1. Introduction
Pulsed electric fields are one of the non-thermal treatment technologies for different products or matrixes for gentle treatment within an electric field [
1,
2]. Typically, the products are placed in a treatment chamber consisting of two electrodes, and high-voltage pulses that are activated for very short periods (ns or ms) permeabilize the membranes of the treated material, resulting in reversible or irreversible cell opening. The applied voltage acts on the naturally occurring transmembrane voltage of cells, which is formed between the interior and exterior of the cells by ion gradients. As soon as a critical threshold value is reached, pores are formed in the membrane and cells are opened [
2,
3]
Thus, PEF treatment is a promising technology for enhancing mass transfer during the processing of foodstuffs and is associated with many benefits, as can be seen in juices or syrups of different origins and an associated improvement in physicochemical properties [
4,
5,
6] but without negative impact on the flavor profile [
7]. This technology has been shown to produce improved peelability in various vegetables, such as tomatoes, peaches, and oranges [
8], while the potato industry is investigating the use of PEFs to pre-treat potatoes for the production of French fries and the degradation of starch to improve their texture [
9]. Reductions in the microbial activity of products, such as blueberries, could be also achieved using PEFs [
10].
The application of PEF treatment is also used in wine production [
2,
11,
12,
13,
14,
15,
16,
17,
18]. Wine is a popular luxury food worldwide with current consumption volumes of 234 million hL in 2021 [
19]. Research has often focused on pretreating red grapes in particular with the aim of increasing their polyphenols, anthocyanins, or color intensity, which are some of the quality parameters of red wine [
11]. In addition, studies have also been conducted on volatiles at different stages of wine production [
12]. A reduction in maceration time [
13] is one positive result achieved by treating red grapes with PEFs. A method for early verification of the effectiveness of PEF treatment was also developed that highlighted the importance of selecting a responsive grape variety [
2,
13], as well as the harvesting time and pulse duration [
13]. Different grape varieties thus showed increases in anthocyanins, polyphenols, and color, with the greatest effect observed in Mazuelo grapes treated with a higher electric field strength of up to 10 kV/cm (compared with 2 kV/cm). Shorter processing times and lower energy consumption were also achieved [
2].
The influence of PEF treatment on wine sensory characteristics was also considered, as can be seen in [
12]. The sensorial aroma of Merlot grape musts examined by trained panelists using a descriptive analysis changed when grapes were treated with E > 40 kV/cm, developing a more intense blackcurrant flavor and odor [
12]. Analyses using headspace (HS) solid phase microextraction (SPME) coupled with gas chromatography (GC) and mass spectrometry (MS) (HS–SPME/GC–MS) also showed reductions in (E)-2-hexenal, the compound responsible for the green aroma, after samples had undergone PEF treatment. More than 40 compounds including terpenoids, alcohols, or carbonyls were identified in grape juice via SPME/GC–MS, with the floral and fruity aroma that was detected resulting from the identified esters and terpenes. Ethanol was also present with a high peak [
20].
In addition to the listed studies on PEF-treated red grapes, this work focused on Thompson Seedless grapes, a seedless white grape also known as Sultania. Thompson Seedless grapes are used as raisins, table grapes, and wine grapes, are available over a wide time period during the year, and a fraction of 14.5 percent of these grapes in the USA and a portion in Europe are used for the production of juices and wines [
21,
22]. Thompson Seedless was the leading raisin-type variety in California (USA) in 2021 and was utilized for raisins, fresh markets, concentrates, and wines [
23]. It is also registered in the European Union for its multipurpose use, including wine production, in Spain, Greece, Turkey, and Croatia [
24].
Up to now, numerous applications for pulsed electric fields have been investigated and the technology even reached a level where it was successfully implemented in industrial processes. As of now, only relatively small pieces of the puzzle can be added to the body of knowledge, and the fields of application need to be systematically broadened. The objectives of the present work were to show the effects of pulsed electric fields on commercially important Thompson Seedless grapes that are known for their multipurpose use. To the best of our knowledge, the application of PEFs on Thompson Seedless grapes has not been investigated before. Electric field strength was varied to understand the influences of PEF treatment on the physicochemical and sensory characteristics of the grapes and the extracted juice. The influence of PEF on white grapes and especially its effect on the sensory profile required for consumer acceptance, has been little studied to date [
14,
15,
16]. The aim of this study is to investigate the physicochemical properties of Thompson Seedless grapes, including any changes in color, pH, and texture as well as the content of sugar and total polyphenols. Another focus is the characterization of aroma profiles using instrument measurements (SPME/GC–MS) as well as human sensory tests, which can determine essential quality parameters that are ultimately of high relevance for consumers.
4. Discussion
The benefits of non-thermal treatment of red grapes with PEFs, which have already been demonstrated in several studies, stimulated this study to test the effect of PEF treatment on white grapes. Unlike the few studies dealing with PEF-treated white grapes and their limitation to sensory or physical changes, this study also compared the physical, chemical, and sensory parameters using instrument-based and human perceptions of Thompson Seedless grapes. This commercially important grape variety is known for its multipurpose use (fresh market, raisins, concentrate, and wine) [
21,
22,
23,
24]. For this purpose, grapes were treated with different field strengths and the resultant specific energy inputs and then analyzed for changes in pH, texture, color, TPI, and reducing sugars. Human and instrumental sensory data complement these results.
Previous studies investigating PEF treatment on different juices of longan [
40], grapefruits [
41], carrots, or oranges [
4] demonstrated that non-thermal treatment does not affect all parameters, such as pH or °Bx. Using a treatment of 20 kV/cm and 600 µs for grapefruit yielded a pH of 4.54 and 10.7 °Bx for the treated juice compared with a pH of 4.51 and 10.0 °Bx for the control. The values observed by Rivas et al. in 2006 [
4] also yielded only slight variations between a pH of 3.83 (9.5 °Bx) for the control juice and a pH of 3.86 (10.2 °Bx) for the PEF-treated juice (25 kV/cm with a maximum temperature of 68 °C). Small differences were partly confirmed in this study, as significant differences were found between the control and treated samples but also between the different field strengths. While the field strengths in this study were 4–12 times lower, representing a significant difference from the study just mentioned, the degree of ripeness of the single grapes might also have been responsible for the variations in the treated samples. Options to investigate the degree of ripeness include NIR analysis as demonstrated by a study on Cabernet Franc grapes [
42]. On the other side, a significant increase in the scale of the processing of the grapes might help to overcome the heterogeneity in terms of different stages of ripeness within each batch, as was carried out by Fauster and co-workers (2020) [
14].
Significantly greater differences were observed in the visual appearance of the treated grapes regardless of the field strength. Color changes in the grape skin were noted within a few minutes, evidenced by increasing a* and b* values, but there were no changes in brightness (L*) over 60 min. Grimi et al. (2011) [
43] detected similar browning behavior in PEF-treated and untreated apple slices. They saw significantly increased browning in the treated slices over a period of 90 min due to tissue damage and the corresponding contact with air. In contrast, the control variant had a lesser visible browning effect and thus corresponded to the results for the control grapes of our study. While the treated grapes are damaged by short pulses and thus come into contact with air, the grape skin darkens within a few minutes over a period of up to 60 min. This could be diminished by the direct use of antioxidants during the PEF treatment. But the question arises whether the diffusion of antioxidants into the whole grapes is sufficient. In this regard, PEF treatment itself promotes rapid browning of the tissue, which is why the color change is most pronounced immediately after treatment. Furthermore, the study of apple slices revealed that the duration of contact with air is not relevant for the browning intensity of apple slices and is thus consistent with the visual appearance of the grape skin treated with different PEF intensities in this study [
43]. On the other side, the brownish coloration and the increased mass transfer after PEF treatment are beneficial in the case of raisin production [
44]. A study showed that a drying time reduction of 20% was possible for raisin production after PEF treatment [
45].
Color changes in the form of a higher red intensity were seen in PEF-treated tomato juice when treated with 40 kV/cm for a short time [
46]. In addition, the decline in the red tone was less for the treated juice (3.75 to about 3.60; control 3.60 to about 3.20) compared with the control after 4 months of storage (4 °C). In contrast, grapefruit juices did not show any changes for treated samples when L*, a*, and b* values were measured [
41].
Regarding the influence on the juice yield, a previous study that focused on PEF-treated white grapes showed an increase in the juice yield of up to 26% to an overall yield of about 81% with settings of E = 0.75 kV/cm and 100 pulses [
15]. Depending on the variety, yields of 51% to 56% were obtained for grapes without PEF treatments. This is also consistent with the yield obtained for the controls in this study (55%). However, the treated samples in our study showed slightly lower yields with values between 50 and 54% but were not significantly different. Similar results were obtained by Fauster and co-workers [
14]. It is possible that the different yields that have been used in different studies can be explained by the different pressing times and technologies applied. While the grapes were pressed with a common kitchen press and the grape marc was pressed three times in total, Praporscic et al. (2007) [
15] pressed grapes for 45 min with the combined PEF chamber. Therefore, the grapes should be pressed for a longer period of time. Easier and higher juice discharge, due to the facilitated cell opening, was also shown by El Kantar et al. (2018) [
5]. A comparison between controls and treated whole fruits showed increases for lemons (39% to 63%), pomelos (54% to 74%), and oranges (48% to 60%) using field strengths similar to this study ranging from 3 to 10 kV/cm. These values were similar to the increases observed in white grapes from previous research [
15]. Increased juice yields were also measured in wild blueberries when treated with field strengths of 1, 3, and 5 kV/cm and a constant pulse number [
47]. A higher extraction yield was explained by the higher water content of grapes and that other vegetables and fruits have tougher tissues [
37]. Bobinaitė et al. (2015) [
47] also mentioned that their most intense field strength of 5 kV/cm did not show the highest extraction yield and cited the work of Jaeger et al. (2012) [
48] by way of explanation: excessively high field strength can lead to compression and closing of the capillaries in the press cake, resulting in an unfavorable effect on the de-stacking.
A consequence of this is also the softening of the texture of treated products, which was already measured in the grapes 5 min after PEF treatment. Significant reductions in the crunchiness (up to 50%) and firmness (up to 45%) were observed between field strengths of 2 to 6 kV/cm compared with the control. The effect of a reduction in firmness by more than 70% was also observed in blueberries treated with a combination of sanitizing solution and PEF treatment of 2 kV/cm for 2, 4, and 6 min [
10]. This reduction could be a response to the cell membrane breakdown, which could also be responsible for the softening of grape texture. However, the greater loss of texture in blueberries could also be due to the treatment time, as they were treated for a few minutes at 100 pulses per second. The grapes, on the other hand, were treated with 50 pulses for just a few seconds [
10].
Reduced firmness of fruits could also negatively affect the products’ sensory quality and, therefore, PEF treatment should only be used for fruits that require a softer texture for processing [
10]. Analyzing the odor of the treated grapes from this study did not reveal any significant sensory differences using the two short-term measurements CATA and RATA. Accordingly, the varying degrees of PEF treatment had no negative effect on the odor of the grape juices, which was also seen in the sensory similarity between treated and untreated carrot and orange juices compared with heat-pasteurized juices [
4]. A possible further analysis would be to check the acceptance of treated and untreated samples using a nine-point scale such as that used by Min and Zhang (2003) [
46] with 30 trained panelists. Tomato juice treated with PEF (40 kV/cm; 45 °C for 90 s) achieved a rating of 6.1 for flavor and 6.4 for color, while juice that had undergone heat treatment (92 °C for 90 s) only achieved ratings of 4.7 for flavor, while color was rated 6.1. Siddeeg et al. (2019) [
49] could not find significant differences in vinegar from date palm fruits treated with PEF, ultrasound, or a combination of both compared with untreated vinegar. Trained panelists also ranked the samples with mean values of 6.90 for taste and color and 7.80 for flavor. Significant results could only be seen in the overall acceptability with a ranking of 8.10 for PEF-treated vinegar while the control was rated slightly lower at 6.00 (nine-point scale). The sensory profile is often also determined via GC–MS as seen in a study with Aglianico wine [
11]. With their aim of identifying a possible negative aroma when grapes were treated with PEFs, field strengths of 1.5 kV/cm did not significantly affect volatiles for the most part but led to a few variations in some compounds compared with the control. For example, the composition of octanoic and hexanoic acid, which is considered negative in the literature, was higher in their treated wine. These findings cannot be confirmed in the white grapes in this study. Despite the fact that only the six most conspicuous peaks were examined more closely in the current study, almost no hits for octanoic or hexanoic acid were detected in the raw data of the replicates. One reason for the failure to detect the two acids is possibly the presence of the final product because this study uses only grape juice and thus a precursor of wine. Octanoic and hexanoic acid could be detected in this study in a total of two replicates of the treated samples but only in a very low abundance. This could be related to the degree of ripeness of the grapes. Ripeness could be successfully verified using SPME/MS in a study by Sánchez-Palomo et al. (2005) [
50] with optimization of the process using additional verification with different HS–SPME fibers. Furthermore, the aroma profile of Muscat grapes identified 16 compounds, with linalool, geraniol, and nerol being the most important volatile compounds. Since these compounds were partially determined in this work but did not reach high peak areas, fiber changes should be considered for further analysis. A systematic analysis of the aroma profile of wines from two white grapes (Grüner Veltliner, and Traminer) was conducted by Fauster and co-workers [
14]. The aim was to investigate the combined application of an enzymatic treatment and a PEF treatment to white wine mash. In this work, nine different terpenes and 18 esters were identified and quantified. Especially, the concentration of volatile esters of the variety Traminer increased significantly after PEF/enzymatic treatment [
14].
The greatest differences between treated and untreated white grapes were seen in the analytical measurements of TPI and reducing sugars. The increases in TPI of about 20% to 30% were seen at the higher treatment strengths of 4.5 and 6 kV/cm and are consistent with the results for red wine [
11]. Field strengths between 0.5 and 1.5 kV/cm with specific energy inputs between 1–50 kJ/kg resulted in an increase in total polyphenols of up to 38% (1.5 kV/cm, 25 kJ/kg) compared with the control. A higher quantity of total polyphenols derived from the PEF treatment could improve the antioxidant properties and maintain acidity or sugar content [
11]. Unfortunately, measurements by these authors on a second grape variety could not achieve this increase in polyphenols, meaning that the grape variety seems to be important for the success of the PEF treatment. At a lower electric field strength of 0.4 to 2.0 kV/cm, the quantity of polyphenols increased (44.61%) when tomatoes were examined [
51]. Higher total polyphenols were also seen in trials with oranges, pomelos, and lemons using treatment strengths of 3 and 10 kV/cm [
5] as well as in wild blueberries, with increases of 43%, where the electric field strength was varied between 1, 3, and 5 kV/cm [
47]. An increase in specific polyphenols was also described in the wine from Grüner Veltliner and Traminer grapes after a combination of PEF and enzymatic treatment [
14].
For the reducing sugars, an electric field strength of E = 4.5 kV/cm also showed significant increases of more than 11% to a total quantity of 220.97 g/L compared with control grapes. The other field strengths also resulted in higher reducing sugar quantities but were not significantly different from this value. This was also confirmed for PEF-treated musts from white grapes with mean values of 182 g/L for the control and 179 to 182 g/L from treated grapes [
16]. In particular, the control values for reducing sugars (182 g/L) of [
16] were in a similar range to the results of this study (199 g/L), whereas their PEF-treated musts differed with almost unchanged values of reducing sugars compared with the obtained increases from this study. Measurements of reducing sugars in wine after storage for 3 months and bottle aging also revealed no significant differences in reducing sugar levels, with values ranging from 1.50 g/L (control) to 1.45 g/L (PEF-treated Cabernet Sauvignon wine) [
18]. Also, in the work of Donsi et al. (2010), no significant increase in reducing sugars was detectable in the wine of PEF-treated Aglianico grapes [
11].