in products with better nutritional and sensorial properties by minimizing the thermal impact. Therefore, much research has focused on studying the impact of ohmic pasteurization on nutritional compounds and sensory properties of the food. Improvements in the color of OH-pasteurized peaches were observed by Rinaldi et al. [120] who found better redness values due to higher phenolic compound content. Debbarma et al. [116] applied an ohmic treatment of 17.10 V/cm for 40 s on orange juices, and improvements in the final values of β-carotene (no degradation), total phenolic content (12.22%), and color were found. However, Dhenge et al. [119] applied ohmic pasteurization to orange juice and observed no improvements in viscosity, total phenolic content, ascorbic acid, total antioxidant capacity, and browning. On the other hand, when MEF treatment at 120 V/cm, 60 kHz, and 80 °C / 7 min was applied to carrot juice, better color, higher antioxidant capacity, and better carotene content were obtained compared to those treated by a conventional pasteurization at 80 °C during 7 min [127].

As already pointed out above, it is very complex to compare between treatments as it has to be done for the same lethality, and in many cases, this has not been calculated or reported. Similarly, treatment conditions may also vary widely: batch vs. continuous, different foodstuff, etc. Furthermore, in some cases, there is missing information about the ohmic parameters applied.

Regarding dairy products pasteurized by OH, Silva et al. [146] analyzed sensory parameters such as bitterness, brightness, and fluidity and concluded that applying electric fields of 10 V/cm and temperature of 90 °C during 3 s improved the consumer acceptance. However, Kuriya et al. [145] observed a negative effect on color, volatile profile, and rheological behavior in ohmic-pasteurized blueberry-flavored dairy desserts. Thus, the application of electric field strengths of 9–20 V/cm and temperatures of 72–75 °C during 15 s increased the degradation of monomeric anthocyanin, total polyphenols, and DDPH as compared to conventional heating.

Finally, in a study conducted by Kanjanapongkul and Baibua [126] on coconut water, they observed that ohmic pasteurization (10 V/cm and 50 Hz) prevented the pinkish discoloration caused by conventional pasteurization, which they attributed to the effects of electric fields on the PPO enzyme.

In summary, ohmic pasteurization would allow shorter heat treatments compared to traditional ones, which would improve the sensory qualities of the food but, depending on the matrices treated, in some cases, the electrical effect might negatively affect certain bioactive compounds.

Extraction

Many waste products from the food industry (skins, seeds, lees, etc.) are a source of valuable compounds including phenolic compounds, digestive fiber, hydrocolloids, pectins, organic acids, carotenoids, among others, which are used as natural additives [147, 148]. However, extraction processes involve the use of solvent that could be a danger for health or considered non-environmentally friendly solvents [149]. OH is one of the emerging technologies being investigated in order to reduce the use of these chemicals and favor a more environmentally friendly process [150].

Several experiments have been performed on OH-assisted extraction of valuable compounds in vegetables and fruits: pectins from the peel of pomegranate, grapefruit, lemon, and oranges [46, 128], anthocyanins from grape peel [42], polyphenols from tomato peel and seeds [45], betalain from red beetroot [129], and phytochemical and steviol from Stevia rebaudiana leaves [32]. For instance, Pereira et al. [42] evaluated the application of two ohmic pre-treatments at 16 and 70 V/cm and 25 kHz (mild heating 40 °C/ 20 min and flash heating from 40 to 100 °C in 20 s) on grape peel to favor the extraction of anthocyanins in water. The results obtained with the mild treatment at 40 °C were not significantly different from the conventional heat treatment at the same temperature; meanwhile, the flash ohmic treatment allowed to increase the concentration to 20% total anthocyanins/TPC (total polyphenol content). This flash OH treatment that applies high temperature during short times allowed to minimize thermal degradation. In this sense, the application of MEF or ohmic-PEF that allows the application of higher electric fields resulting in an increased heating rate and higher electroporation capacity could be of interest for extraction processes. But it is something that requires more research.

At this point, it should be remarked that there are some factors that seem to highly influence the effect of ohmic extraction such as the solvent, applied electric field, and frequency. Cabas and Icier [129] studied the extraction (40 °C / 1 h) of betalain in water, ethanol–water, and acidified water–ethanol from beetroot at 11, 17, and 23 V/cm at 25, 100, and 400 Hz. The extraction medium had a significant effect on the total extraction yield for low gradients (11 V/cm at all frequencies) with the highest extraction being obtained when using acidified water–ethanol. However, at higher fields (23 V/cm) and frequencies (400 Hz), the extraction medium was not a significant factor. Furthermore, depending on the compound (betacyanins, betaxanthins, or betalain), the influence of frequency, extraction medium, and voltage was different. This suggests that OH is a potential technology to apply a sustainable extraction process but that optimization would be required before its industrial implementation.

Drying

Drying process is widely applied for food preservation, but it is a highly energy-consuming process, so any technology that accelerates mass transfer results is very interesting. Since OH has to be applied to the product in contact with the electrodes or immersed in a liquid medium, it has been evaluated as a pre-treatment for this drying application. Hosainpour et al. [151] applied an ohmic pre-treatment to tomato paste using a Teflon chamber with a hole in the center to insert the temperature probe and allow the water to evaporate until the moisture content was reduced to the desired level. Different electric fields (6–16 V/cm) were evaluated to reach a temperature of 95 °C which was maintained until the moisture content decreased from 90 to 70% (w.b.). Increasing the electric field from 6 to 16 V/cm allowed to reduce the processing time by 86.8% and up to 38 times when compared to air drying at 105 °C and 1 m/s. In another study, Soghani et al. [152] applied OH for the blanching process (8.18 V/cm, 15 min) before microwave (MW) drying (900 W) of white mushroom immersed in aqueous NaCl solution (1%). Blanching involved increasing the temperature from 20 to 80 °C in 15 min. No differences in drying rate were observed when applying MW after OH blanching.

There is a study in the literature where OH was applied during drying using needle-shaped electrodes. In that case, Turgut et al. [130] used stainless steel needle-shaped electrodes in which the potato sample was pricked at both ends so that current flowed from one electrode to the other inside the potato. This novel system was able to reduce the drying time by 20–60% compared to the conventional hot air system, but as the drying process progressed and the moisture of the product decreased, the efficiency of the ohmic heating was reduced. Any case, besides the potential of OH for drying, other electro-technologies for drying such as MW or radiofrequency are under research since no contact with the product is required, obtaining more efficient results [153].

Concentration

Concentration processes are highly used in the juice industry due to the restriction of the harvesting period, low cost of packaging, storage, transport, and distribution [132]. It has been observed that the rapid and uniform heating resulting from the application of ohmic technology reduces the fouling and therefore the cost of cleaning while maintaining the original quality of the sour cherry juice [132]. Additionally, the fast heating leads to reduced processing times and energy consumption. Thus, Karakavuk et al. [134] achieved a reduction of 44% in the time required for apple juice concentration; Darvishi et al. [135] reported a 68.5% diminution in mulberry juice concentration time. The higher the electric field, the better the results as higher heating rates were obtained [33, 132,133,134]. Moreover, better final results in terms of TPC and antioxidant activity were achieved. However, it must be noted that during water evaporation processes, the conductivity of the product will increase until it reaches boiling temperatures where the loss of moisture will reduce the electrical conductivity [134].

Thawing

Apart from being a high-cost and time-consuming process, thawing has to be done as quickly as possible in order to maintain the microbial and nutritional quality of the food. Therefore, ohmic technology is an interesting alternative to conventional thawing systems as the heat applied is fast and uniform. There are several studies in the literature describing the thawing of food with OH [30, 136, 137, 154,155,156,157,158]. For instance, thawing time was reduced by 82.44% for pure spinach (15 V/cm) [136] and by 30% for tuna fish (50 V) [137]. In the latter study, a saline solution (0.3% NaCl) at 25 °C was used to immerse the fish during the treatment, although direct contact with the electrodes can also be used [154].

Achieving temperature uniformity is critical for all thermal processes but even more in ohmic thawing due to the large difference in electrical conductivities between a frozen and thawed product [159]. Frozen food is not a conductive material but, gradually, once part of the ice goes into a liquid state, its conductivity will increase, and it will be able to heat up due to the current flow. Liu et al. [158] conducted a study on different tuna pieces (dorsal, lateral, and ventral) using ohmic thawing to increase the temperature from − 30 to 20 °C. The thawing rates were different depending on the piece and the direction of the current to the muscle fibers. The configuration that presented the best uniformity of heating between the three pieces was in parallel with the current flow (the dorsal piece was the fastest, and no differences were observed between the lateral and ventral pieces). Moreover, the electrical conductivity hardly changed (0.00134 mS/cm) up to − 10 °C since water in solid state has very little mobility. But from − 7 °C onwards, an increase was observed, with the range from − 3 to − 1 °C showing the greatest change. This corresponds to the freezing temperature of the tuna (− 1.4 to − 5 °C) and therefore the amount of water in the liquid state begins to increase. The reached values increased to 2 mS/cm at 0 °C and 4 mS/cm at 20 °C. Similarly, Cevik and Icier [30] observed that thawing minced beef with a 2–18% fat content from − 6 °C onwards, the fat content had an impact on the final conductivity by reducing the value.

Furthermore, when OH is applied by direct contact with the electrodes, it is important that the frozen product is completely flat to ensure the perfect contact; otherwise, hot spots will be created, even though this is not very easy with frozen products unless those freeze with plate freezers.

Cooking

The purpose of cooking food is to bring changes in their texture and composition to make them edible, i.e., starch gelatinization (vegetables), softening the texture (pectins, collagen), boosting the flavor, color changes, and improving the digestibility of proteins. However, when the cooking temperature is not properly controlled, undesirable effects may occur, such as the generation of burning compounds on the surface of the cooked food or total loss of texture [160]. Therefore, applying ohmic cooking would aim to have a more uniform temperature evolution avoiding overtreatment in order to improve quality and shorten processing times.

Regarding cooked meat, its major quality parameters are color, tenderness, and juiciness [161]. Bedane et al. [68] investigated the effect of ohmic cooking on beef by applying MEF (175 V/cm) and OH 57.5 V/cm for 2 min at 72 °C. No effect was observed on the tenderness and color of the samples compared to conventionally cooked samples, but quality was improved in terms of reducing cook-loss values (19–41%). Authors associated this improvement to the shortening of the processing time by 93.3% and 91.7% for reaching 72 °C in the center of the sample. Ángel-Rendón et al. [113] applied 21 V/cm for the cooking of marinated pork but instead of having the sample immersed in a salt solution, it was in direct contact with the electrodes. In this case, no improvement in cooking loss, color, or water-holding capacity was observed. These differences in the results may be related to the type of meat, e.g., the color in beef is strongly affected by heat (effect on myoglobin) while in pork, the impact is much lower. Therefore, further studies are needed to evaluate the effects of all critical parameters of OH processes.

Ohmic cooking has also been applied to vegetables found to promote softening of carrot, red beet, golden carrot [162], and potato samples [110] due to a greater damage of vegetable tissue. A relevant aspect of this application is the peel of the vegetables, which acts as an electrical insulator. Its presence generates a greater heterogeneity of temperatures as Gratz et al. [37] observed with potato tubers. They applied OH treatments (55.6–111 V/cm, 2 kW/kg) of potato samples immersed in aqueous NaCl solution with 2.5 mS/cm and after 200 s, the temperatures of the peeled and unpeeled potatoes were 80 and 30 °C for 12 kHz and 100 and 90 °C for 300 kHz, respectively.

Furthermore, a recent application with promising results is the baking of gluten-free bread by ohmic heating [49, 163, 164]. These products find it very difficult to retain CO2 in their structure, resulting in dense and crumbly breads. However, applying ohmic baking (64.9 V/cm, 12 kHz) in gluten-free bread allowed to obtain a significant increase (up to twofold) in crumb volume compared to conventional baking [49]. They explained this result by the fact that as the heating process was much faster, the bread crumb was formed before the dissipation of CO2. In addition, no differences were observed in the color of the crumb, but there were differences in the crust, as the heating process is volumetric, and the evaporation of water occurs in a similar way throughout the product.

Therefore, ohmic technology for food cooking is promising as it is easy to apply and highly reduces cooking times favoring the sensory and nutritional quality of the food. In fact, a domestic ohmic cooking device called Sevvy (https://www.sevvy.nl/) is already on the market.

Peeling

Conventional systems consist of using bleach in the treatment medium which, by diffusion, penetrates the inside of the peel and degrades the plant wall, in particular the pectins. The application of OH favors this diffusion process and reduces the percentage of bleach required, as was observed in the case of tomato peeling by Wongsa-Ngasri and Sastry [139]. They also observed that the electric field strength and the composition of the peeling medium affected the peeling time, i.e., for the same medium concentration, the higher the electric field, the shorter the peeling time. The peeling time was defined as the time needed until the peel cracking. Moreover, for the same electric field, the higher the NaOH concentration, the shorter the peeling time. This was to be expected, as the increase in the electric field favors electroporation and thus the diffusion process, and also the heating is much faster. Meanwhile, increasing the NaOH concentration not only increases the chemical effect but also increases the electrical conductivity of the solution, which makes the heating process faster. All things considered, the application of MEF and ohmic-PEF could have great potential for peeling as the electric fields are higher which would enhance the benefits of ohmic heating.

Protein Gels Formation

Apart from causing thermal denaturation of proteins, it has been observed that OH causes modifications to the structures (secondary and tertiary) of proteins due to exposure to an electric field [142, 165]. These ohmic effects may result in protein aggregates with different characteristics than those formed by conventional thermal systems, and the resulting gels may exhibit different properties, i.e., viscosity, water-holding capacity, solubility, or firmness. Most studies in the literature have evaluated the application of OH as a heat treatment of protein solutions, mainly as an initial heating step for a subsequent cold-set gelation process [142, 143, 166, 167]. Rodrigues et al. [143] evaluated OH (10–20 V/cm and 50 Hz, 20 kHz) for the formation of whey protein gelled by the addition of salts (NaCl). The gels were weaker and more elastic, with higher water retention and swelling capacity and lower solubility [143]. However, Wang et al. [166] applied OH (50 Hz and 200 V) to soymilk for protein thermal denaturation and found that gels were firmer and more resistant. When OH technology was applied during gel formation, it was not possible to induce an adequate elastic structure, resulting in gels with smaller aggregates and a fluid texture [168]. Joeres et al. [144] applied OH (7.7–17.2 V/cm and 4.3–7 V/cm; 10 kHz) during thermal gelification of white egg protein and reported that gels treated by OH showed lower water-holding capacity and lower firmness for longer heating times compared to CH, but the differences were not statistically significant.

The effect of OH on proteins and its impact on the characteristics and properties of gels are still unclear. However, OH can help to obtain gels with new textures and characteristics.

Impact on Food Allergenicity

One of the most current issues regarding novel food processing technologies is their impact on food allergenicity since it is strongly linked to consumer health. Allergenicity is an adverse reaction triggered by the immune system related to the ingestion of certain foods or specific component. The main food allergens are milk, soy, nuts, eggs, fish, shellfish, wheat, and peanuts, with several proteins involved such as β-lactoglobulin, casein, β-conglycinin, glycinin, ovalbumin, ovomucoids, among others. A growing number of studies are evaluating this aspect; however, there is little work on OH and allergenicity [6, 169, 170]. As in the previous section, both electrical and thermal effects must be considered for the assessment of allergenicity in OH-treated foods. Due to the wide variety of allergenic proteins, the effects of heat on them are heterogeneous, as some are more thermo-resistant (caseins, β-lactoglobulin, ovomucoid, ovalbumin), and others are more thermolabile (bovine serum albumin, β-lactalbumin). OH would also be expected to behave similarly to heat due to its mainly thermal effect. The impact of ohmic heating is likely to depend on the specific food, the structure of the allergenic proteins, and the processing ohmic conditions since both electric field strength and frequency can have an effect on protein structure. Pereira et al. [171] evaluated the application of OH (4 V/cm and 25 kHz) on β-lactoglobulin immunoreactivity. They observed that OH decreased the immunoreactivity of the protein compared to conventional heating. However, when 4 V/cm at 25 kHz was applied during thermal treatment at 65 °C for 30 min, immunoreactivity was still increased as compared to the non-treated protein. In another study, Pereira et al. [172] evaluated the impact on trypsin inhibitor protein present in soybean, and it was observed that OH (10 V/cm) at low frequencies of 50 Hz produced higher conformational disturbances, but when two electric fields of 2 and 20 V/cm were evaluated, no effect was observed. This resulted in a 36% reduction in immunoreactivity at 50 Hz. Also, recently, Li et al. [173] applying 100 V at 50 Hz determined that OH at 50 °C required 19.2 × less time and 121.6 × less energy consumption to reduce by 70% eel collagen allergenicity than a heat treatment in a water bath at the same temperature. In a similar way and methodology, these authors observed a 65% reduction of the allergenicity of parvalbumin when using OH heating compared to water bath heating or thermostatic heating [174]. All these studies seem to indicate that OH may help to reduce the allergenicity of certain proteins; however, the studies are too few to make solid conclusions, and more research is still required for this interesting possible application of OH in the food industry.

MEF and Ohmic-PEF Applications

Most studies in the literature focus on OH (E < 100 V/cm). In the case of MEF, some studies have been carried out but the applied electric fields ranging from 1 to 100 V/cm (similar to those of OH) [24, 69, 71, 104].

Considering the previous food applications, the use of higher electric fields would be interesting for those processes involving mass transfer, as it would be promoted by the electroporation phenomenon. Therefore, the application of MEF and ohmic-PEF would be very efficient to improve processes such as component extraction, drying, and concentration. Their application could reduce processing times by increasing the extraction of compounds and water removal, and moreover, the energy transfer could be increased. All of this could make it possible to work at lower temperatures in order to have less impact on food quality.

For other applications such as microbial and enzymatic inactivation, thawing, cooking, and peeling, the application of MEF and ohmic-PEF could improve energy transfer, increase heating speed, and reduce processing times, which could improve food quality by reducing thermal impact.

However, in the case of ohmic-PEF, literature studies are very recent, and current equipment is not optimized for it. Further studies are needed to assess the potential of this technology and its subsequent equipment development.

Future Trends

OH is a thermal technology applied in many processes in the food industry. Although its main effect is thermal, in some cases, such as spore and enzyme inactivation, a non-thermal effect can occur due to the application of an electric field, leading to higher inactivation than in traditional heating. These non-thermal effects are more pronounced at higher electric fields, which is why MEF and ohmic-PEF systems have been gaining interest in recent years. These systems allow higher electric fields to be applied than ohmic heating, which, on the one hand, favors non-thermal effects and, on the other hand, increases the heating rate. Moreover, in the case of eukaryotic cells, these systems induce higher degrees of electroporation, increasing the outflow of intracellular compounds and improving the homogeneity of the conductivity of the food and, consequently, the uniformity of the heating in the whole product. This approach would be very promising for the application of heating in large solids, where huge temperature gradients are generated when conventional heating is used.

Regarding solid foods and due to their complexity, numerical simulation tools are useful to clarify how heat is distributed in the food depending on its components, the treatment chamber, the presence or not of a saline solution (immersion system), and the technological parameters applied. In this way, a more reliable and accurate optimization of the process could be achieved.

Finally, more studies on the application of MEF and ohmic-PEF are required, as these technologies have certain advantages over OH (mainly the electric field effect). Research should be focused on electrical effects, on a more in-depth evaluation of processing parameters on heating uniformity, on the design of new processing equipment for larger foods, and on the impact on the nutritional and sensory properties of the food. Moreover, the role of the electrical effect in this type of heating has not yet been clarified. For this reason, researchers need to make an effort to carry out studies focused on achieving a reliable and valid comparison between conventional and ohmic heating by developing a proper methodology.