Ohmic Heating in Food Processing: An Overview of Plant-Based Protein Modification

Abstract

This review provides an analysis of ohmic heating in food processing and its effect on plant proteins. This study explores the effect of this technology on protein denaturation and aggregation, affecting both non-covalent and covalent bonds. These structural and chemical changes have significant implications for the techno-functional properties of proteins, contributing to their use in food processing. This article emphasizes the need to adjust processing conditions to maximize the benefits of ohmic heating, distinguishing it from other traditional thermal techniques due to its direct and controllable impact. By highlighting these contributions, this review serves as a resource for researchers and professionals interested in innovation and efficiency in food processing through the use of emerging technologies.

1. Introduction

Proteins offer a wide range of nutritional potentialities and promote health and well-being [1]. Furthermore, a protein-rich diet can assist in appetite control, promote weight loss, maintain lean muscle mass, and regulate blood glucose levels [2,3]. Therefore, proteins are essential to a balanced and healthy diet.

Proteins also play a crucial role in the structure and functionality of food. However, they often exhibit technological properties that are not ideally suited for applications in the food industry, mainly the plant-based ones. Proteins can often be sensitive to temperature, pH, and pressure, limiting their usefulness in food formulations [4]. Furthermore, proteins’ properties such as texture, solubility, and flavor, may not meet the desired standards for food products. Therefore, the need to modify the technological properties of proteins through modification processes, such as heat treatment, chemical modification, or food engineering techniques, becomes essential to optimize the functionality of proteins in food production and meet consumer expectations [5]. Improving the functional properties of proteins also enables the creation of plant-based products that resemble animal-based ones in terms of technological, sensory, and nutritional characteristics [4,6].

Plant proteins are categorized into four main types according to Osborne’s solubility classification. These include albumins (soluble in water), globulins (soluble in saline solutions), prolamins (soluble in alcohol), and glutelins (soluble in alkalis). While albumins perform physiological functions, the other three categories serve as storage proteins [7]. Understanding the primary protein fraction is important as it significantly influences functional properties such as solubility, emulsification, foaming, and gelation. Approximately 40% of plant proteins are predominantly globulins, while around 7% are primarily albumins. In specific cases, such as potatoes, the main protein fraction is patatin [8].

Modifying the fractionation conditions during protein extraction can alter the main protein fraction. Studies by Albe Slabi et al. [9] and Subaşı et al. [10] with sunflower flour showed that while globulin predominated in the first study, albumin was the main fraction in the second. Similarly, in quinoa, Mir et al. [11] identified globulin as the primary protein fraction, whereas Van de Vondel et al. [12] found albumin to be predominant. Thus, the primary protein fraction can be adjusted during extraction depending on fractionation conditions and specific functional needs.

Any process that induces structural reconfiguration in proteins impacts the balance between hydrophobic and hydrophilic properties on the protein surface. This can affect both the protein’s solubility and its ability to absorb water and/or oil. Protein modification is a technique used to alter the molecular structure or chemical composition of proteins with the aim of enhancing their functionality [13]. Furthermore, ohmic heating can enhance the bioactivity of proteins by promoting controlled denaturation, which makes previously hidden bioactive regions more susceptible to exposure, and by improving digestibility, thereby releasing bioactive peptides with positive health effects. These modifications make plant proteins versatile and multifunctional ingredients, expanding their use in a variety of functional food systems and overcoming their limitations by adjusting their physicochemical and functional properties [8].

Thermal modifications, whether through conventional heat, microwaves, infrared, radiofrequency, or ohmic heating, induce significant changes in protein structure. These changes result mainly from denaturation and unfolding of amino acid chains, caused by the weakening and breaking of hydrogen and disulfide bonds, leading to the formation of protein aggregates [8].

In this scenario, ohmic heating technology emerges as a promising approach for modifying plant-based proteins. This technology harnesses the direct application of electromagnetic energy to heat protein molecules, enabling precise customization of processing conditions [14,15]. Furthermore, this technique is praised for its energy efficiency, reducing processing time, and thus minimizing environmental impact [16]. The application of ohmic heating in the modification of vegetable proteins offers several advantages, such as altering their techno-functional properties, including solubility, gel formation capacity, and emulsification, without the need for chemical additives. This makes the technology promising for the segment due to the growing demand for plant-based products [17].

In this article, we thoroughly explore the applications of ohmic heating in the modification of plant-based proteins, making them more suitable for a wide range of food applications.

2. Ohmic Heating

In ohmic heating, or Joule heating, an electric current is passed through the material to be treated, generating internal heat. This technology involves the conversion of electrical energy into thermal energy. In contrast to other methods, this approach requires the insertion of electrodes in direct contact with the food (a feature absent in techniques such as microwaves and inductive heating), as well as the control of the applied frequency and the type of wave used, which is usually sinusoidal [18].

The method involves conducting alternating current through a food product. When this electric current passes the food, the displacement of charges within the material results in the agitation of molecules and atoms in the system. During this agitation process, charged particles collide and interact, leading to an increase in temperature. In this way, the particles migrating toward the electrode of opposite polarity are ions or charged molecules, such as proteins. Since the electric current alternates when passing through the food, ions and molecules constantly move [19].

Figure 1 provides a schematic diagram illustrating the working principle of ohmic heating.

Table 1. Advantages and disadvantages of using ohmic heating in food processing [15,20,21,22,23].

Advantages	Disadvantages
Fast and uniform heating: Ohmic heating can heat food quickly and evenly, preventing hot and cold spots. This helps maintain the quality of the final product.	High initial costs: The installation of ohmic heating equipment can be expensive, which can be a barrier for small food processing businesses.
Energy efficiency: Compared to some conventional heating methods, ohmic heating can be more energy-efficient as it directly converts electricity into heat in the food.	Potential quality issues: Depending on the food composition and processing conditions, ohmic heating can affect the texture, color, and taste of food undesirably. Another concern is the migration of metallic ions from the electrodes into the food.
Precise temperature control: Precise temperature control during ohmic heating is possible, which is important for ensuring food safety and the quality of the final product.	Requires conductive foods: Ohmic heating works best with foods that are good electrical conductors. Foods with low electrical conductivity (such as those high in fats) may be less effective in this process.
Reduced nutrient loss: In some cases, ohmic heating may result in less nutrient loss compared to more aggressive thermal processing methods, such as prolonged boiling. This can be achieved thanks to more uniform and rapid heating, which allows for reduced temperature and shorter exposure time of the food to heat. Additionally, the food is heated directly through the passage of electrical current, eliminating the need for heating mediums such as water, which can lead to nutrient loss through leaching.	Need for technical monitoring: Due to the precision required in temperature control, ohmic heating processes demand constant technical monitoring, which can increase operational costs.
Reduced need for additives: Since the process is quick and effective in reducing microorganisms, there may be a reduced need for chemical additives or preservatives. Additionally, the reduced exposure of the food to heat minimizes thermal damage to the food matrix, which decreases the need for added additives to compensate for sensory loss caused by heating.	Potential for electrical burns: There is a risk of electrical shock when using ohmic heating equipment, necessitating the implementation of appropriate safety measures

presents the main advantages and disadvantages of this technology in food processing [15,20,21,22,23].

However, there are challenges associated with ohmic heating. The equipment installation can be costly and a barrier for small food processing businesses. Additionally, the process can affect the texture, color, and flavor of foods, depending on the composition and processing conditions, which can be undesirable. It is most effective in foods that conduct electricity, making it less suitable for foods with low electrical conductivity, such as those rich in fat. The required temperature control accuracy demands constant technical monitoring, which increases operational costs. There is also a risk of electrical shock in the use of ohmic heating equipment, necessitating the implementation of appropriate safety measures [24].

This technique for heating food offers several notable advantages. Firstly, it is highly energy-efficient, converting electricity directly into heat within the food. Additionally, it allows precise temperature control, which is crucial for food safety and product quality [25]. Ohmic heating also results in less nutrient loss compared to more aggressive thermal methods, making it an attractive choice for preserving the nutritional value of foods [26,27]. Furthermore, its effectiveness in reducing microorganisms can reduce the need for chemical additives or preservatives [28].

It is worth noting that the process configuration for ohmic heating, including the current intensity and exposure time, can be adjusted and consistently repeated, ensuring that the treatment conditions are similar across batches.

3. Ohmic Heating Applied to Food Proteins

The literature indicates that ohmic heating can modify the pathways of protein denaturation and aggregation, as well as gelation behavior and interaction mechanisms (Table 2). This enables the creation of different protein systems, such as gels, films, and nano/microstructures, with specific functional properties. Most research on the modification of plant proteins by ohmic heating has focused on soy and its derivatives, and significant changes in techno-functional properties have been observed [29,30]. However, the use of ohmic heating to modify other plant proteins is still a poorly explored area.

Various phenomena can occur during ohmic heating, such as protein aggregation, forming new molecular bonds, rearranging disulfide bridges, cleavage of peptide bonds, and chemical modification of amino acids within the protein structure. These modifications can significantly impact proteins’ functional and structural properties, affecting aspects such as texture, stability, and other characteristics of thermally processed food products [31]. While the literature already contains numerous studies investigating the effects of ohmic heating on proteins of animal origin [31,32], it is crucial to focus more attention on evaluating the application of this process on the secondary structure of plant-based proteins and understanding its effect on the chemical bonds of these proteins.

The information available in the literature is primarily focused on soy protein and its derivative products, such as tofu. The processing of soy milk by ohmic heating to promote coagulation in tofu production has demonstrated promising improvements in the textural properties of the final product [33,34]. Additionally, an inverse relationship has been observed between the applied voltage and the viscosity, precipitation, and surface hydrophobicity of soy protein [33]. In more in-depth studies, Li et al. [35] investigated the effects of ohmic heating on the structure and functional properties of soymilk protein, finding significant changes compared to traditional heating. It has also been suggested that changes in total sulfhydryl content, surface hydrophobicity, and functional properties, such as foaming and emulsification capacities, were induced by the application of the electric fields characteristic of ohmic heating [35].

It is worth noting that the efficiency of this technique on food proteins is influenced by the protein concentration in the sample, ionic strength, and the pH of the solution, all of which are directly related to the thermal conductivity of the material. The type of protein, temperature, and processing time are factors correlated with the degree of thermal degradation of the proteins. Lastly, the homogeneity of the sample is important to ensure uniform heating, as are the equipment settings, such as current intensity and electrode configuration.

Table 2. Key findings from scientific studies on the application of ohmic heating for the modification of food proteins.

Protein	Experimental Process	Ohmic Heating Treatment	Highlight	References
Soy protein isolate	They assessed the effects of ohmic and conventional heating on structural, thermal, and physical changes in soy protein isolate edible films.	The electrical frequency for treatment was 50 Hz, and the electric field intensities used for ohmic treatment were 3, 6, 9, and 12 V·cm−1, compared to conventional heating.	Ohmic-heated films exhibited reduced α-helix content, increased random coil content, smaller average particle size, lower fluorescence intensity, and higher surface hydrophobicity.	[36]
Sunflower protein isolate	They analyzed the structural, thermal, and physical changes in sunflower protein isolate compared to sodium caseinate when subjected to the application of non-thermal moderate electric field (MEF).	Electric field application: sample had 10 V/cm for 1 h (V1) and 10 V/cm for 2 h (V2) applied; sample had 150 V/cm for 5 s (V5) and 150 V/cm for 20 s (V20) applied.	Greater homogeneity was achieved with V20. The secondary and tertiary structures of sunflower protein changed primarily with V20, gaining a bulkier and more flexible structure and increased surface hydrophobicity. V5 had the lowest denaturation enthalpy. V1 also produced sodium caseinate solution with the lowest denaturation enthalpy.	[10]
Soy protein isolate	They studied the effect of ohmic heating on the structure and immunoreactive properties of soy proteins.	Ohmic heating studies were conducted at various frequencies, including 50 Hz, 500 Hz, 2 kHz, and 20 kHz. The electrode distance was consistently maintained at 30 mm for all assessed conditions. When necessary, the electric field intensities during the procedure ranged from 2 V/cm to 20 V/cm. Soybean trypsin inhibitor allergenic protein (STI), also known as Gly m TI, was selected as the allergen of interest due to its remarkable resistance to thermal treatments	Ohmic heating applications at frequencies ranging from 50 to 500 Hz can lead to conformational changes in soy protein fractions, as evidenced by the reduced fluorescence of aromatic amino acids. This effect may be directly associated with the formation of complexes with metals present in the protein solution during ohmic heating. However, there is a lack of detailed information on how electrical variables, such as electric field and frequency, affect soy protein fractions in this specific context.	[31]
Lactoferrin	The purpose of study was to create gel-like gelatin emulsions using lactoferrin dispersions heated through ohmic treatment. The impact of both ohmic and conventional heating on the protein’s secondary structure and the subsequent thermal aggregation of lactoferrin was assessed.	Ohmic heating was applied using a 4 cm electrode gap, with voltages ranging from 36 to 86 V. This treatment was compared to conventional heating. A double-walled glass reactor vessel was used, and a 20 mL lactoferrin dispersion was heated to 90 °C for 30 min for both treatments.	Both ohmic and conventional heating influenced the thermal unfolding and aggregation of lactoferrin molecules. Ohmic heating likely affected the molecular flexibility or the stability of lactoferrin’s hydrophobic groups. Ohmic heating resulted in fewer aggregated protein molecules compared to conventional heating. The difference in aggregation patterns was confirmed by a smaller increase in particle size, turbidity, intrinsic and extrinsic fluorescence, and a distinct dichroic signal, impacting the structural and mechanical properties of the prepared emulsions.	[37]
β—Lactoglobulin	They investigated the effect of moderate electric fields (MEF) during the ohmic heating of purified β-lactoglobulin fractions under different physicochemical conditions of pH and temperature.	The treatments were conducted at temperatures of 50, 60, 70, 80, and 90 °C for 10 min. The applied electric field intensity varied from 80 V/cm during the heating phase to 20 V/cm during the holding step. The electric frequency remained constant at 20 kHz, as the choice of frequencies within the kHz range was adopted to prevent unwanted electrochemical effects, such as electrolysis and electrode oxidation.	The results indicated that the effects of moderate electric field (MEF) are limited to unfolded protein conformations. This suggests that under the specific conditions tested, including electric field intensity, frequency, and exposure period, the imposed disturbances are not significant enough to impede the protein folding process. Thus, the effects of MEF appear to act synergistically with thermal effects. Furthermore, their specific influence and scope varied according to the pH, which may be related to the flexibility of β-lactoglobulin in response to environmental conditions. These findings support theories presented in other studies, which had already demonstrated that moderate electric field (MEF) effects impact the aggregation and gelation of serum proteins.	[38]
Whey protein isolate	They evaluated the formation of soluble whey protein aggregates from whey protein isolated under conditions of near-neutral pH, both in the presence and absence of a moderate electric field (MEF).	The applied voltage and temperature were controlled using a function generator connected to an amplifier system. During the heating and holding phases, the applied voltage ranged from 15 to 22 V/cm and 4 to 8 V/cm, respectively. The electrical frequency was 25 kHz, chosen to reduce electrochemical reactions at the electrode interface, minimizing the potential for corrosion and metal leakage into the medium. The results were compared to conventional heating.	The results indicated that the application of moderate electric fields impacts the unfolding and aggregation processes of whey protein at relatively high temperatures. After heating at 85 °C for 30 s, treatments with moderate electric fields resulted in whey protein isolate solutions with a higher amount of β-Lactoglobulin (8%) and α-Lactalbumin (10%) in their native form, compared to a traditional heating method. The protein aggregates in solutions treated with a moderate electric field increased by up to 78 nm, while conventional heating led to an 86 nm increase in the size of these aggregates. The aggregation of whey proteins under the influence of the moderate electric field was not intense enough to create an elastic gel network. This is due to reductions in the value of storage and loss moduli after treatment with the moderate electric field.	[39]
Lentil protein isolate	They assessed the impact of ohmic heating (temperature, electric field intensity, and frequency) on the structure of lentil protein obtained through alkaline extraction and protein–pectin interactions	The heating was conducted at temperatures of 40, 50, 60, 70, and 80 °C. After reaching the desired temperature, a waiting period of 30 s was observed. The voltages used for heating and maintaining a constant temperature were 5 and 75 V/cm, respectively. In the ohmic heating treatments, the initial phase involved the application of an electric field of 75 V/cm until the solution reached 80 °C. In the first ohmic treatment, the voltage was reduced to 5 V/cm, and the temperature was kept constant at 80 °C. In the second treatment (OH-75 V/cm), electric fields ranging from 10 to 20 V/cm were used while maintaining the temperature at 80 °C.	The results indicated that the characteristics of lentil protein were modified by the thermal processes, with the extent of these modifications being influenced by the treatment temperature, electric field intensity, and pH during protein fraction extraction. Lentil protein extracted at pH 9 showed greater stability to temperature variations during ohmic heating. Under low electric field conditions (5 V/cm), there was an increase in protein surface hydrophobicity and accessibility to sulfhydryl groups. In contrast, lentil protein extracted at pH 7 was significantly affected by all thermal treatments. While some structural changes were observed in specific protein characteristics, no influences on protein–pectin interactions in lentil were identified.	[40]

Table 2. Key findings from scientific studies on the application of ohmic heating for the modification of food proteins.

Protein	Experimental Process	Ohmic Heating Treatment	Highlight	References
Soy protein isolate	They assessed the effects of ohmic and conventional heating on structural, thermal, and physical changes in soy protein isolate edible films.	The electrical frequency for treatment was 50 Hz, and the electric field intensities used for ohmic treatment were 3, 6, 9, and 12 V·cm−1, compared to conventional heating.	Ohmic-heated films exhibited reduced α-helix content, increased random coil content, smaller average particle size, lower fluorescence intensity, and higher surface hydrophobicity.	[36]
Sunflower protein isolate	They analyzed the structural, thermal, and physical changes in sunflower protein isolate compared to sodium caseinate when subjected to the application of non-thermal moderate electric field (MEF).	Electric field application: sample had 10 V/cm for 1 h (V1) and 10 V/cm for 2 h (V2) applied; sample had 150 V/cm for 5 s (V5) and 150 V/cm for 20 s (V20) applied.	Greater homogeneity was achieved with V20. The secondary and tertiary structures of sunflower protein changed primarily with V20, gaining a bulkier and more flexible structure and increased surface hydrophobicity. V5 had the lowest denaturation enthalpy. V1 also produced sodium caseinate solution with the lowest denaturation enthalpy.	[10]
Soy protein isolate	They studied the effect of ohmic heating on the structure and immunoreactive properties of soy proteins.	Ohmic heating studies were conducted at various frequencies, including 50 Hz, 500 Hz, 2 kHz, and 20 kHz. The electrode distance was consistently maintained at 30 mm for all assessed conditions. When necessary, the electric field intensities during the procedure ranged from 2 V/cm to 20 V/cm. Soybean trypsin inhibitor allergenic protein (STI), also known as Gly m TI, was selected as the allergen of interest due to its remarkable resistance to thermal treatments	Ohmic heating applications at frequencies ranging from 50 to 500 Hz can lead to conformational changes in soy protein fractions, as evidenced by the reduced fluorescence of aromatic amino acids. This effect may be directly associated with the formation of complexes with metals present in the protein solution during ohmic heating. However, there is a lack of detailed information on how electrical variables, such as electric field and frequency, affect soy protein fractions in this specific context.	[31]
Lactoferrin	The purpose of study was to create gel-like gelatin emulsions using lactoferrin dispersions heated through ohmic treatment. The impact of both ohmic and conventional heating on the protein’s secondary structure and the subsequent thermal aggregation of lactoferrin was assessed.	Ohmic heating was applied using a 4 cm electrode gap, with voltages ranging from 36 to 86 V. This treatment was compared to conventional heating. A double-walled glass reactor vessel was used, and a 20 mL lactoferrin dispersion was heated to 90 °C for 30 min for both treatments.	Both ohmic and conventional heating influenced the thermal unfolding and aggregation of lactoferrin molecules. Ohmic heating likely affected the molecular flexibility or the stability of lactoferrin’s hydrophobic groups. Ohmic heating resulted in fewer aggregated protein molecules compared to conventional heating. The difference in aggregation patterns was confirmed by a smaller increase in particle size, turbidity, intrinsic and extrinsic fluorescence, and a distinct dichroic signal, impacting the structural and mechanical properties of the prepared emulsions.	[37]
β—Lactoglobulin	They investigated the effect of moderate electric fields (MEF) during the ohmic heating of purified β-lactoglobulin fractions under different physicochemical conditions of pH and temperature.	The treatments were conducted at temperatures of 50, 60, 70, 80, and 90 °C for 10 min. The applied electric field intensity varied from 80 V/cm during the heating phase to 20 V/cm during the holding step. The electric frequency remained constant at 20 kHz, as the choice of frequencies within the kHz range was adopted to prevent unwanted electrochemical effects, such as electrolysis and electrode oxidation.	The results indicated that the effects of moderate electric field (MEF) are limited to unfolded protein conformations. This suggests that under the specific conditions tested, including electric field intensity, frequency, and exposure period, the imposed disturbances are not significant enough to impede the protein folding process. Thus, the effects of MEF appear to act synergistically with thermal effects. Furthermore, their specific influence and scope varied according to the pH, which may be related to the flexibility of β-lactoglobulin in response to environmental conditions. These findings support theories presented in other studies, which had already demonstrated that moderate electric field (MEF) effects impact the aggregation and gelation of serum proteins.	[38]
Whey protein isolate	They evaluated the formation of soluble whey protein aggregates from whey protein isolated under conditions of near-neutral pH, both in the presence and absence of a moderate electric field (MEF).	The applied voltage and temperature were controlled using a function generator connected to an amplifier system. During the heating and holding phases, the applied voltage ranged from 15 to 22 V/cm and 4 to 8 V/cm, respectively. The electrical frequency was 25 kHz, chosen to reduce electrochemical reactions at the electrode interface, minimizing the potential for corrosion and metal leakage into the medium. The results were compared to conventional heating.	The results indicated that the application of moderate electric fields impacts the unfolding and aggregation processes of whey protein at relatively high temperatures. After heating at 85 °C for 30 s, treatments with moderate electric fields resulted in whey protein isolate solutions with a higher amount of β-Lactoglobulin (8%) and α-Lactalbumin (10%) in their native form, compared to a traditional heating method. The protein aggregates in solutions treated with a moderate electric field increased by up to 78 nm, while conventional heating led to an 86 nm increase in the size of these aggregates. The aggregation of whey proteins under the influence of the moderate electric field was not intense enough to create an elastic gel network. This is due to reductions in the value of storage and loss moduli after treatment with the moderate electric field.	[39]
Lentil protein isolate	They assessed the impact of ohmic heating (temperature, electric field intensity, and frequency) on the structure of lentil protein obtained through alkaline extraction and protein–pectin interactions	The heating was conducted at temperatures of 40, 50, 60, 70, and 80 °C. After reaching the desired temperature, a waiting period of 30 s was observed. The voltages used for heating and maintaining a constant temperature were 5 and 75 V/cm, respectively. In the ohmic heating treatments, the initial phase involved the application of an electric field of 75 V/cm until the solution reached 80 °C. In the first ohmic treatment, the voltage was reduced to 5 V/cm, and the temperature was kept constant at 80 °C. In the second treatment (OH-75 V/cm), electric fields ranging from 10 to 20 V/cm were used while maintaining the temperature at 80 °C.	The results indicated that the characteristics of lentil protein were modified by the thermal processes, with the extent of these modifications being influenced by the treatment temperature, electric field intensity, and pH during protein fraction extraction. Lentil protein extracted at pH 9 showed greater stability to temperature variations during ohmic heating. Under low electric field conditions (5 V/cm), there was an increase in protein surface hydrophobicity and accessibility to sulfhydryl groups. In contrast, lentil protein extracted at pH 7 was significantly affected by all thermal treatments. While some structural changes were observed in specific protein characteristics, no influences on protein–pectin interactions in lentil were identified.	[40]

3.1. Ohmic Heating in Coagulated Products

This technology may be used for the thermal denaturation of proteins and posterior gel formation. The gelling properties of vegetable proteins are important for the development of dairy analogs, especially plant-based cheeses [41].

Wang et al. [34] applied one- or two-stage ohmic heating processing on soymilk to produce soft-tofu. The binomial temperature × time was an important factor to be evaluated, and better results were obtained with two-stage processing at 70 °C for 10 min and 100 °C for 5 min. The products showed increased yield, solid recovery, and reduced syneresis, which is important from the industry’s view. The authors discuss that 7S globulin subunits dissociate/associate selectively at the first heating stage. Then, in the second heating stage, the 7S subunits and/or newly dissociated 11S subunits associate cooperatively [34]. Pare et al. [42] observed that ohmic heating had a higher yield in tofu production compared to other heating methods (conventional and microwave) but presented lower textural properties (hardness, springiness, chewiness, cohesiveness, and gumminess).

The application of ohmic heating also contributed to potato protein gels by preserving the integrity of native protein when compared to conventional heating (evaluated using SDS-PAGE and gel permeation chromatography analyses), resulting in fewer hydrophobic interactions and gels with lower rigidity and water-holding capacity [43]. These findings emphasize the importance of these structural alterations as a determining factor in developing foods and products with specific characteristics, and the ohmic heated potato protein gels could be applied in products in which soft gels are desired. The results suggest the application of ohmic heating in soft plant-based cheeses, resulting in increased yields and improved texture properties.

3.2. Thermal Treatment Using Ohmic Heating to Improve Protein Stability and Protein–Molecule Interactions

Ohmic heating allows for more precise temperature control, promoting controlled protein denaturation. This probably can reduce the exposure of heat-sensitive regions or increase the surface hydrophobicity of proteins, which reduces interaction with water and, therefore, decreases heat sensitivity. Additionally, greater accessibility of sulfhydryl groups (-SH) in proteins can be favored, promoting the formation of disulfide bonds (-S-S) that stabilize the structure against heating. Exposure to the electric field can also induce the formation of aggregates, whose intermolecular interactions may strengthen the overall protein structure. In summary, these events, whether combined or not, enhance the thermal stability of proteins by reducing the exposure of heat-sensitive regions or facilitating the formation of more stable structures. Additionally, the structural modifications induced by heating allow for increased chemical interactions between proteins and other molecules such as carbohydrates and polyphenols, for example.

In a study conducted by Miranda et al. [40], the impacts of various thermal processes, including conventional and ohmic heating, on the structure of lentil proteins and their interactions with pectin were investigated. The results revealed that thermal processes could potentially remodel the structure of lentil protein, with the extent of these modifications being intrinsically linked to the treatment temperature, the applied electric field intensity, and the pH used in protein fraction extraction. Notably, protein extracted from lentils at pH 9 demonstrated remarkable stability to temperature variations during ohmic heating, suggesting potential industrial applications involving thermal variations. Additionally, applying a low-intensity electric field (5 V/cm) increased hydrophobic surface and accessibility to the protein’s sulfhydryl groups. These specific structural changes, including their possible influence on α-helix and beta sheet content, were significant evidence of the impact of ohmic heating. This technology exhibits considerable potential for modifying the structure of lentil proteins. However, it should be noted that results may be sensitive to the extraction method used, which can determine different structural arrangements in the native protein fraction.

The positive impact of this technology on protein denaturation may be used to improve the properties of protein-based films. Soybean protein isolate films have a high potential for food packaging applications and, it has been reported that the electrical field was an important processing parameter [44]. Soy protein isolate films modified by ohmic heating with added catechin were produced by Wang et al. [45] and, the pre-treatment with this technique made the soybean protein isolate structure more conducive to covalent bonding with catechin.

In general, ohmic heating can modify the technological properties of protein biofilms, such as improving tensile strength, elongation break, and crystallinity index and reducing water vapor permeability and solubility [46]. Therefore, the application of this technique may promote the utilization of plant-based proteins in films.

3.3. Effect on Chemical Bonds

Li et al. [35] evaluated the changes in the chemical bonds of soymilk proteins treated with ohmic heating. Detailed comparisons were made between ohmic heating and traditional resistance heating using different voltage levels (17, 23, 30, and 37 V/cm). The results showed significant changes in the properties of soymilk proteins subjected to ohmic heating, including a 14% increase in free amino groups, indicating modifications in chemical bonds. This technology can increase the number of free amino groups in proteins by unraveling polypeptide chains, thereby exposing previously hidden functional groups. The process can also cause partial hydrolysis of peptide bonds, releasing free amino groups. Additionally, the disaggregation of protein complexes, facilitated by uniform and rapid heating, can result in the release of subunits with accessible amino groups. Changes in local pH during heating may promote deamination reactions or other chemical transformations, further increasing the presence of free amino groups.

Still in the same study Li et al. [35], there was a decrease in the total content of sulfhydryls and surface hydrophobicity. These findings highlight the impact of ohmic heating on the chemical properties of proteins, suggesting that intramolecular bonds undergo substantial changes during the process.

Pereira et al. [31] expanded the understanding of ohmic heating by exploring its impact at different electrical frequencies (50 Hz–20 kHz) and moderate electric field intensities (up to 20 V/cm) on soy protein isolate (SPI). This study addressed issues such as electrode metal leakage and immunoreactivity. It was observed that the technique at 50 Hz and 95 °C led to significant changes in the intrinsic fluorescence of SPI and the release of detectable amounts of Fe/Ni. This phenomenon resulted in a 36% reduction in the immunoreactivity of Gly m TI, indicating an impact on chemical bonds and the interaction between proteins and trace metals. Thus, this research provides valuable insights into how process parameters, such as electrical frequency and electrochemical reactions, can influence the structure and immunoreactivity of SPI fractions, emphasizing the complexity of the physicochemical changes caused by the technique. It is important to note that the effect on food proteins can vary considerably depending on processing conditions, including temperature, exposure time, and protein concentration. Therefore, optimizing these parameters is essential to meet the specific needs of the final product.

3.4. Techno-Functional Properties

3.4.1. Water-Holding Capacity and Solubility

Food proteins’ water absorption capacity and solubility are fundamental characteristics that play a crucial role in food formulation and the texture of food products [47]. In the context of ohmic heating, when proteins undergo this process, remarkable transformations occur in their structure and functionality, significantly impacting their water absorption capacity. During ohmic heating, the heat leads to the denaturation of proteins and, as a result, food proteins become more predisposed to interact with water and absorb it more efficiently [18]. Additionally, ohmic heating can also favor breaking non-covalent bonds in proteins, making them more susceptible to interaction with water.

On the other hand, solubility refers to the ability of peptides or proteins to dissolve in a solution, balancing with other solid substances like salts under specific temperatures and pH conditions. This parameter plays a critical role in the food industry, affecting essential functional properties such as emulsification and foam formation, and provides insights into denaturation and peptide interactions. This property is crucial when using proteins and peptides as functional ingredients in food systems [48]. In this context, ohmic heating is a promising technique to modify the solubility of food proteins since it promotes structural modifications in these molecules, making them more susceptible to effectively interacting with water and other food components. As a result, solubility improves, enabling the formulation of more stable products.

3.4.2. Oil-Holding Capacity and Emulsification

Ohmic heating can impact the emulsifying properties of food proteins, although the effect depends on various factors, including the type of protein, heating conditions, and the emulsified system at hand. Here are some ways in which this technology can affect the emulsifying properties of food proteins: (i) changes in the protein’s net charge; (ii) alteration of the isoelectric point; (iii) protein denaturation; (iv) protein solubility; (v) modifications in protein–water interactions; and (vi) alterations in interfacial properties. These effects will be discussed below.

This technique can influence the net charge of proteins, as the charge is related to the medium’s pH. When these changes are close to the isoelectric point’s pH, proteins have lower solubility in water, which can affect the electrostatic interaction between proteins and oil molecules. Electrostatic repulsion and steric hindrance play an important role in maintaining the stability of oil droplets [49]. When proteins undergo ohmic heating, these chemical bonds can be cleaved, or new bonds can form, altering their conformation and interaction with oil droplets [35].

Ohmic heating generally involves directly applying heat to foods through high-frequency electric fields. This heat can result in protein denaturation, a change in their three-dimensional structure. When proteins denature, their emulsifying properties can be affected because the original protein structure stabilizes emulsions. Consequently, ohmic heating can also affect the interactions between proteins and water since changes in protein hydration properties may occur, affecting their emulsifying capacity [32].

The increased emulsifying capacity of a protein can be advantageous in products where stability, creamy texture, and uniformity are desired, contributing to a longer shelf life and enhanced sensory perception. However, for certain foods, high emulsifying capacity may negatively affect texture, making it denser, or hinder a desired phase separation. Additionally, high emulsification can encapsulate flavor molecules, reducing flavor intensity and altering the food’s sensory profile.

Ohmic heating alters the surface properties of proteins, affecting their ability to absorb oil. The formation of new functional groups on the protein’s surface or changes in its hydrophobicity can positively or negatively influence oil absorption [37]. Lima Becerra et al. [50] evaluated the modification of bean flours by ohmic heating and observed that flours treated with ohmic heating exhibited better emulsifying properties. The authors also mention that although the compact and rigid structure of globulins (the main protein fraction in beans) does not have good emulsifying/foaming properties, the results found indicate that samples processed by ohmic heating showed better properties, confirming the potential of this technology to improve protein techno-functional properties.

3.4.3. Gelling Properties

In recent years, there has been a significant increase in the development of protein-based food products, driven by the evolving demands of a new generation of consumers. Plant-based proteins play a crucial role in this context, as they have the potential to fulfill various functions, such as texturization and gelation, which are largely dependent on their protein structure [51,52]. It is important to note that the thermal treatment of protein-rich foods is a pivotal step in food processing, significantly influencing the textural properties of the final product [43].

Protein gels treated with ohmic heating exhibit distinguished gel properties, displaying a notably distinct texture compared to conventionally treated protein gels. These distinct properties hold great promise for the development of innovative approaches, especially concerning the creation of new food products and the exploration of promising applications. These findings suggest a promising avenue for further research, aiming for scientific and technological advancements [53]. Hydrophobic interactions and electrostatic forces are crucial for developing a three-dimensional protein network and, consequently, gel formation [54]. When combined, moderate electrical field and heat treatment (as in ohmic heating) lead to disturbances and rearrangements of hydrophobic residues and charge redistributions, affecting the establishment of a three-dimensional network [32].

Joeres et al. [43] in their comparative study investigating the impact of moderate electric fields on potato protein’s characteristics, isolated gels using both ohmic and conventional heating methods and identified significant variations in textural attributes, particularly gelation functionality and gel stiffness. Gels produced with the electric field technique are smoother and more flexible than those treated conventionally, supporting the authors’ conclusions. Furthermore, the results suggest that this method can effectively preserve proteins during thermal treatment, minimizing denaturation and making it suitable for the food industry. The formation of a less dense network, likely due to disruptions during protein aggregation and gelation from the electric field treatment, leads to more flexible gel structures due to reduced hydrophobic interactions in the protein matrix, resulting in observable impacts on gel textural properties [43].

The study by Joeres et al. [55] describes gels produced through ohmic heating as exhibiting fluid gel characteristics or having a pro-gel state texture. The authors emphasize that these findings can be relevant in developing innovative food products, especially concerning texture and the incorporation of solubilized nutraceutical components, whose accessibility can be optimized.

Additionally, it is noted that ohmic heating-treated gels provide a higher amount of native protein, which remains soluble in the water retained within the gel matrix. It is important to highlight that fluid gels exhibit smaller network domains, surrounded by a non-gelled phase, resulting in spread or poured gel properties [56]. These characteristics and insights provided by the study are valuable for understanding and enhancing protein gel systems under ohmic heating conditions. Other research efforts are dedicated to exploring this technology processes focusing on soy milk and other raw materials, aiming to induce coagulation for tofu production. These studies reveal a beneficial effect on the textural properties of the product [33]. Such research provides a solid foundation for understanding the textural alterations resulting from ohmic heating and its implications for tofu quality, contributing to the evolution of knowledge in food science.

4. Final Considerations

In conclusion, the electric field technique represents a promising and innovative approach for modifying plant protein structures. This method can potentially induce significant changes in proteins’ secondary structure and chemical bonds, ultimately affecting their functional and reactivity properties. The impact varies depending on temperature, pH, electric field intensity, and treatment duration. Notably, proteins extracted under specific pH conditions may exhibit greater stability during this process.

The alterations in protein structure induced by this technique can have broad implications for the food industry, particularly in developing plant-based protein products such as meat analogs, enzyme preparations, and food additives. However, optimizing process parameters is essential to harness the full potential of this method for these applications.

Overall, the research discussed in this article highlights the intricate interplay between the electric field technique and plant proteins. Understanding how this method affects the structural and functional properties of proteins is crucial for tailoring the characteristics of plant-based foods to meet specific industrial and consumer needs. Further investigations and advancements in this field hold great promise for developing innovative, structurally enhanced plant protein products.