Pulsed Electric Fields as an Infrared–Convective Drying Pretreatment: Effect on Drying Course, Color, and Chemical Properties of Apple Tissue

Abstract

In this research, the impact of the pulsed electric fields (PEF) pretreatment on the infrared–convective drying (IR-CD) of apples and selected quality indicators (antioxidant capacity, color, and total phenolic content) of dried apples was evaluated. PEF pretreatment was carried out at the following parameters: electric field strength of 1 kV/cm, specific energy inputs of 1, 3.5, and 6 kJ/kg. Moreover, variable IR-CD process parameters were also assessed (peak wavelength of 1.2 µm, distance between the infrared lamps and the apple slices of 10, 20, and 30 cm). PEF pretreatment, by disintegrating apple cells, reduced the drying time of that fruit by 11–20%. The IR-CD of apples was the most effective at the shortest distance (10 cm) between the infrared source and the apple slices; it was associated with the intense heating of its surface. PEF-pretreated samples exhibited lower retention of antioxidants and were darker and redder than untreated ones. PEF could increase the activity of enzymes responsible for oxidizing phenolic compounds to brown pigments. The use of a medium distance (20 cm) during IR-CD promoted the highest retention of antioxidant compounds (relatively prompt drying with moderate heating of the sample surface).

1. Introduction

Drying involves the movement of heat from its source to a given material in order to remove a specific amount of water from said material. This can be achieved solely by radiation, convection, or conduction, as well as by using two of the aforementioned methods at the same time, or all of them simultaneously. As a result, a product with reduced water content is obtained, which reduces the risk of moisture-mediated spoiling reactions [1,2,3].

In infrared drying (IR), infrared radiation energy (as electromagnetic wave) is delivered directly to the surface of the material being dried without heating the surrounding air. This energy is absorbed mainly by the surface layers of the material, but depending on the properties of the matrix and IR wavelength, it can penetrate to a small depth. The infrared wavelength radiation produced by the source interacts with the internal structure of the material, which undergoes the drying process. An increase in the material’s surface temperature increases the vapor pressure, which intensifies the diffusion of water molecules from deeper layers of the material towards its surface. The induced molecular excitations provide rapid heating of the material, which, in turn, increases mass transfer. Compared to conventional convective drying (CD), also called hot-air drying (HAD) because of the use of hot air as the drying medium, infrared drying is generally characterized by higher drying efficiency, which results from a higher energy transfer rate. In addition, reduced energy consumption, more efficient transmission through the air (less energy waste), and better product quality should also be acknowledged [1,2,3,4,5,6,7,8,9].

Combining both techniques as a hybrid method (IR and CD/HAD) appears to be superior to other methods due to its synergistic effect. The surface of the material being dried absorbs infrared radiation, which, in turn, results in an increase in its temperature. Subsequently, there is an intense movement of water molecules towards the surface, from where they are evaporated by a flow of air (which additionally reduces the temperature of the material’s surface, simultaneously reducing the risk of burning it) [8]. Regarding this hybrid method, it is worth considering using a relatively slow air flow to reduce the risk of a cooling effect [5]. It is also necessary to take into account the risk of thermal degradation of the material in the event of incorrect selection of the radiation intensity or the distance between the infrared source and the material being dried [4,6,10].

Both the course of infrared–convective drying and the properties of the dried product obtained in this way can be adjusted by appropriately modifying the tissue at the pretreatment stage. So far, among the nonthermal pretreatments, mainly the effect of ultrasound (US) on IR-CD of matrices such as blackberry [11], kiwi [12], strawberry [13], and scallion stalk [14] has been studied. The results are promising: shortening of the drying time of strawberries; lightening of their color and increasing their redness; higher retention of anthocyanins and total phenols [13]; increased drying efficiency in the case of kiwi [12]; and reducing the color difference in relation to both fresh blackberry [11] and scallion stalk (additionally shortening the drying time of this material) [14]. Loosening of the apple tissue, subjected to IR-CD via electroporation, the primary mechanism of pulsed electric fields (PEF), has not yet been comprehensively analyzed. This is another promising nonthermal pretreatment that, unlike US, which creates microscopic channels in solid-like matrices [13,15], generates pores in the cell membrane [16,17].

Despite the many benefits of assisting convective drying with infrared radiation, there are many process parameters whose incorrect settings may cause undesirable effects. Therefore, the purpose of this study was to evaluate the effect of pulsed electric fields pretreatment and infrared–convective drying parameters on the drying course, color, and chemical properties of apple tissue.

2. Materials and Methods

2.1. Materials

Meticulously selected apples v. Golden Delicious (the Experimental Fields of the Department of Fruit Growing of the Warsaw University of Life Sciences, Warsaw, Poland) were chosen for the experiment. After being taken out of cold storage (approximately 5 °C), apples were left on the countertop until they reached room temperature (approximately 20 °C). They were then rinsed with tap water and proceeded to further processing.

All chemical reagents required to perform chemical analyses (total phenolic content, antioxidant capacity) were purchased from Sigma Aldrich (Saint Louis, MO, USA).

2.2. Pulsed Electric Fields Pretreatment

The procedure of the pulsed electric fields (PEF) treatment required the use of the PEFPilot™ Dual System (Elea Technology GmbH, Quakenbrück, Germany). This process was carried out in a batch system, which means that one uncut apple and tap water (conductivity and temperature equaled approximately 718 μS/cm and 21 °C, respectively) were put into the PEF treatment chamber (capacity of 2000 cm³, fixed distance between two parallel electrodes of 24 cm) to reach approximately 1.5 kg of total input. Before supplying monopolar, rectangular pulses, electric field strength, electrode voltage, pulse frequency, and pulse width were set to 1 kV/cm, 24 kV, 20 Hz, and 7 μs, respectively. The number of supplied pulses depended on the specific energy input (1, 3.5, and 6 kJ/kg), and it was calculated as shown below:

$${\mathrm{W}}_{\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{c}}={\displaystyle \frac{\mathrm{I}\mathrm{U}\mathrm{t}\mathrm{n}}{\mathrm{m}}}$$

(1)

where W_spec refers to the specific energy input [kJ/kg], I means the current [A], U indicates the electrode voltage [kV], t stands for the pulse width [s], n represents the number of supplied pulses [-], and m corresponds to the total input of the treatment chamber [kg].

The cell disintegration index (CDI) was used to assess the efficacy of the pulsed electric fields treatment. It was calculated on the basis of electrical conductivity measurements (pH/conductivity meter CPC-401, Elmetron, Zabrze, Poland) in accordance with Equation (2). Supplying 1, 3.5, and 6 kJ/kg of energy to the treated apples resulted in CDIs of approximately 0.1, 0.2, and 0.4, respectively.

$$\mathrm{C}\mathrm{D}\mathrm{I}={\displaystyle \frac{\mathsf{\sigma }-{\mathsf{\sigma }}_{\mathrm{i}}}{{\mathsf{\sigma }}_{\mathrm{d}}-{\mathsf{\sigma }}_{\mathrm{i}}}}$$

(2)

where σ refers to the electrical conductivity of the PEF-treated (W_spec = 1 kJ/kg, W_spec = 3.5 kJ/kg, and W_spec = 6 kJ/kg) apple [µS/cm], σ_i indicates the electrical conductivity of intact (untreated) apple [µS/cm], and σ_d represents the electrical conductivity of totally destroyed (W_spec = 63 kJ/kg) apple [µS/cm].

2.3. Infrared–Convective Drying

In order to perform the drying process, PEF-pretreated and untreated apples needed to be comminuted. For that purpose, whole fruits were sliced into 0.5 cm thick pieces. Parts of the apple other than the flesh and skin were removed with a cork borer; then, slices were cut into quarters. Approximately 0.127 kg of apple pieces were organized into a single layer on the drying tray.

Infrared–convective drying, or infrared-assisted convective drying (IR-CD), was accomplished using a laboratory dryer (Warsaw, Poland). That dryer was supplied with nine lamps (infrared radiation source), organized in three rows, with a total power of 7.875·10⁻⁴ kW/cm², and a peak wavelength of 1.2 µm (NIR—near-infrared radiation [1,10]). Air flowed parallel to the material being dried with a velocity of 50 cm/s. The distance between the infrared lamps and the apple slices was variable, and took values of 10, 20, and 30 cm depending on the process variant. Each drying variant was executed three times. The load of the drying tray was equal to approximately 6.3·10⁻⁵ kg/cm². Records of the material weight every 5 min of the process allowed us to estimate the drying time, i.e., the time required to obtain a moisture ratio (MR) equal to 0.02, calculated as shown below:

$$\mathrm{M}\mathrm{R}={\displaystyle \frac{{\mathrm{M}}_{\mathrm{t}}}{{\mathrm{M}}_0}}$$

(3)

where M_t means the moisture content in apple during infrared–convective drying [g H₂O/g dry matter], and M₀ stands for the moisture content in fresh apple [g H₂O/g dry matter].

2.4. The Analysis of Dry Matter Content

Stabilization of the dried apples lasted one week, during which they were kept in tightly sealed foil pouch packaging (PET/AL/PE), impermeable to external gas, light, and vapor.

After that, all materials were subjected to analyses. First, dry matter (d.m.) content was determined following the protocol AOAC 920.15 [18]. That gravimetric method was performed in three repetitions for each sample. The moisture content in a fresh apple equaled 5.90 ± 0.04 g H₂O/g d.m. (85.5 ± 0.1%), while for all dried samples, it was in the range of 0.07–0.10 g H₂O/g d.m. (6.2–8.9%).

2.5. The Analysis of Color

The CR-5 colorimeter (Konica-Minolta, Tokyo, Japan) was used to assess the color of both fresh apple and all dried samples. It was described in the CIELab color space and was measured by the reflectance method, using the aperture of 0.3 cm in diameter, illuminant D65, and the CIE 2° standard observer. Ten replicates were carried out for each sample. Based on the recorded L*, a*, and b* color parameters, the total color difference (ΔE) in relation to fresh apple and browning index (BI) were determined through the following calculations:

$$\Delta \mathrm{E}=\sqrt{{\left(\Delta \mathrm{L}\mathrm{*}\right)}^2+{\left(\Delta \mathrm{a}\mathrm{*}\right)}^2+{\left(\Delta \mathrm{b}\mathrm{*}\right)}^2}$$

(4)

$$\mathrm{B}\mathrm{I}={\displaystyle \frac{\left[100\left(\mathrm{x}-0.31\right)\right]}{0.17}}$$

(5)

$$\mathrm{x}={\displaystyle \frac{(\mathrm{a}\mathrm{*}+1.75\mathrm{L}\mathrm{*})}{5.645\mathrm{L}\mathrm{*}+\mathrm{a}\mathrm{*}-3.012\mathrm{b}\mathrm{*}}}$$

(6)

where ΔL*, Δa*, and Δb* refer to the differences in the values of the L*, a*, and b* color parameters between dried and fresh apples [−], respectively.

2.6. The Analysis of Total Phenolic Content

The first stage of chemical analyses was the extraction of particular compounds from the apple tissue. The whole procedure of preparing ethanolic extracts has already been described in detail [19]. The extraction was carried out twice for each sample, and the obtained extracts were used in all chemical analyses.

Total phenolic content (TPC) in the fresh and dried apples was evaluated on the basis of the spectrophotometric method proposed by Singleton et al. [20], with the use of gallic acid (GAE) as a standard. Both the amount and order of added reagents (distilled water, prepared ethanolic extracts, Folin and Ciocalteu’s phenol reagent, supersaturated sodium carbonate solution) and the reaction time have already been described by Matys et al. [19]. At the final stage of this analysis, the absorbance of prepared solutions needed to be verified (in relation to the blank sample, with the lack of the ethanolic extract) at the wavelength of 750 nm (Thermo Electron Corporation Helios Gamma Spectrophotometer, Thermo Electron Corporation, Waltham, MA, USA). Each sample was analyzed in four repetitions.

2.7. The Analysis of Antioxidant Capacity

Antioxidant capacity analyses (ABTS and DPPH assays) were performed on the same ethanolic extracts as in the TPC analysis. The aim of those determinations was to assess the scavenging degrees of the ABTS^•+ and DPPH^• free radicals by chemical compounds with antioxidant properties contained in the fresh and dried apples [21]. Both stock solutions and working solutions of ABTS^•+ and DPPH^• free radicals were prepared by strictly following the protocol given by Matys et al. [22]. Further procedures involving the addition of reagents in a specified amount and order (ethanolic extracts, ethanol–water solution, working solutions of ABTS^•+ and DPPH^• free radicals), as well as the reaction times were performed in accordance with the methodology described in detail by Matys et al. [19]. The last stage of those analyses was the measurement of the prepared solutions absorbances (in relation to the blank sample, with the lack of the ethanolic extract) at the wavelengths of 734 and 515 nm for ABTS and DPPH assays, respectively (Thermo Electron Corporation Helios Gamma Spectrophotometer, Thermo Electron Corporation, Waltham, MA, USA). Each sample was analyzed fourfold.

2.8. The Statistical Analysis

The experiment was organized using a two-way factorial randomized design with two numerical factors, each having three levels (factor 1: distance between the infrared lamps and the apple slices, 10, 20, and 30 cm; factor 2: specific energy input, 1, 3.5, and 6 kJ/kg). Assumptions for ANOVA, such as normality (Shapiro–Wilk test) and homogeneity of variance (Levene’s test) were checked. In order to compare the results of the analyses, a multi-way analysis of variance with Tukey’s test (Statistica, version 13, TIBCO Software Inc., Palo Alto, CA, USA) was implemented. Results of that analysis are presented in Figures as uppercase and lowercase letters. They show the impact of two independent variables (1: distance between the infrared lamps and the apple slices, represented by the uppercase letters; 2: specific energy input, indicated by the lowercase letters) on the analyzed dependent variables (drying time, L*, a*, b*, ΔE, BI, TPC, EC₅₀ ABTS, and EC₅₀ DPPH) (α = 0.05). Moreover, Pearson correlation analysis was utilized to assess the possible correlation between antioxidant capacity (measured via ABTS and DPPH assays, expressed as EC₅₀) and total phenolic content (TPC).

3. Results and Discussion

3.1. Drying Kinetics and Drying Times

The course of the infrared–convective drying (IR-CD) of apples with variable process parameters is presented in Figure 1. The obtained curves were characterized by a similar course; at the initial stage, a period of constant drying rate (linear decrease in material moisture) could be observed, followed by a period of decreasing drying rate (curvilinear decrease in material moisture). This is a phenomenon typical for drying this type of matrix, where the free surface water initially evaporates, followed by the water in deeper layers of the material, which is limited by its cellular structure (a barrier to the diffusion process) [23,24]. As can be seen, the shorter the distance between the infrared lamps and the apple slices, the faster the water removal. A shorter distance from the heat source means higher surface heating and, consequently, more intense water evaporation [9,25,26,27].

In order to thoroughly compare the differences in the efficiency of the water removal from apples depending on the IR-CD and PEF pretreatment parameters, Figure 2 shows the time required to dry the material to MR = 0.02. Drying time was significantly dependent on both the distance between the infrared lamps and the apple slices (η² = 0.862) and the specific energy input (η² = 0.864). According to the drying curves (Figure 1), the shorter the distance between the infrared lamps and the apple slices, the faster the water removal. Increasing the distance from 10 to 20 cm and from 20 to 30 cm led to an extension of the drying time by an average of 7.4 and 15.2%, respectively. As mentioned above, the closer the infrared lamps were placed to the drying tray, the more intensely they heated the surface of the material being dried, which, in turn, increased the evaporation rate [9,26,27]. Moreover, the drying time of apples was reduced by applying pretreatment, i.e., pulsed electric fields (PEF). Supplying specific energy to apples at the levels of 1, 3.5, and 6 kJ/kg resulted in reduction of the drying time of that fruit by approximately 20, 11, and 19%, respectively (in comparison to untreated samples dried at the same IR-CD conditions). Based on those observations, two conclusions can be drawn. First, electroporation of the apple tissue cell membrane reduced the resistance to water diffusion, making it occur faster and more intensely in PEF-pretreated samples [28,29]. Second, the degree of cell disintegration (CDI) increasing with the amount of supplied energy during PEF treatment had no direct impact on the drying time results. The values of this index were approximately 0.1, 0.2, and 0.4 for the samples with an energy input of 1, 3.5, and 6 kJ/kg, respectively. At the same time, the drying times of the minimally (1 kJ/kg) and maximally (6 kJ/kg) PEF-processed apples were statistically identical, despite significant differences in CDI. This indicates that it is worth considering milder processing of apples with PEF treatment in order to obtain satisfactory results with lower energy consumption [19,30].

3.2. Color

Lightness (L*) in the CIELab color space indicates a coordinate for black (0) and white (100) [31]. For fresh apple, L* took a value of 77.4 ± 0.3, while for all dried samples, values of that color parameter were in the range of 59.5–75.0. L* color parameter was significantly influenced not only by the distance between the infrared lamps and the apple slices (η² = 0.093) and the specific energy input (η² = 0.644), but also by the interaction of these two independent variables (η² = 0.231). Nevertheless, increasing the distance between the infrared lamps and the apple slices from 10 to 20 cm and from 20 to 30 cm did not cause significant changes in the lightness of the samples (Figure 3). In general, all PEF-pretreated apples were darker after drying than untreated samples dried in the same way. Electroporation of the cell membrane could induce leakage of non-enzymatic browning substrates, e.g., sugars, from the internal environment to the extracellular fluid [32]. This, combined with the heating of the sample’s surface in the proximity of infrared lamps [27], could promote the occurrence of non-enzymatic browning [33]. PEF could also increase the activity of the enzymes responsible for the oxidation of phenolic compounds to quinones (brown pigments)—polyphenol oxidase or peroxidase [7,19,25,32,34,35]. It was also noticed that milder processing of apples with PEF (1 kJ/kg) significantly reduced their darkening (in relation to treatment with specific energy inputs of 3.5 and 6 kJ/kg).

Another coordinate in the CIELab color space, a*, is associated with the red–green opponents. Positive values of this color parameter indicate red, while negative values represent green [36]. For fresh apples, a* was equal to −2.9 ± 0.4. In turn, for dried apples, values of that color parameter were in the range of −0.4–5.1. The a* color parameter was significantly affected not only by the distance between the infrared lamps and the apple slices (η² = 0.197) and the specific energy input (η² = 0.747), but also by the interaction of these two independent variables (η² = 0.206). As can be seen from Figure 4, the highest distance between the apple slices and the infrared lamps during drying (30 cm) resulted in products with significantly lower redness (compared to the distances of 20 and 30 cm). Intense heating of the samples in the proximity of infrared lamps could promote the occurrence of non-enzymatic browning due to the elevated temperature of the material [27,32,33]. It is worth mentioning that compared to PEF-pretreated samples, untreated apples exhibited negative values of a* after drying, which proves that they were green. The application of the PEF as a treatment prior to IR-CD resulted in obtaining apples with significant redness. The electric field-induced damage to the cell membrane increased its permeability. This, in turn, could have led to the release of non-enzymatic browning substrates from the cellular structure of the apple tissue, increasing its redness after drying. PEF could also promote enzymatic browning of treated apples due to potentially boosting polyphenol oxidase or peroxidase activity [7,19,25,32,34,35]. Moreover, milder-processed apples (1 kJ/kg) were significantly redder after drying than apples with the highest specific energy input (6 kJ/kg).

The last coordinate in the CIELab color space is b*, which indicates yellow (positive) and blue (negative) opponents [36]. For fresh apples, b* took a value of 17.7 ± 1.9, while for all dried samples, values of that color parameter were in the range of 21.1–25.5. The b* color parameter was significantly dependent only on the distance between the infrared lamps and the apple slices (η² = 0.137). As can be seen in Figure 5, the shortest distance between the apple slices and the infrared lamps during drying (10 cm) resulted in products with significantly higher yellowness (in relation to the distances of 20 and 30 cm). Similarly to the previous color parameter in the CIELab color space (a*), the increase in the yellowness of the material exposed to elevated temperature could be related to the Maillard reaction and caramelization process [27,32,33]. It is worth mentioning that the application of the pulsed electric fields did not have a negative impact on the color of dried apples in terms of their yellowness. Similar conclusions were drawn by other authors in studies conducted on hot-air-dried carrots [37] and purple potatoes [38]. Moreover, apples supplied with 3.5 kJ/kg energy before IR-CD exhibited significantly lower values of the b* color parameter after drying than the untreated ones.

Based on the obtained values of L*, a*, and b* color parameters, the total color difference (ΔE) was calculated with reference to the fresh apple. This parameter allows for the assessing of the color deviation of a given material (in this case dried apples) from a particular standard (fresh apple) [39]. For all dried samples, it took values in the range of 7.8–20.5. It means that the differences in color between raw material and final dried products were clearly visible [40]. ΔE was significantly influenced not only by the distance between the infrared lamps and the apple slices (η² = 0.146) and the specific energy input (η² = 0.637) but also by the interaction of these two independent variables (η² = 0.249). As can be seen from Figure 6, IR-CD carried out at the shortest distance between the material and the infrared lamps (10 cm) resulted in obtaining dried apples with the highest total color difference (in relation to processes performed at the distance of 20 and 30 cm). As mentioned above, a shorter distance from the heat source means higher heating of the apple’s surface, which could promote browning reactions [27,32,33]. Application of PEF as a pretreatment before IR-CD resulted in obtaining dried products with a higher total color difference from the raw material than untreated samples. This was caused by a significant reduction in lightness and an increase in redness of PEF-pretreated apples, probably due to an increased rate of non-enzymatic browning reaction. In addition, there are studies indicating an increase in the activity of certain enzymes due to the action of the pulsed electric fields (including those responsible for enzymatic browning) [7,19,25,32]. Moreover, milder-processed apples (1 kJ/kg) exhibited significantly lower ΔE after drying than apples with the highest specific energy input (6 kJ/kg).

Considering the non-enzymatic and enzymatic browning processes occurring during drying, it seems crucial to calculate (based on the obtained values of L*, a*, and b* color parameters) the browning index (BI), which is a measure of the purity of the brown color [41]. For fresh apple, BI was equal to 22.5 ± 2.9, while for all dried samples, it took values in the range of 34.3–53.6. Even a twofold increase in the BI value for dried apples in relation to fresh material confirms the occurrence of browning processes during water removal. BI was significantly affected by both the distance between the infrared lamps and the apple slices (η² = 0.160) and the specific energy input (η² = 0.246). As can be seen in Figure 7, the shortest distance between the apple slices and the infrared lamps during drying (10 cm) resulted in products with the highest values of browning index (compared to the distances of 20 and 30 cm). Intense heating of the samples in the proximity of infrared lamps could promote the occurrence of non-enzymatic browning due to the elevated temperature of the material [27,32,33]. PEF-pretreated apples exhibited higher BI after drying than untreated samples dried in the same way. Electroporation of the cell membrane could induce leakage of non-enzymatic browning substrates, e.g., sugars, from the internal environment to the extracellular fluid [32]. This, combined with the heating of the sample’s surface in the proximity of infrared lamps [27], could promote the occurrence of non-enzymatic browning [33]. PEF could also increase the activity of the enzymes responsible for the oxidation of phenolic compounds to quinones (brown pigments), i.e., polyphenol oxidase or peroxidase [7,19,25,32,34,35].

3.3. Chemical Properties

Apples contain significant amounts of polyphenols, which are plants’ secondary metabolites. Those phytochemicals have strong antioxidant effects [42,43,44]. Total phenolic content (TPC) in fresh apple was equal to 797.5 ± 44.0 milligrams of gallic acid (GAE) per 100 g of dry matter (d.m.). In turn, the content of those bioactive compounds in dried apples was in the range of 312.5–937.0 mg GAE/100 g d.m. TPC was significantly dependent not only on the distance between the infrared lamps and the apple slices (η² = 0.828) and the specific energy input (η² = 0.846) but also on the interaction of these two independent variables (η² = 0.684). As can be seen in Figure 8, increasing the distance between the infrared lamps and the apple slices from 10 to 20 cm resulted in higher retention of TPC. Polyphenols are compounds sensitive to elevated temperatures [42]; therefore, more intense heating of the surface of apples being dried could have led to their higher degradation. Interestingly, a further increase in the distance between the infrared lamps and the samples (from 20 to 30 cm) resulted in a significant reduction of TPC for all samples. As can be seen from Figure 2, the drying of apples was the longest when the distance to the infrared lamps was equal to 30 cm. Longer drying means longer heating of the apple surface and therefore an increased risk of thermal degradation of polyphenols. Application of the PEF as a treatment prior to IR-CD resulted in obtaining apples with lower retention of TPC than that obtained for untreated ones. The electric field-induced damage to the cell membrane increased its permeability. This, in turn, could have led to the release of polyphenols from the apple’s cellular structure, increasing the risk of their thermal degradation during drying. As mentioned above, PEF could also increase the activity of enzymes responsible for the oxidation of phenolic compounds, i.e., polyphenol oxidase or peroxidase [7,19,25,32,34,35]. Moreover, milder-processed apples (1 kJ/kg) exhibited significantly lower retention of polyphenols after drying than other PEF-treated apples.

As pointed out above, apples are a rich source of antioxidants. Therefore, the fresh raw material and the dried apples were analyzed for their ability to scavenge ABTS^•+ and DPPH^• free radicals. Considering that the results of those analyses were presented as the EC₅₀ coefficient, which means the lowest concentration of the sample extract required to reduce the initial content of ABTS^•+ and DPPH^• radicals by 50%, it is assumed that a lower EC₅₀ value indicates a higher antioxidant capacity. For fresh apple, the EC₅₀ of ABTS and EC₅₀ of DPPH were 0.37 ± 0.03 and 0.79 ± 0.08 mg d.m./mL, respectively. In turn, for dried apples, those values were in the ranges of 0.39–1.26 and 1.13–3.62 mg d.m./mL, respectively. EC₅₀ of ABTS and EC₅₀ of DPPH were significantly influenced not only by the distance between the infrared lamps and the apple slices (η² = 0.840 and η² = 0.910 for ABTS and DPPH, respectively) and the specific energy input (η² = 0.884 and η² = 0.933 for ABTS and DPPH, respectively) but also by the interaction of these two independent variables (η² = 0.755 and η² = 0.826 for ABTS and DPPH, respectively). Generally, as can be seen in Figure 9a and Figure 9b, increasing the distance between the infrared lamps and the apple slices from 10 to 20 cm and from 20 to 30 cm caused a decrease and an increase in EC₅₀, respectively, which means an increase and decrease in the antioxidant capacity, respectively (for both ABTS and DPPH assays). The use of a medium distance (20 cm) during IR-CD promoted the highest retention of antioxidant compounds (the lowest EC₅₀) probably due to the relatively prompt drying with moderate heating of the sample surface. Application of the PEF treatment before infrared–convective drying resulted in obtaining apples with lower antioxidant capacity than that obtained for untreated ones. Moreover, the lower the specific energy input, the lower the antioxidant capacity of dried apples. Determined r-Pearson correlation coefficient was equal to −0.913 and −0.915 for TPC-EC₅₀ of ABTS and TPC-EC₅₀ of DPPH, respectively (p < 0.05). The obtained results demonstrate a negative correlation between antioxidant capacity (measured via ABTS and DPPH assays, expressed as EC₅₀) and total phenolic content, which is consistent with the conclusions of other authors [45,46]. Sultana et al. [45] found a good correlation (r = 0.957) between the results of DPPH scavenging capacity (in %) and TPC (in GAE g/100 g of dry matter) contained in dried strawberry, mulberry, plum, apple, and apricot. In another study, Zhou et al. [46] claimed that correlation coefficient between the results of TPC (in mg gallic acid/g d.b.) and antioxidant capacity measured by ABTS (in µmol Trolox/g d.b.), DPPH (in µmol Trolox/g d.b.), and FRAP (in µmol Trolox/g d.b.) assays was equal to 0.8467, 0.8282, and 0.9042, respectively. This may suggest that both properties are strongly related; higher TPC in the tested material is associated with a higher ability to neutralize free radicals.

4. Conclusions

This research demonstrates that the pulsed electric fields (PEF) technology has a positive effect on the infrared–convective (IR-CD) drying process of apples, but it may also be associated with the risk of deterioration of their quality. It was proven that the electric field-induced damage to the cell membrane resulted in a reduced drying time of apples by up to 20%, but at the same time, the physicochemical characteristic indicated more intensive browning of the samples and lower retention of antioxidants. The distance between the infrared lamps and the apple slices during drying was also significant; the shortest (10 cm) favored quick water removal, the highest (30 cm) limited color changes, but it was the medium (20 cm) that proved to be the most beneficial for the preservation of antioxidants.