1. Introduction
Pulsed electric field (PEF) is a non-thermal technique that relies on the application of high-intensity electrical pulses for short duration times to biological tissues placed between two electrodes [
1]. Albeit this technology was first adopted in the food industry about 50 years ago, PEF is still considered an emerging technology and is increasingly raising the interest of the scientific community due to its sustainability, environmental friendliness, and recent advancements in industrial applications [
2,
3]. PEF is applied as a single treatment or in combination with other technologies to synergistically enhance product quality, microbial stability, and process yields [
4,
5]. The abovementioned goals can be achieved through the modulation of several process parameters (e.g., electric field strength, number of pulses, time of treatment, etc.) leading to the electrical breakdown of cell membranes (i.e., electroporation) and resulting in the creation of pores, which work as conductive channels [
6,
7].
Although in the past 50 years several mechanisms about pore formation have been supposed [
8,
9,
10], recent advancements in the field highlighted that the main mechanism of electroporation involves the development of aqueous pores in the lipid bilayer of the cell membrane, along with variations to individual membrane lipids and proteins, as suggested by Kotnik et al. [
11]. In more detail, the same authors reported that pulsed electric fields trigger peroxidation to the membrane lipids, which causes deformations at the level of the lipid tails, thus increasing the permeability of the bilayer to water, ions, and other small molecules [
12]. Depending on the electric field strength, the electroporation process can be either reversible or irreversible [
13]. In the first case, the cell might be damaged but still able to fully recover, while in the second the membrane integrity is lost, causing an extensive leakage of intracellular content and, eventually, cell death [
6,
14]. Based on the reversibility of the process, PEF technology can have different applications in the food industry. Irreversible electroporation at high field strengths (e.g., 10–50 kV/cm) is particularly studied as an alternative to traditional thermal processing for microbial inactivation, with the advantage of minimally altering sensorial and nutritional characteristics of foods [
1,
14,
15]. On the contrary, irreversible electroporation at lower field strength values (0.5–5 kV/cm), might be fruitfully exploited to enhance mass transfers during further technological processes such as drying, freezing, freeze-drying, and osmotic dehydration [
8,
16,
17,
18].
Having this in mind, the attraction towards the application of PEF in the meat industry has increased in recent years, as evidence grows on its ability to induce microstructural changes that might lead to the improvement of meat functional properties [
19]. However, based on the available literature, a remarkable number of studies concerning the application of pulsed electric fields on pork and beef has been carried out over the past 20 years, while studies regarding the effect of PEF on poultry meat are still scarce. According to the last reports of the Organization for Economic Co-operation and Development (OECD), poultry is the most widely eaten type of meat worldwide [
20]. With growing trends in further processing and increasing rates of broiler breast muscle abnormalities [
21], meat technological properties (e.g., water holding capacity, emulsifying, and gelling properties, etc.) are gaining progressively more importance and the application of innovative technologies is actually considered as the most promising option for the improvement of such properties in poultry processing plants [
1].
Considering these aspects, the knowledge concerning the feasibility of emerging technologies on chicken meat should be deepened. For this purpose, this study aimed at evaluating the implications of different PEF treatments on the main quality traits and the technological properties of chicken meat.
4. Discussion
The application of PEF is drawing the interest of the meat industry thanks to its ability to accelerate mass transfers during drying and brining, improve tenderization, enhance the micro diffusion of water-binding agents and increase meat safety [
4]. However, the utilization of PEF to improve the technological properties of chicken meat is still an unexplored field. Overall data evidenced that PEF treatments tested within the experiments do not affect meat pH, besides the number of pulses and the electric field strength applied. This result corroborates what was found by Khan et al. [
28], where the application of pulsed electric fields at both 0.30 and 1.25 kV/cm did not change the pH of chicken breast muscles. Similar results were also observed on beef [
22,
29]. However, the application of PEF with high total specific energy input (>150 kJ/kg) was found to be associated with a significant decrease in muscular pH values, due to a modification in meat conductivity caused by the leakage of intracellular liquids following cellular electroporation [
30]. Considering that the highest total energy input achieved within this study was 2.42 kJ/kg (
Table 1 and
Table 2), it is reasonable to assume that the treatments performed in the experiments were not strong enough to induce cell breakdown and thus the leakage of cellular fluids.
Only few studies are available in the literature concerning the effect of PEF on meat color, which is known to be the main factor leading customers’ purchasing choices. A recent study carried out in the U.S. reported that consumers have a clear preference towards lighter colored poultry meats over darker ones, especially for breast meat [
31]. The effect of PEF on meat color strongly depends upon the magnitude of the temperature increase during the treatment, which might alter the redox state of myoglobin [
19]. Indeed, high-intensity treatments coupled with elevated numbers of repeats may increase samples’ temperature, thus promoting myoglobin oxidation and meat discoloration [
5]. Data from the available literature showed that PEF treatments with high total specific energy input (>50 kJ/kg) caused a significant decrease of meat lightness [
32], while milder settings did not affect color parameters of both beef and turkey meats [
33,
34]. Although low-intensity treatments (<5 kJ/kg) were performed within this study, PEF slightly changed meat lightness and yellowness, while redness was not affected. However, the trends observed for meat color variations were controversial between the two experiments and might be complex to interpret. Indeed, while in the first experiment a lower electric field strength and number of pulses were associated with an increase in meat lightness, in the second trial they were linked to a decrease in both lightness and yellowness of samples. Considering the low total specific energy input of the treatments (see
Table 1 and
Table 2) and the poor content of myoglobin in chicken meat [
35], it is reasonable to assume that color variations were not likely due to an alteration of meat pigments caused by an increase in sample temperature during the process, but it might be linked to a possible redistribution of water within cellular compartments after the PEF application. Indeed, PEF might have favored the movement of water within cellular spaces, thus leading to a change in the refractive properties of the tissue [
36]. Albeit further research should be performed to test this hypothesis, it is noteworthy to mention that meat color variations induced by PEF treatments in this study were negligible and probably not detectable by the human eye.
In muscular tissue, water is distributed in several compartments and can be present either as bound or free water: the first is tightly fastened to meat proteins, while the second is held between myofilaments and myofibrils, as well as outside the fibers [
37]. There is growing evidence concerning the ability of PEF to modify the microstructure of meat through the formation of pores, thus suggesting its potential to influence the water holding capacity of meat by facilitating water movements [
5]. Overall outcomes of this study indicated that low-intensity PEF treatments can significantly improve water holding capacity in chicken breast fillets by reducing the loss of liquids from meat from 13% up to 28.5% during 4 days of refrigerated storage. Moreover, in both the experiments, doubling the number of pulses reduced drip losses to a greater extent if compared to increasing the electric field strength. There are conflicting reports in the literature concerning the role of PEF on the water holding properties of meat; the divergences found among the studies are ascribable to the intrinsic properties of the muscle (i.e., amount of fat and connective tissue, pH, fiber diameter, muscle contractile state), the dimension and the initial moisture of the samples as well as PEF processing conditions (i.e., total specific energy input, number of pulses, frequency, etc.) [
19]. It is generally believed that high-intensity PEF treatments leading to irreversible electroporation (i.e., irreversible electrical breakdown of cell membranes) are associated with an increase of drip loss from meat due to a number of mechanisms including protein denaturation, myofibril fragmentation as well as cell rupture and leakage of cell fluids into extracellular spaces [
38]. Considering the utilization of low total specific energy inputs and short exposure times, it can be hypothesized that PEF treatments performed within this study have induced reversible cell electroporation, meaning that cell membranes momently modified their permeability without loosening their integrity [
4,
7]. Within this scenario, the reasons behind the remarkably reduced drip losses of meat following low-intensity PEF treatment might be different. The first hypothesis deals with a possible re-compartmentalization of moisture following cellular electroporation that might have facilitated water movements within extra- and intracellular compartments [
1,
5]. Indeed, the exposure of skeletal muscle tissue to a sufficiently high external electric field likely caused a rapid increase in membrane permeability (i.e., membrane electroporation) due to the formation of temporary pores in the phospholipid bilayer of the cell membranes [
39]. The abovementioned pores are defined as “aqueous” since they are particularly hydrophilic; indeed, the application of the electric field causes the exposure of the polar heads of the membrane’s phospholipids allowing greater interaction with the water molecules [
7]. Most theoretical works on electroporation suggest that following a period ranging from milliseconds up to few minutes after the field is removed, the pores reseal [
39]. Given these compulsory details, it could be hypothesized a water re-compartmentalization following the temporary changes in membrane permeability that might have favored the transition of water from extra- to intramyofibrillar spaces within skeletal muscle tissue. Thus, water molecules penetrated into the lipid bilayer might be trapped into the pores, thus leading to a reduced water loss from meat. The second hypothesis is related to the potential of PEF to change the polarity of amino acids’ side chains, responsible for their hydrophilic or hydrophobic behavior [
40]. In 1999, Yeom et al. [
41] put forward the theory that high intensity PEF treatments are able to modify secondary and tertiary structures of proteins by increasing the content of β-sheets to the detriment of α-helices. This theory was further validated in 2008 by Zhao and Yang [
3], who suggested that, as a direct consequence of proteins’ conformational changes, PEF might influence proteins’ hydrophobicity. Later on, the ability of PEF to break covalent bonds and generate new sorts of interactions within the peptide chains was reported by Poojary et al. [
15]. Within this context, we could hypothesize that low intensity PEF treatments performed within this study may have induced proteins’ conformational changes which likely caused a modification of the attraction/repulsion interactions between polar and apolar amino acids, thereby enhancing the interactions between proteins and water molecules. However, based on the available literature and considering the different PEF intensity applied, it is not possible to recognize which of the two abovementioned mechanisms, taken individually or jointly, is responsible for the increased water holding capacity of meat following PEF. Thus, further studies must be carried out to deeply investigate water distribution in the muscle after the application of PEF. From a technological point of view, the potential of PEF to reduce water loss from meat during refrigerated storage might reduce the presence of undesired liquid inside the meat packages, improving consumer acceptance.
Within this experiment, the application of PEF did not exert any effect on the solubility of chicken meat protein fractions. Results in the literature concerning the role of PEF on muscular protein solubility are lacking, however, those available highlighted that the functional properties of proteins are drastically impaired as the intensity of PEF and the treatment time increase [
42]. Accordingly, a recent study performed on proteins isolated from pale, soft, and exudative (PSE) chicken meat reported that the solubility of the myofibrillar fraction improved with the application of 18 kV/cm, while a further increase in the electric field strength was associated to a worsening of protein functionality, due to the occurrence of protein denaturation and aggregation [
43]. Thus, since myofibrillar proteins are of great importance for meat technological properties (e.g., water holding capacity) [
44], it is essential to modulate process parameters in order to avoid detrimental effects on meat proteins and their ability to hold water molecules. Considering the low electric field intensities applied within this study (<2.5 kV/cm), it might be reasonable that PEF did not negatively affect proteins solubility, thus preserving their functional properties. Moreover, DSC technique allowed to evidence that the application of PEF did not trigger protein denaturation processes, thus corroborating the outcomes concerning protein solubility which is generally considered as an indicator for protein denaturation level. However, considering the explorative approach of this study, further research should be carried out to validate these results and investigate the effective potential of this emerging technology to be applied in the poultry field.