The Impact of Pulsed Electric Field Treatment of Beetroots on the Physicochemical Properties of Juice, Dried Juice, and Dried Pomace

Abstract

The aim of this work was to determine the impact of pulsed electric field treatment of beetroots on select physicochemical properties of the obtained juice, spray-dried juice, and freeze-dried pomace. Pulsed electric field pretreatment of beetroots was conducted with a different number of pulses (3 or 44 pulses) at a constant electric field strength of E = 1.07 kV/cm, which resulted in the total energy levels of 0.5 and 4 kJ/kg. The juice was dehumidified and air spray-dried with NUTRIOSE^® as a drying carrier at 90/60 °C, while pomace was freeze-dried at 0.63 Pa and a shelf temperature of 20 °C. Pulsed electric field (PEF) treatment caused an increase in pigment content in beetroots and the juice. In the case of powders, the following effects were noted: a decrease in drying efficiency, changed powder color, decreased powder particle diameter. Dried juice and pomace treated with a higher pulsed electric field (PEF) treatment were characterized by the highest hygroscopicity.

1. Introduction

Pulsed electric field (PEF) is considered to be a non-thermal food preservation method and is classified as an alternative food processing method [1]. This treatment is based on the transmission of short electric pulses to the raw material placed between two electrodes. The range of electric field intensity is most often between 100 V/cm and 80 kV/cm at a given temperature and time [2]. Electric pulses used during PEF treatment can take various forms such as exponential, rectangular, or other [3]. The use of electric pulses with an intensity exceeding 20 kV/cm is a method that can be considered a non-thermal food processing technique ensuring microbiological stability [4]. There are two types of mechanisms that are considered to be the most common with regard to PEF’s impact on cells. The first type is the acceleration of chemical reactions, and the second is electroporation, which, in consequence, leads to the destruction of cells. It should be underlined that the phenomenon of electroporation can be either permanent or reversible [5,6]. Reversible cell electroporation occurs when a short-term PEF is applied to cells, and breaks in the continuity of cell membranes are created, through which the exchange of ions, dyes, DNA, and RNA can take place. Moreover, the production of secondary metabolites increases, which, in consequence, results in an increased antioxidant activity. Irreversible electroporation occurs when the cell membrane is permanently damaged by PEF, the intensity of which is higher than its natural potential [7,8,9]. These properties of the PEF are the leading cause of this method’s application in the food industry as it is highly suitable for microbial inactivation, bioactive compound recovery, and pretreatment before multiple drying techniques without the use of thermally induced methods [5].

It is well-known that production of high-quality frozen food products is determined by many factors, including the time required for freezing the chosen material, the size of ice crystals, and their arrangement inside the raw material. PEF treatment prior to freezing can lead to the formation of pores in the cell membrane or wall and, therefore, serve as a center for crystallization. The formation of pores in the cell membrane or wall also results in the improvement of heat transfer, which affects thermal conductivity and, in consequence, reduces freezing time [10]. Ammar et al. [11] reported on PEF pretreatment prior to freezing potato tissue. They noted that this technique reduced the phase transition time by 15% compared to the potatoes that were not subjected to PEF treatment. A similar relationship occurs in the case of drying using different methods when the material has been treated with PEF. Liu et al. [12] vacuum-dried carrots and observed that the drying time of the treated material was reduced by 33–55% at drying temperatures varying from 25 to 90 °C. Giancaterino et al. [13], who freeze-dried kiwi fruits, red bell peppers, and red beetroots, observed that the freeze-drying time of red bell peppers was shorter when they were treated with PEF. The application of a PEF to raw materials accelerated processing, thus leading to lower costs, which is economically crucial for the food industry.

The quality of raw materials is important in food production. Drying can cause negative physical and chemical changes in products, which is often the result of the application of high drying temperatures and uneven moisture distribution in the product during drying. The consequences of drying may include shrinkage or change in color, texture, or taste. Liu et al. [14], who dried apple tissue, observed that PEF application facilitated the preservation of the shape of dried samples as it lowered shrinkage and increased the size of the pores in the tissue. The phenomenon of electroporation, which follows PEF application, also contributes to the increased extraction of beneficial bioactive compounds. Low efficiency of extraction and the need to use chemical solvents may cause a decrease in the quality of products. The application of a PEF may be a solution to this problem as it can improve the efficiency of obtaining bioactive compounds from plant tissue, such as polyphenols and anthocyanins from wine, as presented in the study by Yang et al. [15]. Therefore, the aim of this research was to determine the impact of PEF pretreatment of beetroots on select physicochemical properties of the obtained juices, powders, and pomace. Drying beetroot juice using an innovative spray drying method with dehumidified air allows the use of a small addition of drying carriers. This is particularly important if the beetroot is intended to be applied as a natural dye. The possibility of using PEF and dehumidified air spray drying methods in the production of concentrated dyes appeared valuable. NUTRIOSE^® was used as a drying carrier in the production of powders, and PEF pretreatment was applied with different numbers of pulses (3 or 44 pulses), the total energy of 0.5 and 4 kJ/kg, and a constant electric field strength of E = 1.07 kV/cm.

2. Materials and Methods

2.1. Materials and Processing

2.1.1. Materials

The research material consisted of beetroots (Beta vulgaris L.), Wodan F1 variety, purchased at a local market in Warsaw, Poland. This cultivar is characterized by round, uniformly developed roots that achieve a spherical shape with smooth skin. To account for the seasonal availability of the raw material and to ensure consistency in results, only beetroots from a single batch were used. Prior to experimentation, the material was stored under refrigeration at a temperature of 5 ± 1 °C and a relative humidity of 85 ± 5%. Before analysis, the roots were rinsed with tap water and gently dried, and excess surface water was removed with a paper towel.

2.1.2. PEF Pretreatment

The pretreatment parameters were determined based on preliminary studies by Nowacka et al. [16], where changes in the material’s conductivity were monitored through the cell disintegration index (CDI) following PEF treatment. The electrical conductivity of beetroot tissues, both before and after treatment, was measured with a conductometer [CPC-401 Elmetron, Zabrze, Poland] equipped with a custom-built dual-needle probe made of platinum. An energy input range of 0.5 to 4 kJ/kg was selected for optimization, as this range promotes an increase in the CDI without reaching the “saturation” threshold, beyond which further energy input yields minimal CDI enhancement. The PEF processing was carried out using an ELEA GmBH (Quakenbrück, Germany) PEF Pilot device. The main PEF parameters were configured as follows: an electrode voltage of 24 kV, an electric field strength of 1.07 kV/cm, a pulse frequency of 20 Hz, and a pulse width of 7 µs. The energy dose was regulated by varying the number of applied electrical pulses. The treatment cell design consisted of two parallel stainless-steel electrodes spaced 24 cm apart. The material was placed in a Teflon chamber and submerged in tap water at 20 ± 1 °C to reach a total mass of 1 kg [17].

2.1.3. Juice Preparation

The beetroots were cut into smaller pieces and placed in a vertical, slow-speed juicer (Kuvings D9900, Wasilków, Poland) with an auger rotating at 48 rpm. Immediately after pressing, the juice was directed for analysis or further processing. Juice extraction was performed three times for each material batch.

2.1.4. Spray Drying

The feed solution was prepared by combining the drying carrier, NUTRIOSE^® (Roquette, Lestrem, France) with the ratio of beetroot juice d.m. to NUTRIOSE^® d.m. of 75:25. Drying was conducted using a Mobile Minor spray dryer (GEA, Soeborg, Denmark) equipped with an external air dehumidification system, including a TAEevo TECH020 cooling unit (MTA, Tribano, Italy) and an ML270 condensation–adsorption unit (MUNTERS, Kista, Sweden). During the DASD process, the maximum air humidity was maintained below 1 g/m³ [18]. The feed rate was set at 0.3 mL·s⁻¹, with the atomizing disk rotating at 26,000 rpm under a compressed air pressure of 4.5 bar. The drying parameters were as follows: inlet air temperature of 90 °C and outlet air temperature of approximately 60 °C. Each drying variant was performed in duplicate, and the resulting powders were stored in sealed light- and air-barrier bags at a temperature of 4 °C.

The powder recovery (drying yield) was calculated to evaluate the effectiveness of spray drying. This value is defined as the ratio of the solid content in the solution prior to spray drying to the solid content in the resulting powder.

2.1.5. Pomace Lyophilization

The pomace, obtained from juice extraction, was placed in a single layer with a thickness of 1 cm on a metal tray and transferred to a shock freezer (HCM 51.20, IRINOX, Conegliano, Italy) at −40 °C. After 4 h, the material was transferred to a laboratory freeze dryer (Gamma 1–16 LSC, CHRIST, Osterode am Harz, Germany). The lyophilization process was carried out at a pressure of 0.63 Pa, with a shelf temperature of 20 °C and a condenser temperature of −55 °C for 48 h [19]. Following the process, the material was ground in a food processor (Thermomix, VORWERK Elektrowerke GmbH & Co, Wuppertal, Germany) at a speed of 10,000 rpm for 1 min. In

Table 1. Experimental materials and treatments.

Sample Name	Material	Number of Pulses	Electric Field Strength [kV/cm]	Total Energy [kJ/kg]	NUTRIOSE (Carrier Agent)
B0	Beetroots	0	0	0	-
B1		3	1.07	0.5	-
B2		44		4	-
J0	Beetroot juice	0	0	0	-
J1		3	1.07	0.5	-
J2		44		4	-
JC0	Beetroot juice with a carrier	0	0	0	+
JC1		3	1.07	0.5	+
JC2		44		4	+
DASDJC0	Dehumidified air spray-dried beetroot juice with a carrier	0	0	0	+
DASDJC1		3	1.07	0.5	+
DASDJC2		44		4	+
FDP0	Freeze-dried pomace	0	0	0	-
FDP1		3	1.07	0.5	-
FDP2		44		4	-

, the sample code and treatment parameters were presented.

2.2. Analytical Methods

2.2.1. Dry Matter Content

The dry matter content in the spray-dried and freeze-dried powders was determined with a gravimetric method using a laboratory dryer (SUP-65G, Warsaw, Poland) with natural air circulation. Drying was performed at 105 °C for 4 h [20]. The determination was carried out in four replicates.

2.2.2. Water Activity

The water activity of the dried samples was determined using a HygroLab C1 meter (Rotronic AG, Bassersdorf, Switzerland) with a cylindrical probe at 25 °C [21]. The measurements were performed in quadruplicate for each powder.

2.2.3. Sugar Content

The sugar content was determined using high-performance liquid chromatography (HPLC) with a refractive index detector [22]. The system included a quaternary pump (Waters 515, Milford, MA, USA), an autosampler (Waters 717, Milford, MA, USA), a Waters Sugar Pak I column (300 × 6.5 mm) with a Sugar-Pak precolumn, and a detector (Waters 2414, Milford, MA, USA). The sample was ground in an analytical mill, and 0.3 g were weighed into centrifuge tubes and diluted to 50 mL with Milli-Q redistilled water at 80 °C. The extraction was carried out on a mechanical shaker (Shaker Multi Reax; Heidolph Instruments, Schwabach, Germany) for 4 h at ambient temperature. The solution was centrifuged, and the supernatant was filtered through a 0.45 µm PTFE syringe filter before being placed in a cooled autosampler. Milli-Q redistilled water was used as the mobile phase, with a flow rate of 0.6 mL/min. A 10 µL aliquot of the solution was injected into the column. The column and detector were thermostated at 80 °C and 50 °C, respectively. The content of sugar was calculated based on calibration curves for individual sugar standards: glucose, fructose, and sucrose (Sigma-Aldrich, Steinheim, Germany). Measurements were performed in triplicate.

2.2.4. Color

The color of the samples was evaluated using the CIE L*a*b* color system, where L* represents lightness, a* denotes the chromatic component indicating the red (+a*) or green (−a*) hues, and b* indicates the yellow (+b*) or blue (−b*) chromatic parameter. Color measurements were conducted with a chromameter (CR-5, KONICA-MINOLTA, Osaka, Japan) in the reflectance mode. The CIE D65 illuminant, with diffuse illumination and a 0° viewing angle (d:0), a CIE 2° standard observer, and an 8 mm measurement area were used [23]. Measurements were repeated 10 times.

2.2.5. Polyphenol Content

The determination was performed on raw materials, juices, and dried samples using the Folin–Ciocalteu method [24], with gallic acid as the standard. The lyophilized pomace was ground in a laboratory mill (IKA A11 Basic, Staufen, Germany). A sample of 0.5 g of dry material and approximately 1 g of wet samples were weighed into glass beakers. Extraction was carried out using 25 mL of 80% ethanol. The beakers were placed on a heating plate and covered with a watch glass, and the mixture was brought to a boil. The mixture was then filtered, and the volume was adjusted to 50 mL. In glass test tubes, 0.18 mL of the extract, 4.92 mL of distilled water, and 0.3 mL of the Folin–Ciocalteu reagent were pipetted and mixed, and after 6 min, the solution was alkalized with 0.6 mL of a 17.7% sodium carbonate solution. The contents of the test tubes were mixed again and left at room temperature, protected from light, for 1 h. The absorbance of the solutions was measured at a wavelength of 750 nm, with the reagent blank serving as the reference [25]. The measurement was performed in duplicate. The results were expressed as mg of gallic acid per 100 g of dry solid material.

2.2.6. Betalain Content

The material was subjected to pigment extraction using a phosphate buffer at pH 6.5 [26]. Approximately 0.1 g of the powder and pomace, as well as approximately 1.2 g ± 0.0001 g of the fresh beetroots and juice, were weighed and placed into 50 mL volumetric flasks, then filled up to the mark with the buffer solution. The extraction was carried out in a laboratory shaker (Heidolph Multi Reax, Schwabach, Germany) for 15 min. The absorbance of the supernatant was measured using a spectrophotometer (SPECTRONIC 200; Thermo Fisher Scientific Inc., Carlsbad, CA, USA). The concentration of betalains, including red and yellow pigments, was quantified as betanin (mg betanin/100 g d.m.) and vulgaxanthin-I (mg vulgaxanthin-I/100 g d.m.), respectively. The pigment content was calculated using the absorption coefficients (A1%) of 1120 for betanin at 538 nm and 750 for vulgaxanthin-I at 476 nm. Additionally, absorbance at 600 nm was measured to account for the correction of interfering substances, ensuring accurate pigment quantification. The measurement was performed in duplicate.

For the calculation of red and yellow pigments, the following equations were used [27]:

Red = D·(1.095(A₅₃₈ − A₆₀₀)/M·DM·1120,

(1)

Yellow = D·(A₄₇₆ − A₅₃₈ + 0.677·1.095(A₅₃₈ − A₆₀₀))/M·DM·750,

(2)

where D represents the sample dilution factor; A₄₇₆, A₅₃₈, and A₆₀₀ denote the absorbance values of the solution measured at 476, 538, and 600 nm, respectively; M refers to the sample mass (in grams); and DM indicates the dry matter content (in g/g). The factor 1.095 is used to correct for the increased absorbance at 538 nm due to the presence of impurities; and 1120 is the molar extinction coefficient for a 1% betanin solution at 538 nm in a 1 cm cuvette.

Similarly, the factor 0.677 adjusts for the absorbance increase at 476 nm caused by impurities, and 750 is the molar extinction coefficient for a 1% vulgaxanthin solution at 476 nm in a 1 cm cuvette.

2.2.7. Antioxidant Activity

A stock solution of the radical was prepared 16 h prior to the analysis. To create the solution, 38.4 mg of ABTS and 6.6 mg of potassium persulfate were dissolved in 10 mL of distilled water. The working solution was prepared immediately before the analysis by diluting the stock solution approximately 100 times with 80% ethyl alcohol. The absorbance of this working solution, measured at 734 nm, should fall within the range of 0.700 ± 0.020. For the reaction, 10, 20, 30, and 40 µL of the ethanolic sample extract were added to test tubes, along with 3 mL of the radical working solution. The mixtures were allowed to react for 6 min, after which the absorbance was measured. This procedure was repeated for each measurement. The antioxidant activity was evaluated by measuring the ability of the ethanol extracts from dried peppers to neutralize the ABTS^•+ radical cation (2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonate)). All measurements were performed in triplicate. The results are expressed as the concentration of the sample required to reduce the radical concentration by 50% [28].

2.2.8. Powder Morphology

The particle morphology was characterized by analyzing the scanning electron microscopy (SEM) images (Phenom XL, Phenom World, Eindhoven, The Netherlands). The material was placed on a carbon tape affixed to a metal stub, followed by coating with a 5 nm gold layer using an auto sputter coater (Cressington Scientific Instruments, Watford, UK). The analysis was conducted under a voltage of 10 kV, a vacuum of 1 Pa, and at a magnification of 1000× [29].

2.2.9. Powder Particle Size

The particle size distribution was determined using a Cilas 1190 laser diffraction instrument (CILAS, Orleans, France). The powder samples were suspended in ethanol (in triplicate) within a flow-through cell, achieving an obscurance value of 10%. The particle size distribution was recorded three times for each sample. The results were expressed as the volume-weighted median diameter (D) [30].

2.2.10. Powder Hygroscopicity

The measurement of the hygroscopic properties of the powders was performed by weighing an appropriate amount of the sample and placing the samples in aluminum containers. The containers were positioned above a saturated sodium chloride solution with a water activity of 0.75 [31]. The samples were weighed after 1, 3, 6, 9, 24, 48, and 72 h. The measurements were conducted in two replicates.

2.2.11. Powder Density and Cohesiveness

The bulk densities of both loose (DL) and tapped (DT) powders were determined using a STAV 2003 automatic tapper (Engelsmann AG, Ludwigshafen, Germany). The measurements were carried out by assessing the volume occupied by 10 g of powder, with the tapped density recorded after 100 taps. The cohesiveness and flowability of the powders were characterized by calculating the Hausner ratio (HR), defined as the ratio of tapped density (DT) to loose density (DL): HR = DT/DL [32].

2.3. Statistical Analysis

The results were analyzed using Statistica 13.1. One-way analysis of variance (ANOVA) was performed at a significance level of α = 0.05, and homogeneous groups were determined using Tukey’s test.

3. Results and Discussion

3.1. Beetroots

The advantages of phenolic compounds being present in beetroots include improved metabolism, blood clotting regulation, removal of free radicals, and protection against cancer [16,33]. In this research, beetroots were characterized by polyphenols varying from 2547 to 2751 mg of chlorogenic acid/100 g d.m. The highest value was obtained for the sample treated with the lowest number of electric pulses (B1) (2751 mg/100 g d.m.), the lowest—for the untreated sample (B0) (2547 mg/100 g d.m.) (

Table 2. Polyphenol content (TPC) and RC in relation to juices (in brackets), betalain content: red pigments (BC), yellow pigments (VC), antioxidant activity (ABTS) of the beetroots (B), juice (J), dried juice (DASDJC), and dried pomace (FDP).

Sample Code	TPC [mg Chlorogenic Acid/100 g Solids]	BC [mg Betanin/100 g Solids	VC [mg Vulgaxanthin/100 g solids]	ABTS [mg Solids/100 mL]
B0	2547 ± 50 a	778 ± 9 a	261 ± 4 a	14.1 ± 0.8 b
B1	2751 ± 12 b	898 ± 7 b	311 ± 3 b	10.7 ± 0.4 a
B2	2645 ± 11 a,b	848 ± 23 b	361 ± 13 c	12.1 ± 0.4 a,b
J0	958 ± 19 a	2615 ± 83 a	743 ± 21 a	8.7 ± 0.6 c
J1	1030 ± 11 a	2867 ± 37 b	848 ± 20 b	2.1 ± 0.3 b
J2	978 ± 21 a	2783 ± 17 a,b	878 ± 5 b	5.2 ± 0.8 a
DASDJC0	2070 ± 43 b (288%)	496 ± 3 b (25%)	390 ± 6 b (70%)	36.0 ± 18.0 a
DASDJC1	1479 ± 73 a (191%)	418 ± 5 a (19%)	292 ± 3 a (46%)	36.1 ± 18.1 a
DASDJC2	1992 ± 82 b (272%)	491 ± 5 b (24%)	400 ± 8 b (61%)	36.0 ± 16.7 a
FDP0	1532 ± 69 a	542 ± 42 a	241 ± 10 a	26.2 ± 1.6 b
FDP1	1523 ± 42 a	619 ± 12 a	282 ± 5 b	17.4 ± 1.3 a
FDP2	1705 ± 47 a	576 ± 10 a	245 ± 11 a,b	21.6 ± 0.6 a,b

Note: ^a–c—the same symbols in the columns indicate homogeneous groups (for each group of materials separately).

). Kannan [33] observed that PEF application increased the extraction of phenolic content from beetroots by 51% in comparison to the untreated sample. However, it should be underlined that a similar effect of a PEF was not noted in this research, which could have been an effect of the use of different procedures of treatment (PEF treatment in a 7% sodium chloride solution, which could have prevented the leakage of bioactive components from the material during the treatment), different numbers of pulses used (20 pulses), different pulse characteristics (pulse width of 15 µn, pulse frequency of 1 Hz), different electric field strength (1.5 kV/cm), different equipment with a distance between the electrodes of 6 cm, or different techniques for preparing samples for the analysis.

The beetroots that were not exposed to the PEF treatment (B0) were characterized by a betalain content of 778 mg betanin/100 g d.m. and 261 mg vulgaxanthin/100 g d.m. (Table 2). Both PEF treatments (B1 = 0.5 kJ/kg and B2 = 4 kJ/kg) significantly increased the availability of the tested compounds. In the case of betanins, no effect of the intensity of exposure to the PEF on the extractability was demonstrated, while in the case of vulgaxanthins, increasing the number of electric pulses resulted in a better extraction of these pigments. Nowacka et al. [16] observed a similar relationship when treating beetroots with a different number of pulses. Thus, it can be concluded that the application of a PEF increases the extraction of betalains as an effect of better permeability of the cell membrane.

The antioxidant activity of the tested beetroots varied from 10.7 ± 0.4 mg d.m./100 mL to 14.1 ± 0.8 mg d.m./100 mL (Table 2). The obtained results showed that the control sample (B0) had the lowest antioxidant activity. More free radicals were neutralized in the samples treated with PEF. These results are in compliance with the analysis of the total phenolic content and betalain content, as both groups of compounds contribute to antioxidant activity. Due to the increased permeability of the cell membrane, the beetroots with an increased content of bioactive compounds presented a better antioxidant activity than the untreated samples. Kannan [33] noted the same relationship when applying a PEF to beetroots, as the author observed more juice extracted after the treatment. In that case, the application of a PEF to beetroots led to a 38.5% increase in juice yield, which was significantly higher compared to the control group.

3.2. Juice

3.2.1. Water Content in Juice

The juices were characterized by a water content of 90.9–91.3% (

Table 3. Sugar content: sucrose (S), glucose (G), fructose (F), and L*a*b* color parameters in the beetroot juices.

Sample Code	Water Content [%]	S [g/100 g Solids]	G [g/100 g Solids]	F [g/100 g Solids]	L*	a*	b*
J0	91.3 ± 0.5 a	2.22 ± 0.04 b	1.74 ± 0.02 b	1.42 ± 0.02 a	4.48 ± 0.12 c	15.46 ± 0.18 c	3.04 ± 0.12 b
J1	90.9 ± 0.3 a	1.81 ± 0.03 a	1.49 ± 0.05 a	1.12 ± 0.16 a	3.74 ± 0.07 b	10.53 ± 0.42 b	1.63 ± 0.19 a
J2	91.0 ± 0.4 a	1.69 ± 0.04 a	1.39 ± 0.03 a	1.13 ± 0.01 a	2.71 ± 0.08 a	9.13 ± 0.28 a	1.65 ± 0.16 a

Note: ^a–c—the same symbols in the columns indicate homogeneous groups (for each group of materials separately).

). Statistical analysis did not show any differences between the tested variants. This can be interpreted as a lack of influence of PEF pretreatment, regardless of its parameters, on the water content.

3.2.2. Sugar Content in Juice

The sugar content of the beetroot depends on the variety [34] as well as the pretreatment, as noted in our study. Table 3 presents the sugar content in the beetroot juices. The initial sugar content in the juice obtained from the beetroots not subjected to PEF treatment (J0) was 2.22 g/100 g solids for sucrose, 1.74 g/100 g solids for glucose, and 1.42 g/100 g solids for fructose. The content of total sugars in the beetroot juices was influenced by the PEF treatment of the raw materials. The content of sucrose and glucose decreased on average by 22 and 19% after the application of a PEF, while there were no significant differences in the fructose content. However, there was no effect of the number of pulses on the sugar content in the beetroot juices. It should be underlined that the decrease in the content of glucose and fructose, which are simple sugars of the low glass transition temperature, in liquid feed was a beneficial phenomenon for the spray-drying process. As a consequence, it lowered the risk of stickiness that might occur during spray drying.

3.2.3. Color in Juice

Table 3 presents the L*a*b* color parameters of the beetroot juices, beetroot powders, and freeze-dried samples. Generally, samples of all the tested beetroot juices were characterized by dark color following low values of the L* parameter at the level of 2.71–4.48. The a* parameter values decreased from 15.46 to 9.13, while the b* parameter decreased from 3.04 to 1.63, with the lowest average values observed for the variant pretreated with the highest number of PEF pulses (J2) and the highest values observed for the raw, untreated beetroot juice (J0). The color parameters of the beetroot juices differed statistically. It was observed that increasing the number of electric pulses caused a decrease in the a* and b* color parameters (indicating the shares of the red and yellow colors, respectively) of the beetroot juices. The obtained results contradict previous studies. Rivas et al. [35], who conducted research on blended orange and carrot juice treated with a PEF, observed no differences in lightness between samples with higher and lower intensities of the PEF. Bi et al. [36] reported that apple juice was lighter with a higher PEF intensity, and at the same time, there was no influence on the a* parameter.

3.2.4. Phenolic Content in Juice

The amount of polyphenols in beetroot differs based on its variety [34]. For example, Vasconcellos et al. [37] obtained in beetroot juice polyphenols content in amount of 367 mg/100g. In our study, beetroot juice (J0) contained a polyphenols content of 958 mg of chlorogenic acid/100 g solids. However, the highest value was recorded for the juice subjected to 3 electric pulses (J1), in which the content was 1030 mg of chlorogenic acid/100 g solids, and the lowest value was noted for the juice not subjected to PEF treatment (J0)—however, the total phenolic content did not differ significantly among the researched juices (Table 2). Thus, it can be concluded that PEF treatment did not influence the extractability of phenolics.

3.2.5. Content of Betalain Pigments in Juice

Table 2 presents the content of betalain pigments (betanin and vulgaxanthin) in the raw beetroots and juices. The beetroots that were not pretreated with a PEF (B0) were characterized by a pigment content of 778 mg betanin/100 g solids and 261 mg vulgaxanthin/100 g solids, and the obtained results showed that the use of PEF treatment significantly increased the betalain content. This concurs well with previous findings of Nowacka et al. [16], who conducted research on the PEF’s influence on beetroots’ bioactive compounds. In the case of betanins, no effect of the number of applied pulses on the extractability was observed, while in the case of vulgaxanthins, increasing the number of electric pulses resulted in a better extraction of these pigments. These findings confirm that the PEF increased the cell membrane permeability as a result of electroporation.

Generally, the highest content of betalain pigments among all the tested samples was noted in juices (betanins: 2615–2867 mg/100 g solids, vulgaxanthin: 743–878 mg/100 g solids). Variants J1 and J2 were characterized by a higher content of pigments than the untreated variant J0. As stated before, PEF treatment increased the extractability of pigments from the vegetable tissue.

3.2.6. Antioxidant Activity in Juice

Among all the tested beetroot products, the juice had the greatest ability to fight free radicals, followed by the beetroots from which it was obtained, the powder from freeze-dried pomace, and the powder obtained by spray drying (Table 2).

In the case of juices, a significant improvement in antioxidant properties was observed after PEF treatment—samples J1 and J2 were characterized by lower ABTS values. Dziadek et al. [38] showed that the use of a PEF reduces the loss of antioxidants in juices during storage. During the experiment, there was a decrease in antioxidant activity immediately after PEF treatment, but the subsequent decrease was much smaller than in the samples not subjected to PEF treatment. The exception was the variant in which 400 pulses were used. In that case, an increase in activity by several units was observed, followed by a decrease that did not exceed the initial antioxidant content. Mannozzi et al. [39] observed that PEF treatment combined with high temperatures increased the antioxidant capacity of fruit and vegetable juices.

3.3. Dried Juice and Pomace

3.3.1. Powder Recovery of Dried Juice and Pomace

Powder recovery for the juice obtained from the non-PEF treated materials (SDJC) was 86.9 ± 1.4%, while for the PEF-treated materials, it was 73.9 ± 1.5% (DASDJC1) and 77.8 ± 1.6% (DASDJC2), respectively. According to the criteria presented by Bhandari [40], successful pilot-scale spray drying is characterized by a powder recovery of a minimum of 50%. The obtained values, which were higher than this limit, indicate that the process ran without problems, i.e., without stickiness issues, which are usually observed in the case of sugar-rich materials such as fruit juices, honey, or molasses. The sugar content in beetroots is not high in comparison to these types of materials (Table 3). However, the feed solutions contained 25% of the carrier (in solids, w/w) to prevent the possible deterioration of powder recovery, which could result from sugar content, even if it was quite low. Such a low carrier content is not sufficient to reduce stickiness in the case of fruit juices during conventional high-temperature spray drying, where usually at least 50% is required [41]. The powder recovery of the juices obtained from the PEF-treated beetroots was significantly lower than that obtained from the non-treated sample. This could result from the differences in sugar content, which in turn could affect stickiness and particle size. Lower sugar (sucrose and glucose) content in the PEF-treated samples decreased stickiness, and this reduced stickiness resulted in a smaller particle size (p. 3.3.2.). A similar observation of smaller particles after drying a material with lower stickiness was previously reported, e.g., by Samborska et al. and Barańska et al. [18,42]. This smaller particle size affects powder recovery because the cyclone recovery for smaller particles is not as efficient.

3.3.2. Morphology and Particle Size of Dried Juice and Pomace

SEM microphotographs of the powder particles obtained after spray drying are presented in Figure 1, while the particle size distribution and the cumulative particle size distribution, with the values of the median diameter D50 and the polydispersity index (PDI)—in Figure 2. The morphology of the particles was typical for spray-dried material, and the shape of the particles was regular. The presence of small particles among bigger ones could be observed. The morphology of all the samples in the SEM pictures was similar, while after the determination of particle size by laser diffraction, some differences were noted—sample DASDJC had significantly bigger particles than DASDJC1 and DASDJC2. As mentioned above, this could result from the differences in sugar content after PEF treatment (Table 3). Lower sugar content of the treated samples decreased the stickiness effect, and this resulted in smaller particles. Such a behavior of smaller particles after decreasing stickiness had been observed before, e.g., by Samborska et al. for white mulberry molasses powders [43].

3.3.3. Water Content and Water Activity of Dried Juice and Pomace

The stability of powder is strongly impacted by water activity and moisture content [44]. In the case of powders obtained by spray drying of juice, an increase in water content was observed with more intensive exposure of beetroots to the electric field. The subsequent variants (DASDJC0, DASDJC1, DASDJC2) reached higher values than the others, amounting to 9.9, 10.9, and 11.3%, respectively (

Table 4. Water content (MC), color parameters (L*, a*, b*), water activity (a_w), loose density (ρL) of the dried juice (DASDJC) and pomace (FDP).

Sample Code	MC [%]	L*	a*	b*	aw [-]	ρL [g/cm3]
DASDJC0	9.9 ± 0.4 a	23.88 ± 1.09 b	34.48 ± 1.29 a	7.62 ± 0.11 a	0.247 ± 0.017 a	0.68 ± 0.04 a
DASDJC1	10.9 ± 0.7 a,b	22.62 ± 0.75 a	40.86 ± 0.79 b	8.67 ± 0.12 b	0.265 ± 0.003 b	0.74 ± 0.01 b
DASDJC2	11.3 ± 0.8 b	23.58 ± 0.39 a,b	42.00 ± 0.19 c	11.52 ± 0.21 c	0.238 ± 0.002 a	0.65 ± 0.02 a
FDP0	7.4 ± 1.6 a	42.26 ± 0.48 a	26.70 ± 0.18 a	0.13 ± 0.03 b	0.030 ± 0.002 a	0.32 ± 0.01 b
FDP1	6.5 ± 0.9 a	42.92 ± 0.59 a	27.16 ± 0.21 b	−0.92 ± 0.20 a	0.080 ± 0.002 c	0.29 ± 0.00 a
FDP2	5.8 ± 0.7 a	42.85 ± 0.67 a	26.91 ± 0.19 a,b	0.05 ± 0.11 b	0.040 ± 0.002 b	0.36 ± 0.01 c

Note: ^a–c—the same symbols in the columns indicate homogeneous groups (for each group of materials separately).

). From a statistical point of view, powders DASDJC0 (without PEF treatment) and DASDJC2 (44 pulses with intensity E = 1.07 kV/cm) differed from each other, while powder DASDJC1 (3 pulses with intensity E = 1.07 kV/cm and the total energy of 0.5 kJ) was similar in terms of water content both to the variant without PEF pretreatment and to sample DASDJC2 obtained from the beetroots with the total given energy of 4 kJ/kg.

The freeze-dried pomace was characterized by a water content ranging from 5.8% for FDP2 to 7.4% for FDP0. In the case of pomace, no statistically significant differences were found. All the variants were classified into one homogeneous group.

The water content of beetroot powders dried using a foam mat drying was 8.27% [45], while beetroot powder obtained by the sublimation method with a simultaneous micronization process contained about 10% water [46]. Gawałek and Bartczak [47] obtained spray-dried beetroot powders based on maltodextrin, with a moisture content of 0.98–1.25%.

Analyzing the water activity, it was noticed that samples DASDJC0 and DASDJC2 belonged to the same homogeneous group; the results were at the level of 0.23 and 0.24. The highest water activity coefficient was observed in sample DASDJC1, for which the result was 0.26. The use of PEF treatment (for DASDJC2) had no significant effect on the water activity compared to the control sample DASDJC0 (without treatment).

Much greater differences in water activity were observed in the beetroot pomace. In the tested samples, the value of 0.080 was obtained by sample FDP1, which was the highest value. For FDP0, the value was 0.030, for FDP2—0.040. Each of the samples was classified into a different homogeneous group. The pomace achieved visibly lower water activity values compared to the spray-dried juice powders. Comparing the results of the pomace and spray-dried juice powders, it can be seen that the water activity values of the pomace were almost 10 times lower than those of the spray-dried juice powders. This can probably be explained by the differences in drying, as well as by the different drying materials (pomace, juice) with different chemical composition (juice—more low-molecular-weight sugars; pomace—more high-molecular-weight compounds, such as fiber).

3.3.4. Hygroscopicity of Dried Juice and Pomace

Hygroscopicity (H) is a parameter that indicates food stability, and is particularly important in the case of powders. It allows for determining the product’s ability to absorb water from the environment, which is crucial with regard to changes in the powder’s properties during storage and for the selection of appropriate packaging. Hygroscopic properties depend on the composition of the raw materials and, in the case of powders, also on the content and type of the carrier used [48]. In order to assess the hygroscopicity of the beetroot powders, they were stored in an environment with a water activity of 0.756 for 72 h. The powder that was produced using spray drying and not subjected to PEF pretreatment (DASDJC0) was characterized by an H of 26.5% (Figure 3). A similar H (27.4%) was noted for the powder pretreated with 3 electric pulses (DASDJC1). An increase in H (32.8%) was observed for the powder that was subjected to PEF pretreatment in the amount of 44 pulses (DASDJC2). After 72 h, the samples did not reach equilibrium water content, and their mass was constantly increasing. Therefore, it was noted that the more intensive PEF treatment of the beetroots that were used for juice production caused an increase in the H of the powders obtained by spray drying, which is not a favorable effect as it decreases the powders’ storage stability. The greatest increase in powder mass was observed during the first 10 h of the experiment.

In the case of the beetroot pomace that was freeze-dried, the H of the sample that was not treated with PEF reached 21.4% (FDP0). The pomace that was pretreated with three electric pulses was characterized by an H of 22.2% (FDP1), and the beetroots subjected to 44 electrical pulses (FDP2)—by an H of 26.2% (Figure 3). The pomace powders, similar to the spray-dried juices, absorbed water the fastest during the initial 10 h. The equilibrium moisture was not reached either, and their mass continued to increase.

The water vapor sorption kinetics for the powdered freeze-dried beetroot pomace (FDP0, FDP1, FDP2) differed from the curves for the spray-dried juices. However, in both cases, the material subjected to the highest amount of electrical pulses (DASDJC2 and FDP2) absorbed significantly more water than the samples that were not pretreated (DASDJC0, FDP0) and those treated with only 3 pulses (DASDJC1, FDP1). These results correlate fairly well with Rybak et al. [49], where red bell pepper juice was spray-dried, and a lower H of powders was observed when 1 kJ/kg PEF was applied prior to pressing bell peppers in order to produce a liquid feed compared to the materials that were not treated or treated with a higher-intensity PEF (3 kJ/kg). Lammerskitten et al. [50] freeze-dried apple tissue and observed a similar correlation regarding the H of samples. They noted a 4.5% higher H of the samples that were not pretreated with a PEF compared to the apples that were treated prior to freeze drying. It was concluded that this phenomenon was a consequence of electroporation that enhanced the soluble solids migration to the surface, which affected the H of dried samples. As expected, the results obtained in this research prove that PEF pretreatment can influence the hygroscopic properties and enhance the H of dried samples to some extent. However, it should be emphasized that it depends on the intensity of the PEF applied and the raw material type.

3.3.5. Density and Cohesiveness of Dried Juice and Pomace

The loose density of the beetroot juice powders obtained by spray drying was 0.68 g/cm³ for the variant without PEF treatment (DASDJC0), 0.74 g/cm³ for variant DASDJC1, and 0.65 g/cm³ for variant DASDJC2 (Table 3). In terms of loose density, the powders were divided into two homogeneous groups. One group included variant DASDJC2 and control sample DASDJC0, while the other group included powder DASDJC1.

The freeze-dried beetroot pomace powders (FDP) were characterized by significantly lower values of loose density, which ranged from 0.29 to 0.36 g/cm³. Additionally, no similarities were observed between the individual samples (they were in different statistical groups). Based on the obtained results, it cannot be stated whether the treatment of raw materials with a PEF had an effect on the loose density of the dried pomace. Gawałek and Bartczak [47] showed that the loose bulk density determined for spray-dried beetroot powders was in the range of 0.62–0.70 g/cm³ depending on the amount of carrier added (maltodextrin), the number of dryer disk revolutions, and the inlet air temperature. Janiszewska and Włodarczyk [51] spray-dried beetroots using dehumidified air and a drying carrier (maltodextrin). Depending on the temperature and feed rate, they achieved a loose density from 0.41 to 0.61 g/cm³.

3.3.6. Color of Dried Juice and Pomace

The L* parameter values ranged from 22.62 to 23.88 for the beetroot powders (Table 4). Variants DASDJC0 and DASDJC1 differed from each other, but DASDJC2 was similar to the control sample (DASDJC0), which was not subjected to PEF pretreatment. The a* parameter values ranged from 34.48 to 42.00, while the b* parameter values ranged from 7.62 to 11.52. It was noted that PEF treatment significantly increased the a* and b* color parameters, which translated to a higher share of red and yellow pigments in the powders that were treated with more pulses. This supports the idea that PEF pretreatment led to more efficient extraction of betalains responsible for color as a consequence of electroporation.

The freeze-dried beetroot pomace (FDP) was characterized by L* parameter values ranging from 42.26 to 42.92 (Table 4). The color parameter a* ranged from 26.70 to 27.16, with the highest value for variant FDP1 and the lowest for FDP0. Variant FDP2 was similar to the others regarding the share of red color. As for the b* parameter, there was no difference between the control sample (FDP0) and the variant treated with the highest amount of pulses (FDP2). However, the samples treated with the lowest amount of pulses (FDP1) were characterized by a larger share of blue color than the FDP0 and FDP2 variants. This study did not confirm the previous research on freeze-dried beetroots pretreated with a PEF conducted by Ammelt et al. [52], who observed that the samples treated with a PEF were darker in color than the untreated ones, while in our study, no differences were observed. Moreover, they noted more reddish freeze-dried beetroots which were pretreated with a PEF prior to drying.

3.3.7. Polyphenol Content in Dried Juice and Pomace

The spray-dried powders had an almost approximately 2–3 times higher polyphenol content compared to the juices (Table 2). They contained from 191% to 288% more polyphenols in terms of chlorogenic acid than the juices from which they were obtained. The highest value was recorded for the control sample (DASDJC0), 2070 mg chlorogenic acid/100 g d.m., and the lowest value was obtained for sample DASDJC0 treated with a PEF with 3 pulses (1479 mg chlorogenic acid/100 g d.m.).

The lyophilized pomace (FDP), similarly to the juices, belonged to one homologous group, showing no statistical differences. The range for this group was 1523–1705 mg chlorogenic acid/100 g d.m. These values were lower than in the case of the spray-dried juices, but still about 2 times higher than in the case of the initial juices.

It is worth noting that after the PEF pretreatment of beetroot tissue, there is cell membrane pervaporation, which results in greater solvent penetration and more polyphenols passing into the extract. At the same time, no such correlations in the case of powders and juices can be explained by the fact that these are different materials, and they react with Folin’s reagent in different ways. Folin’s reagent reacts with a wide variety of compounds [53]. In powders, the drying carrier could react with Folin’s reagent. Beetroot juice also reacts differently with Folin’s reagent than cut fresh beetroots.

3.3.8. Content of Betalain Pigments in Dried Juice and Pomace

The content of betanin pigments in the spray-dried juices (DASDJC) was in the range of 418–496 mg/100 g d.m., and the content of vulgaxanthins was 292–400 mg/100 g d.m. (Table 2). In the case of freeze-dried samples (FDP), the betanin content varied from 542 to 619 mg/100 g d.m, and vulgaxanthins ranged from 241 to 285 mg/100 g d.m. The degradation of pigments was observed in all of the dried samples regardless of the method used, unlike in the raw materials. However, considering that the freeze-dried pomace, from which some betalains were removed during processing along with the juice, it can be stated that the freeze-drying process itself caused smaller losses of these compounds. Moreover, regardless of the method, the drying process caused greater losses of red pigments than yellow ones. It can be observed that in the spray-dried samples, both betanin and vulgaxanthin pigments were the highest in the variants that were not pretreated with a PEF (DASDJC0) and when the highest amount of pulses was applied (DASDJC2). Concerning the freeze-dried samples, it was noted that PEF pretreatment did not influence the betalain content. This study does not support previous research in this area. In fact, Ammelt et al. [52], who freeze-dried beetroots pretreated with a PEF, noted that the samples that were treated with a PEF showed a higher pigment yield, which was the effect of increased permeability of the cells.

3.3.9. Antioxidant Activity in Dried Juice and Pomace

Spray drying caused a decrease in the antioxidant capacity of powders from 4.5 (variants J0 and DASDJC0, no PEF treatment) to 18 times (variants J1 and DASDJC1, 3 PEF pulses) compared to raw juice. Kidoń and Czapski [54] observed the opposite relationship—convective drying of beetroots caused an increase in antioxidant capacity.

An increase in the free radical binding capacity after PEF treatment was confirmed in the case of dried pomace. The control samples of pomace (FDP0) showed the lowest antioxidant properties. The results obtained for the materials treated with a PEF were lower (therefore, more free radicals were neutralized by the materials), although the FDP2 samples were statistically similar to the control samples (FDP0).

4. Conclusions

In this study, a statistically significant increase in polyphenols was found in fresh beetroots as a result of PEF treatment, but this was not confirmed in the case of other materials. In the case of juices and beetroots, an increase in the content of pigments was found as a result of the use of PEF treatment; however, this trend was not observed in the case of powders.

As for the powders, it should be emphasized that it was possible to obtain beetroot powders by dehumidified air-spray drying at low temperatures using a low content of fiber as a drying carrier (the ratio of dry juice mass to dry carrier mass was 75 to 25). The use of a PEF, regardless of the variant, significantly reduced the spray drying efficiency, but the process was still characterized by high efficiency (over 70%). Moreover, the particle size and the average particle size distribution decreased after PEF pretreatment.

Both the powders and the pomaces obtained from the beetroots subjected to a higher-intensity PEF treatment (44 pulses and the total energy of 4 kJ) were characterized by the highest hygroscopicity.

When taking into consideration the future application, the powders obtained with a low carrier content as a consequence of dehumidified application make it possible to reduce the dosage of the finished product. This is particularly important in the case of beetroot powders, which are used as natural dyes.