Modelling and Evaluation of the Effect of Pulsed Electric Fields and High Pressure Processing Conditions on the Quality Parameters of Osmotically Dehydrated Tomatoes

Abstract

This study explores the osmotic dehydration (OD) of fresh-cut cherry tomatoes through the application of Pulsed Electric Fields (PEF) and High-Pressure (HP) pre-treatments. Untreated, PEF-treated (1.8 kV/cm, 0–300 pulses), and HP-treated (0–600 MPa, 5 min) tomatoes were subjected to osmotic dehydration at 35 °C for up to 3 h. The results reveal that a 100-pulse PEF treatment and HP treatment at 600 MPa yielded optimal outcomes in terms of both OD enhancement (with effective moisture diffusion coefficients of 7.91 · 10⁻¹⁰ m²/s for PEF and 7.40 · 10⁻¹⁰ m²/s for HP-treated tomatoes compared to 5.17 · 10⁻¹⁰ m²/s for untreated samples) and product acceptability (achieving overall acceptance scores between 7 and 8). Applying PEF (100 pulses) and HP (600 MPa) pre-treatments reduced the water activity (a_w) to 0.887 and 0.760, respectively, after 3 h of OD, compared to a_w = 0.923 for untreated OD samples. The selection of these pre-treatment conditions enabled effective dehydration and quality retention, extending the shelf life by up to 40 days under chilled storage.

1. Introduction

Osmotic dehydration (OD) is a well-established food processing technique involving the immersion of food items in a hypertonic solution. Key process parameters include treatment temperature, duration, solid/liquid ratio, and the properties of the osmotic solution, such as water activity and formulation [1,2]. The selection of appropriate conditions depends on achieving effective dehydration within a reasonably short time while preserving the quality characteristics of the food product. In addition, the combination of mild OD conditions and the selective enrichment of the osmosed tissue with solutes of the osmotic solution (humectants, including carbohydrates, salts and other small molecular weight compounds, novel added-value ingredients, such as food ingredients with functional properties, e.g., trehalose, fructo-oligosaccharides, galacto-oligosaccharides, and food industry by-products e.g., strained yoghurt whey, sugar beet molasses) can produce a modified food commodity with the desired quality attributes [3].

Tomatoes, a globally significant agricultural commodity, are prized for their high nutritional value and appealing sensory attributes, and are commonly consumed fresh or in processed forms like dried tomatoes. The appeal of tomatoes hinges largely on their unique texture and palatability, qualities susceptible to damage during processing [4]. The challenge lies in the waxy cuticle on the tomato peel, which acts as a substantial barrier to water transfer, impeding moisture removal during OD. Since tomatoes are often utilized in their cut form in ready-to-eat products, OD is preferably performed on cut vegetables. Nevertheless, moisture transfer rates can be limited, resulting in prolonged dehydration times [5,6]. To address this, suitable pre-treatments that enhance water transfer can be employed to improve the outcomes of the OD process.

Pulsed Electric Fields (PEF) has emerged as a significant pre-treatment method for traditional dehydration processes [7,8]. Its mechanism of action involves electroporation, wherein an external electric field induces the formation of pores on cell membranes and cell walls. This phenomenon increases cell permeability, facilitating the movement of intracellular material, primarily water, to the extracellular matrix [9]. While this acceleration of water loss significantly expedites dehydration, it can also lead to a loss of cell turgor pressure, resulting in tissue softening. Consequently, one common side effect of PEF treatment is the textural deterioration of plant tissues [10]. In the case of tomatoes, this can include peel detachment, which, while beneficial in some applications, is highly undesirable in the present application [11,12]. Therefore, selecting appropriate PEF treatment conditions that balance effective water removal with minimal textural alteration is essential. The degree of plant cell disruption caused by PEF treatment is often assessed based on physicochemical parameters influenced by the treatment, with plant tissue firmness serving as a reliable indicator of the percentage of cells that have undergone electroporation. As an initial step, we sought to determine the range of PEF treatment conditions that effectively soften tomato tissue and establish the maximum specific energy input that plant tissue can tolerate.

High Hydrostatic Pressure (HP), ranging from 100 to 600 MPa, is another technology that significantly disrupts plant cells, leading to the loss of intracellular water during subsequent processing [1,13,14]. HP treatment also affects the activity of endogenous pectic enzymes, particularly Pectin Methyl-esterase (PME). In natural conditions, these enzymes work in tandem to hydrolyze pectic polysaccharides, leading to tissue softening. When cells are disrupted, this process is accelerated. For tomatoes, it has been shown that treatment pressures exceeding 500 MPa inactivate PG while leaving PME activity relatively unaffected. When PME acts alone, it releases free carboxyl groups along pectin polysaccharide chains, promoting cross-linkages between polysaccharides and increasing tissue firmness [15,16]. Consequently, selecting appropriate HP treatment conditions must strike a balance between effective dehydration and texture retention.

In summary, OD is a widely employed technique for extending the shelf life of various food materials, including fruits and vegetables. However, cell disruption technologies like PEF and HP have the potential to enhance OD by increasing cell permeability (both PEF and HP) and enzyme inactivation (HP only). Despite this potential, research on the effect of PEF as an OD pre-treatment for plant tissues is limited [17,18], and the application of HP as an OD pre-treatment remains largely unexplored. High-intensity treatments can lead to excessive disruption of plant tissues, resulting in unacceptable products due to textural deterioration. Hence, a comprehensive investigation into the impact of these cell disruption technologies on OD is of high importance. This study aims to evaluate the effect of PEF and HP treatment conditions on the quality parameters, both sensory and physicochemical, of osmotically dehydrated tomatoes. Successfully dehydrated tomatoes could potentially find application in ready-to-eat products with an extended shelf life.

2. Materials and Methods

2.1. Raw Material Selection

Cherry tomatoes were cultivated hydroponically in western Greece (Bousi-Valtoulia, Mesolongi). On each batch of tomatoes received, a preliminary characterization of the raw material was performed to establish the natural variability in quality parameters. Tomatoes were characterized in terms of size, total soluble solids (TSS) and CIELab color (color parameter a/b). Tomatoes were homogeneous in size, exhibiting an equatorial diameter in the range of 2.5 to 3.0 cm. Total soluble solids ranged between 6.5 and 7.5 °Bx while the color parameter a/b was within acceptable limits of ripeness, exceeding the value 0.5 [19].

2.2. Selection of Appropriate PEF Pre-Treatment Conditions

Prior to the application of PEF pre-treatment to OD, a screening of a wide array of treatment conditions was tested, in order to establish the relationship between PEF treatment and quality deterioration. PEF treatments were performed on whole tomatoes submerged in tap water at an electric field strength of 1.8 kV/cm and a number of pulses ranging from 0 (untreated) to 1000, with specific energy inputs up to 12 kJ/kg, respectively. The selection of this treatment condition range was based on previous studies of the cell disintegration index in whole tomatoes [11]. Treatments were performed using the Elcrack-5 kW device (DIL, Quackenbrück, Germany) using a stainless-steel treatment chamber with an electrode gap of 8 cm and a total volume of 300 mL. The pulse width and pulse frequency were fixed at 15 μs and 20 Hz, respectively. After each treatment, color, firmness and sensory attributes were determined (Section 2.4). Based on the results of this experiment, suitable PEF treatment conditions were chosen such that adequate tissue alteration would occur, contributing to an acceleration in dehydration rates but not leading to unacceptable textural degradation.

2.3. Selection of Appropriate HP Pre-Treatment Conditions

For HP pre-treatment, whole tomatoes (approximately 200 g) were packaged in polyethylene bags filled with water, to ensure a homogeneous application of pressure. High Pressure treatments were performed using a pilot scale HP device capable of achieving pressures up to 1000 MPa and temperature up to 90 °C (Food Pressure Unit FPU 1.01, Resato International BV, Roden, Holland), using polyethylene glycol as a pressure transfer medium in a pressure vessel with a total volume of 1.5 L. The heating rate of the samples inside the HP vessel due to adiabatic heating during pressurization was measured via a CR1000 thermocouple data logger system (Campbell Scientific, Logan, UT, USA) connected to the vessel and was equal to 3 °C per 100 MPa. This temperature increase was quickly equilibrated through the temperature control system described above, with a cooling rate equal to about 5 °C/min for the large pressure vessel and 20 °C/min for the small pressure vessels. The pressurization rate achieved with the HP system used in the present study was 13 MPa/s. Depressurization of the chamber was achieved almost instantly (1–3 s). Samples were treated at 100, 200, 400 and 600 MPa for 5 min. These treatment conditions were selected based on the results of our previous study where tomato enzyme HP inactivation was explored in depth [16]. The following treatments, for enzyme activity (PME, PG) and firmness, were performed based on the protocols described in Section 2.4.

2.4. Osmotic Dehydration of Untreated and PEF/HP Pretreated Samples

Untreated, PEF- and HP-treated samples were osmotically dehydrated. The osmotic dehydration medium contained 10% w/w vinegar (8% acetic acid), 10% w/w maltodextrin (DE 47), 3.5% sodium chloride, 1.5% calcium chloride and glycerol as the main dehydrating component, at concentrations ranging from 50 to 70% w/w [20]. Vinegar serves as a flavoring agent and also contributes to the reduction of the treated food’s pH and thus to shelf-life extension. Maltodextrin increases the solution’s dehydration capacity, together with glycerol. Glycerol acts as a humectant and plasticizer, enabling the preservation of the food’s moist appearance and mouthfeel. Sodium chloride enhances mass transfer, and calcium chloride significantly contributes to the preservation of plant tissue texture due to its interaction with endogenous pectic substances. The temperature of osmotic dehydration was fixed at 35 °C, a temperature that strikes a balance between acceleration of water transfer and minimal pectic enzyme activity, which would eventually lead to loss of firmness. Halved tomatoes were pre-weighed (approximately 2–5 g) and placed in cylindrical glass containers (Ø8.5 × 13 cm). The containers were filled with preheated osmotic medium to yield a liquid to solid ratio of 5:1. Containers were placed in a thermo-stated water bath under agitation (240 rpm). At selected times (30, 60, 90, 120, 150 min) triplicate samples were removed from the osmotic solution, blotted gently to remove the excess coating solution and weighed. During osmotic dehydration, water content, water activity, sensory evaluation and quality indices were examined.

2.4.1. Determination of Water Loss and Solids Gain and Mathematical Modelling of Osmotic Dehydration

For each sample, both water loss (WL) and solids gain (SG) during osmotic dehydration were determined gravimetrically after drying at 105 °C for 24 h. Water loss and solids gain represent the mass of initial water that is lost during dehydration and the mass of solids taken up by the sample during osmotic dehydration, respectively, calculated from the following equations (Equations (1) and (2)):

$$WL=\frac{m_0-m_0·D{W}_{wb}-(m_{wet}-m_{dry})}{m_0·D{W}_{wb}}$$

(1)

$$SG=\frac{m_{dry}-D{W}_{wb}·m_0}{D{W}_{wb}·m_0}$$

(2)

where m₀ is the initial wet weight of the sample before submersion in the osmotic medium, DW_wb is the dry weight of the untreated sample in g water/g sample wet base, m_wet is the wet weight of the treated sample in g and m_dry is the dry weight of the treated sample in g.

Osmotic dehydration is based on the diffusion of water from the plant tissue towards the osmotic medium and the diffusion of solids towards the tissue. For this reason, the kinetics of water loss and solids gain are commonly described mathematically by the solution of Fick’s second law of diffusion. This mathematical model is usually applied as the following infinite series (Equation (3)):

$$\frac{X-X_0}{X_{\infty }-X_0}=\frac{8}{{\pi }^2}{\sum }_{n=0}^{\infty }\frac{1}{{\left(2n+1\right)}^2}{e}^{-(2n+1)·\frac{{\pi }^2{D}_{eff}t}{L^2}}$$

(3)

where X is the Water Loss (WL) or Solids Gain (SG) at time t, X₀ is the corresponding values at t = 0, X_∞ is the corresponding values for long dehydration times, L is the mean thickness of the hemispherical cut tomato morsel assuming an infinite slab, and D_eff is the effective diffusion coefficient. For very long dehydration times, only one term of the series is adequate to model the process, in which case the equation is simplified as follows [21] (Equation (4)):

$$1-\frac{X}{X_{\infty }}=\frac{8}{{\pi }^2}exp\left(-\frac{{\pi }^2{D}_{eff}t}{L^2}\right)$$

(4)

In the case of halved cherry tomatoes, moisture diffusion occurs from the cut side, while the peel side exhibits negligible mass transfer. Therefore, the diffusion of moisture from the tomatoes was adequately described by the infinite slab model. The effective diffusion coefficients D_eff for moisture and solids were determined by nonlinear regression for the experimental data using the SPSS version 19 software package (IBM, Armonk, NY, USA). The dehydration efficiency index (DEI), defined as the ratio of diffusivity of water (D_eff for moisture) to diffusivity of solute (D_eff for solids), was calculated.

2.4.2. Determination of Physicochemical Parameters

Total soluble solids (TSS) were measured on homogenized samples using a benchtop Abbe refractometer and were expressed as °Bx. Water activity was measured using a portable water activity meter (Rotronic AM3 Hygrometer, Bassersdorf, Switzerland). Prior to measurement, all samples were equilibrated at room temperature. Readings were obtained after adequate equilibration of the samples in the device after 15–20 min.

2.4.3. Determination of Tomato Firmness

Firmness was measured according to the modified protocol established by Artés et al. [22]. Tomatoes were compressed to a deformation of 20% by a cylindrical probe (diameter of 1 cm) fitted on a Texture analyzer (TA-XT2i, Stable Micro Systems, Godalming, UK). Compression was performed at a probe speed of 0.1 mm/s and firmness was recorded as the maximum compression force. At least 5 replicates were performed per measurement. Halved tomatoes were placed on the instrument pedestal with the cut face facing downwards and were compressed by applying force to the intact side.

2.4.4. Determination of Objective Color

Objective color was measured on the surface of the tomatoes using an Xrite-i1 portable digital colorimeter (Gretag-Macbeth, Grand Rapids, MI, USA) and expressed in the CIE-Lab scale. Color was measured at three equidistant equatorial points on the fruits on at least five replicates. Color difference between treated and untreated samples was expressed using the total color difference ΔΕ [23], given by the following equation (Equation (5)):

$$\Delta E=\sqrt{{\left(a-a_0\right)}^2+{\left(b-b_0\right)}^2+{\left(L-L_0\right)}^2}$$

(5)

where L, a, b are the measured CIELab color parameters and L₀, a₀, b₀ are the measured color parameters of a reference sample.

2.4.5. Determination of PME and PG Activity

Pectin Methyl-esterase (PME) activity was measured as described by Andreou et al. [14] based on the titration of the pH drop caused by PME on a pectin substrate, using a Radiometer Analytical TIM854 auto-titrator (Lyon, France). For the measurement, an apple pectin substrate (1% w/v, 1.2 M NaCl) was used. Fifty millilitres of substrate were placed in a thermostatic (30 °C) cup and placed on the auto-titrator. After addition of 2 mL of appropriately diluted tomato puree, the sample was titrated for 6 min to a constant pH of 7.5 using a 0.02 N NaOH titrant. The slope of titrant consumption was recorded (mL/min) and was subsequently converted to enzyme units. One unit of enzyme requires the addition of 1 μmol of titrant per minute to achieve a constant pH of 7.5 and is calculated according to the following equation (Equation (6)):

$$\frac{\mathrm{U}}{\mathrm{m}\mathrm{L}}=\frac{20·q}{V}$$

(6)

where q is the volumetric flow of titrant in mL/min and V is the volume of the sample.

For polygalacturonase (PG) activity, the method was according to that of Andreou et al. [16]. Fifty millilitres of homogenized tomato puree were centrifuged at 10,000 rpm for 10 min and the supernatant was replaced by cold distilled water (1:1) adjusted with 0.1 M HCl at pH 3.0 and homogenized for 30 min. After further centrifugation at 10,000 rpm for 10 min, the supernatant was discarded and PG was extracted from the pellets using 1.2 M NaCl (1:4) for 1 h. The mixture was recentrifuged at 10,000 rpm for 10 min and the supernatant was assayed for PG activity. All steps were performed at 4 °C. PG activity was measured using a spectrophotometric assay. One hundred microliters of the extracted enzyme solution were incubated with 300 μL of 0.2% w/v poly-galacturonic acid at 35 °C for 10 min. To stop the reaction, 2 mL of 0.1 M borate buffer, pH 9.0 and 400 mL of 1% w/v cyano-acetamide were added to the reaction mixture and boiled for 10 min. After cooling, the absorbance was measured at 276 nm. Blank samples were determined in the same way using extracts inactivated by heating (90 °C, 15 min).

2.4.6. Sensory Evaluation of Tomatoes

For all untreated and pre-treated tomatoes, sensory evaluation was carried out using a panel of 8 members. Tomatoes were scored by the panel for the characteristics: glossiness, deformation/shrinkage, peel detachment, characteristic tomato aroma, off-flavor intensity, bite hardness, crunchiness, characteristic tomato flavor and color, and total acceptance on a hedonic scale of 1–9 (Supplementary Table S1).

2.5. Shelf Life Determination of Untreated and Treated Tomato Samples

Untreated, OD-treated (180 min at 60% glycerol), PEF-OD-treated (100 pulses at 1.8 kV/cm, OD for 90 min at 60% glycerol) and HP-OD-treated (600 MPa for 5 min, OD for 90 min at 60% glycerol) halved tomatoes at conditions selected based on the experiments described in previous sections were subjected to shelf life determination. Approximately 100 g of each product were packed into polyethylene-polypropylene sachets (15 × 15 cm) and stored in the dark in incubators at constant temperatures between 0 °C and 25 °C. At regular time intervals samples were withdrawn and analyzed in terms of total aerobic viable counts (TVC) according to Dermesonlouoglou et al. [18], and sensory attributes based on the characteristics described in Section 2.4.6. Microbial growth versus storage time for each storage temperature was mathematically modelled using the Baranyi growth model [24]. The evolution of the sensory total acceptance versus storage time for each storage temperature was mathematically modelled using a simple zero order equation of the form S = S₀ − k_s t, where S is the sensory attribute at time t, S₀ is the sensory attribute at time zero and k_s the deterioration rate constant. The dependence of the rate constant and lag phase parameters of the Baranyi equation, as well as the dependence of the sensory deterioration rate constant from storage temperature, was mathematically modelled using the Arrhenius equation. Combining the mathematical models used, the following expressions for the shelf life were derived, similar to Dermesonlouoglou et al. [20]:

$$t_{S{L}_S}=\frac{S_0-S_L}{k_{ref,S}\exp \left(-\frac{E_{aS}}{R}\left(\frac{1}{T}-\frac{1}{T_{ref}}\right)\right)}$$

(7)

$$t_{S{L}_{MG}}=L\left(T\right)+\frac{\mathrm{l}\mathrm{o}\mathrm{g}{N}_L-\mathrm{l}\mathrm{o}\mathrm{g}{N}_0}{k_{ref,S}\exp \left(-\frac{E_{aS}}{R}\left(\frac{1}{T}-\frac{1}{T_{ref}}\right)\right)}$$

(8)

where $t_{S{L}_S}$ is the shelf life determined in terms of sensory deterioration, $t_{S{L}_{MG}}$ is the shelf life determined in terms of microbial growth, S₀ is the initial sensory score, S_L is the acceptance limit of the sensory score, logN_L is the limiting microbial population (set at 6.5 log(CFU/g), logN₀ the initial microbial population, R is the universal gas constant and E_a and k_ref the activation energy and rate constants at the reference temperature, respectively, for each measured parameter. In Equation (8), the parameter L(T) represents the dependence of the microbial growth lag phase from storage temperature via the Arrhenius equation. Since different processing conditions were expected to lead to different shelf life estimations based on the two parameters, both values were calculated. The parameter leading to the lowest shelf life estimate at each storage temperature was considered as the limiting parameter.

2.6. Statistical Analysis

Results were expressed as means ± standard deviation of three experimental replicates. For the estimation of the main interaction effects of the investigated factors, factorial analysis of variance (Factorial ANOVA) was used. As a post-hoc analysis for the separation of means with significant differences (p < 0.05), Duncan’s multiple range test was used. For all statistical analyses, the Statistica 7 software package was used.

3. Results

3.1. Selection of PEF Pre-Treatment Conditions for Tomatoes

The degree of electroporation caused by Pulsed Electric Fields (PEF) in plant tissues is well represented by the decrease in tissue firmness. Before the application of PEF as a pre-treatment to Osmotic Dehydration (OD), a wide array of treatment conditions (1.8 kV/cm field strength, 0–1000 pulses of 15 μs width and 20 Hz frequency) was applied in order to establish the limits within which the treatment remains effective, while over-processing is avoided. Figure 1a shows the dependence of tomato firmness on the number of applied pulses. At a number of pulses below 200, firmness drastically decreases as processing intensity increases. On the contrary, for a PEF treatment exceeding 500 pulses, tissue firmness does not significantly (p > 0.05) vary with the increase in process intensity. This signifies that a maximal level of electroporation has been achieved that cannot be exceeded with further increase in pulse delivery. Therefore, a treatment at 300 pulses is adequate to cause the highest attainable degree of electroporation at 1.8 kV/cm field strength. Increasing pulse number above this value leads to unnecessary energy expenditure.

The dependence of the total color difference of PEF treated tomatoes on the number of applied pulses is given in Figure 1b. According to the CIELab specification, a value of ΔΕ exceeding 2.5 corresponds to a color difference perceivable by the human eye [25]. PEF treatment did cause a slight discoloration of tomatoes as processing condition intensity increases. A loss of redness was also reported by the sensory evaluation, as described below. This effect may be attributed to the fact that electroporation also disrupts chromoplasts among other organelles, leading to a diffusion and perceived “dilution” of red pigments across the peel.

Figure 1c presents the dependence of the sensory acceptance on the number of applied pulses. An increase in processing condition intensity leads to deterioration of color, textural consistency and flavor. The value of 300 pulses can be considered the value above which neither textural properties (crispiness, bite hardness) nor color intensity further deteriorate. This outcome is not surprising since tissue softening and the repartitioning of water, solutes and pigments within the tissue are a natural consequence of plant cell electroporation. It is evident that the application of PEF alone on fresh tomatoes would lead to the production of an unacceptable product (overall acceptance falls below 5 for tomatoes treated at 300 pulses). Since PEF is to be applied as a pre-treatment to osmotic dehydration, it is expected that this deterioration will be offset by OD. This is based on two hypotheses: (a) the presence of calcium ions in the osmotic medium in combination with the increased cellular permeability of PEF treated tissues may increase tomato firmness due to the increased interaction of calcium ions with pectic substances within the cells; (b) the removal of water may restore part of the tissue’s firmness and concentrate red pigments leading to a brighter red color. Red pigments of tomato (mainly lycopene and β-carotene) are fat soluble and thus are not expected to be lost to the osmotic medium with the loss of water. From the results of this preliminary screening, PEF treatment conditions as a pre-treatment to OD were narrowed down to the following: 0, 20, 50, 100, 300 pulses at 1.8 kV/cm. Even though 300 pulses were recognized to be an intense treatment condition leading to an unacceptable final product, it was selected nonetheless to serve as a benchmark for effective dehydration.

3.2. Osmotic Dehydration of PEF Pre-Treated Tomatoes

3.2.1. Water Loss and Solids Gain of PEF Pre-Treated Tomatoes during OD

PEF pre-treatment had a significant (p < 0.05) effect on both water loss (WL) and solids gain (SG) during osmotic dehydration. Dehydration curves in terms of WL and SG for untreated and PEF treated samples are presented in Figure 2. Regarding untreated cut tomato morsels, WL and SG reached values up to 4.4 and 1.6 g/g DW, respectively. These results are in agreement with Dermesonlouoglou et al. [18], who reported a range of WL and SG during OD (60% glycerol, 35 °C) of tomato slices up to 9 and 3 g/g DW, respectively. The difference between the values reported in the literature may be due to the different geometric characteristics of the tomato samples. The diffusion from cut tomato morsels takes place only from one side (the area where the cherry tomato was cut). For tomato slices, the diffusion takes place from both sides, a fact that justifies that WL and SG for these samples were twice as high. Increasing PEF processing intensity up to 100 pulses led to an increase in WL and SG. However, a further increase in applied pulses did not lead to further acceleration of dehydration.

Fitting Fick’s law to the experimental data enabled the calculation of the effective diffusion coefficients (D_eff) for both moisture and solids (

Table 1. Effective moisture diffusion (D_eff for Water Loss) and solids diffusion (D_eff for Solids Gain) coefficients, and dehydration efficiency index (DEI) for different PEF treatment conditions as calculated from fitting Fick’s law to the experimental data.

Treatment	Deff Water Loss (m2/s · 10−10)	R2	Deff Solids Gain (m2/s · 10−10)	R2	DEI
Control	5.17 ± 0.69 a	0.942	3.82 ± 0.42 a	0.899	1.35 ± 0.03 a
20 pulses	6.16 ± 0.67 ab	0.945	4.10 ± 0.35 ab	0.855	1.46 ± 0.04 b
50 pulses	6.75 ± 0.52 bc	0.972	4.64 ± 0.39 b	0.896	1.50 ± 0.01 b
100 pulses	7.91 ± 0.62 c	0.982	4.52 ± 0.40 b	0.820	1.62 ± 0.02 c
300 pulses	7.40 ± 0.54 c	0.951	4.58 ± 0.43 b	0.871	1.75 ± 0.03 d

Different superscript letters indicate significant differences between means D_eff ± standard deviation and DEI ± standard deviation as calculated by Duncan’s multiple range test for a significance level of p = 0.05.

). The effective moisture diffusion coefficient exhibited a significant increase of up to 53% for WL and 20% for SG by applying PEF treatment of 100 pulses. This observation corresponds to an accelerated dehydration caused by the varying degrees of electroporation achieved. Calculated DEI values also indicated that WL was higher than SG, and enhanced by the electroporation achieved (from 20–50 to 300 pulses) (Table 1). Dermesonlouoglou et al. [17] reported similar results for OD (30% glycerol, 35 °C) of untreated and PEF-treated kiwifruit disks. Specifically, PEF treatment of kiwifruit at 1.8 kV/cm for 250 pulses increased D_eff for moisture and solids by 120% and 46%, respectively. In another work, Dermesonlouoglou et al. [18] reported that PEF pre-treatment of goji berry (2.8 kV/cm, 750 pulses) significantly increased D_eff for WL from 0.28 × 10⁻⁹ to 1.81 × 10⁻⁹ m²/s and for SG from 0.95 × 10⁻⁹ to 1.81 × 10⁻⁹ m²/s after OD with 60% glycerol at 55 °C. Increasing the applied pulses from 100 to 300 did not further increase the moisture diffusion coefficient, signifying that the maximum obtainable degree of electroporation at an electric field of 1.8 kV/cm has been already attained. This observation is also consistent with the results pertaining to tissue softening (decrease of firmness).

3.2.2. Water Activity, pH and Titratable Acidity of PEF Treated Tomatoes during OD

The results concerning water activity of untreated and PEF-treated tomato samples are presented in Figure 3a. Pulsed Electric Fields treatment led to a significant change in a_w evolution only at treatments exceeding 50 pulses. At 100 and 300 pulses, initial water activity (after treatment and instantaneous submersion in the osmotic medium) is significantly (p < 0.05) lower than the corresponding value of the untreated sample. This observation signifies an instantaneous release of free water immediately after processing, caused by electroporation. A swift submersion in the osmotic medium is adequate to remove this released free water and decrease water activity. Although, overall water activity values are lower at more intense processing conditions, the temporal evolution (rate of a_w decrease) was not affected by the treatments. Finally, a treatment intensity exceeding 100 pulses did not lead to a significant (p > 0.05) reduction in water activity during the course of dehydration. Water activity values below 0.91 are commonly adopted as minimum values where bacterial proliferation is significantly reduced [26]. In combination with a low pH value (less than 4.6), OD of PEF treated tomatoes at 100 pulses for 120 min or more is suitable for obtaining a product of high quality (Figure 3b).

3.2.3. Objective Firmness and Color of PEF Treated Tomatoes during OD

The evolution of objective firmness and total color difference ΔΕ of untreated and PEF-treated samples during OD is presented in Figure 3c,d, respectively. Firmness values are expressed as relative firmness compared to the untreated sample at 0 min of osmotic dehydration. Although water is lost during OD and pectic substances form gels with calcium ions from the osmotic medium, both untreated and PEF treated samples exhibited a decrease in objective firmness during OD. For a treatment at 300 pulses and an OD time exceeding 60 min, firmness has decreased by 50% compared to the untreated sample. Color difference is calculated with regards to the untreated sample at OD time 0 min. A perceivable color difference is achieved for the untreated sample as OD progresses, attributed to the increase in redness (a parameter) due to water loss. PEF-treated samples exhibited an almost constant color difference immediately after processing, which is also maintained during OD, regardless of processing intensity.

3.2.4. Sensory Evaluation of PEF Treated Tomatoes during OD

The results for total sensory evaluation acceptance of untreated and PEF-treated tomatoes during OD are presented in Figure 3e. It was observed that the perceived intensity of glossiness and deformation (Supplementary Figure S1) exhibit an inverse trend: as OD time increases, deformation increases and glossiness decreases due to the shriveling of tomatoes, due in turn to dehydration. Glossiness did not differ between PEF treatment conditions for all OD times, while deformation was more intensely perceived at a treatment of 300 pulses, attributed to the extensive degree of cell disintegration. A slight increase in the perceived peel detachment was observed as OD time increases, for all PEF treatment conditions studied. No treatment condition led to unacceptable levels of peel detachment. Color intensity remained relatively constant during OD for all treatment conditions studied. This is in contrast to the determination of the objective color, where color difference was determined to be noticeable. It is possible that shrinkage and glossiness caused by the osmotic medium masks the perception of this color difference. In terms of tomato flavor and other flavor intensity, a slight loss of tomato flavor was observed as OD progresses, accompanied by an increase in the intensity of other flavors perceived. The panel reported an increase in vinegar aroma with the increase of OD time, which is consistent with the decrease of pH, as discussed above. A marked improvement was observed for bite hardness and crunchiness of the samples. Immediately after processing (OD time 0), both characteristics deteriorate with increasing PEF processing intensity. However, as OD progresses, this deterioration is reversed. This is attributed to the increase of sample cohesiveness caused by the interaction of endogenous pectins with the calcium ions present in the OD medium. Although this is not reflected in the measurements of objective firmness discussed in Section 3.1, it seems that perceived texture cannot be fully represented by firmness alone, as the complex physicochemical interactions taking place within the tissue leads to the improvement of other textural characteristics not detectable by the measurement protocol used. All samples exhibited a high degree of overall acceptance (7–8), regardless of OD time or PEF processing conditions, leading to the conclusion that PEF as a pre-treatment to OD does not lead to quality deterioration.

3.3. Selection of HP Pre-Treatment Conditions for OD of Tomatoes

The results for the selection of HP pre-treatment conditions in terms of enzyme activity and relative firmness are presented in Figure 4. Pressure treatment time was kept constant for all experiments and equal to 5 min. Increasing treatment pressure up to 400 MPa causes significant (p < 0.05) decrease in firmness, due to the increasing levels of cell disruption caused by the pressure. Enzyme activity for both PME and PG remains unaffected. However, a treatment at 600 MPa, causes a significant (p < 0.05) restoration in firmness, accompanied by complete inactivation of PG and retention of PME activity. As discussed earlier, PG is a lytic enzyme which depolymerizes pectic substances, leading to textural deterioration. Its inactivation leaves PME to de-esterify pectin side groups, leaving them available to be crosslinked via reaction with calcium ions. At the same time, a pressure of 600 MPa causes a maximal degree of cellular disruption.

Regarding sensorial characteristics, the most salient deterioration is the perception of peel detachment at a treatment pressure of 400 MPa. The intense degree of cellular disruption caused by HP in combination with the action of pectic enzymes causes an almost complete detachment of the peel, leading to an unacceptable appearance. Both bite hardness and crunchiness decreased with increasing treatment pressure up to 400 MPa but were restored at 600 MPa to levels comparable to those of the untreated sample. This is consistent with the measurements of objective firmness discussed above. Although treatment at 600 MPa causes a high degree of cell disruption, the concomitant inactivation of PG prevents the hydrolysis of pectin and preserves firmness. Based on the intense degradation observed at 400 MPa, treatment at this level was excluded as a viable pre-treatment for OD.

3.4. Osmotic Dehydration of PEF Pre-Treated Tomatoes

3.4.1. Water Loss and Solids Gain of HP Pre-Treated Tomatoes during OD

The evolution of water loss and solids gain during OD of untreated and HP pretreated tomatoes are presented below in Figure 5.

Fitting Fick’s law to the experimental data for both WL and SG enabled the calculation of the effective diffusion coefficients, presented in Figure 5 and

Table 2. Effective moisture diffusion (D_eff for Water Loss) and solids diffusion (D_eff for Solids Gain) coefficients, and dehydration efficiency index (DEI) for different HP treatment conditions as calculated from fitting Fick’s law to the experimental data.

Treatment	Deff Water Loss (m2/s · 10−10)	R2	Deff Solids Gain (m2/s · 10−10)	R2	DEI
Control	5.17 ± 0.69 a	0.955	3.82 ± 0.42 ab	0.920	1.35 ± 0.03 a
100 MPa/5 min	5.74 ± 0.67 a	0.975	3.50 ± 0.70 a	0.855	1.64 ± 0.14 b
200 MPa/5 min	6.15 ± 0.52 a	0.966	4.61 ± 0.79 ab	0.896	1.33 ± 0.12 a
600 MPa/5 min	7.40 ± 0.54 b	0.966	5.07 ± 0.85 bc	0.874	1.46 ± 0.14 ab

Different superscript letters indicate significant differences between means D_eff ± standard deviation and DEI ± standard deviation as calculated by Duncan’s multiple range test for a significance level of p = 0.05.

(as well as the dehydration efficiency index). As was observed for PEF pretreated tomatoes, increasing processing intensity (treatment pressure in the case of HP) led to a progressive increase of the diffusion coefficients.

3.4.2. Water Activity, pH and Titratable Acidity of PEF Treated Tomatoes during OD

The evolution of water activity of untreated and HP-treated samples during OD is presented in Figure 6a. It was observed that increasing treatment pressure led to a decrease in water activity during OD. A treatment at 600 MPa leads to a drastic decrease in water activity from 0.93 down to 0.76 after 150 min of OD. At 600 MPa, water activity drops below 0.90 after only 40 min of OD, whereas the same value is achieved for the untreated sample at OD times exceeding 3 h. The evolution of pH values for untreated and HP pretreated tomatoes during OD are presented in Figure 6b. Increasing treatment pressure led to a decrease in pH values over the course of OD.

3.4.3. Objective Firmness and Color of HP Treated Tomatoes during OD

Objective firmness and total color difference of untreated and HP-treated tomatoes during OD is presented in Figure 6c,d, respectively. As mentioned earlier, OD alone causes a certain degree of tissue softening. This effect is enhanced as treatment pressure increases. At 600 MPa, firmness at t = 0 of OD is restored to 70% of the untreated sample. The restoration of firmness at pressures over 500 MPa has already been discussed and attributed to the selective inactivation of pectic enzymes. As with PEF pre-treatment, total color difference increases with increasing processing intensity and does not vary with OD time, in contrast to the untreated sample. This color difference was attributed to an initial decrease in redness (CIELab-a parameter) due to HP treatment.

3.4.4. Sensory Evaluation of HP Treated Tomatoes during OD

The results of the total sensory evaluation of untreated and HP treated tomatoes during OD is presented in Figure 6e. The intensity of perceived deformation increases with increasing OD time as tomatoes start to shrivel due to reduction of water content (Supplementary Figure S2). Interestingly, perceived acidity does not change over OD for untreated and HP treated samples at 600 MPa, signifying that the flavor of vinegar is masked by the presence of glycerol. This underlines the successful selection of the OD medium formulation. However, the intensity of other flavors increases over OD time for all samples, as panelists report a slight loss of tomato aroma and the flavor of vinegar. Nevertheless, total acceptability of treated samples is higher at 3 h of OD for samples treated at 600 MPa, compared to OD time 0 min. Bite firmness and crunchiness, although severely deteriorating immediately after processing at all treatment pressures, are both restored to the levels of the untreated sample at 3 h of OD.

3.4.5. Selection of the Appropriate Treatment Conditions

The analysis provided above has explored a wide range of treatment conditions for PEF and HP as a pre-treatment to osmotic dehydration. From all the results presented, the most suitable PEF treatment conditions are 100 pulses (15 μs pulse width, 1.2 kJ/kg specific energy input) at an electric field strength of 1.8 kV/cm. At these conditions dehydration is accelerated, and quality characteristics of the product are retained. In terms of the HP treatment conditions tested, the condition which leads to the best dehydration outcomes while preserving the quality parameters of the tomatoes is 600 MPa-5 min. Both treatments achieve a significant acceleration in water loss and can reduce the water activity of the OD treated products below 0.9. Also, this effect is achieved at significantly shorter dehydration times. Therefore, in terms of subsequent dehydration conditions, an OD duration of 90 min was selected for both HP and PEF treated samples with an osmotic solution containing 60% glycerol, while untreated samples require an OD time of 180 min. Although an in-depth cost and equipment analysis is beyond the scope of this work, both processes are expected to be applicable to industrial treatment of whole tomatoes, since there are industrial systems available that can process high product throughputs [27,28]. The selected treatment conditions fall very well within the processing capabilities of such equipment. In an industrial setting, the nonthermal pre-treatment of the whole tomatoes would take place regardless of the subsequent dehydration steps. Both treatments can be performed on whole fruits after the washing step, as they both require the submersion of the fruits in processed water.

3.5. Shelf Life of Untreated and Treated Tomato Samples

The dependence of shelf life on storage temperature based on microbial growth and sensory deterioration (overall acceptance) as calculated from Equations (7) and (8) is presented in Figure 7a,b, for untreated, OD-treated, PEF-OD-treated and HP-OD-treated samples. A marked difference can be observed overall between untreated tomatoes and tomatoes that have undergone osmotic dehydration. For both characteristics, the shelf life curves are shifted towards higher storage temperatures, indicating that all treated products are significantly less perishable compared to the fresh-cut produce.

Based on the calculation of shelf life from the two quality parameters (sensory deterioration and microbial growth), it was evident for all samples that the defining characteristic for shelf life was sensory acceptance. The shelf life for each final product and each storage temperature is presented in

Table 3. Shelf life of untreated (Control), osmo-dehydrated (OD) and PEF- and HP-treated and osmo-dehydrated (PEF-OD and HP-OD, respectively) halved tomatoes at different storage temperatures.

T (°C)	Control	OD	PEF-OD	HP-OD
0	44.7 ± 3.2 j	-	-	-
5	9.8 ± 0.6 bc	37.8 ± 3.9 i	44.8 ± 4.5 j	50.4 ± 5.3 k
10	4.2 ± 0.1 a	22.6 ± 3.2 fg	26.3 ± 3.6 gh	29.6 ± 3.0 h
15	-	10.9 ± 3.1 cd	15.7 ± 2.7 de	17.7 ± 1.9 ef
25	-	5.0 ± 1.1 ab	5.8 ± 1.7 abc	6.7 ± 0.7 abc

Different superscript letters indicate significant differences between means of shelf life ± standard deviation as calculated by Duncan’s multiple range test for a significance level of p = 0.05.

. At typical chilled storage temperatures (5 °C), it was observed that all OD treated samples exhibited a significant (p < 0.05) shelf life extension by up to four-fold. The increase in shelf life of nonthermally treated samples could be attributed to the solids uptake, lower water activity and pH reduction. It must also be noted that the required duration of the OD process for PEF and HP pretreated samples was half (90 min vs. 180 min) compared to the untreated samples which were not able to achieve adequate moisture loss even after 3 h of OD. Therefore, apart from offering a significant shelf life extension, nonthermal treatments can have a profound impact on the duration of dehydration, possibly leading to energy savings and increases in productivity. The increased resilience of the product’s quality at higher temperatures means that food waste could be reduced along the cold chain, as the quality deterioration of the product would be greatly limited. These observations might also pave the way for incorporating fresh-like vegetables, such as cherry tomato, into long shelf life, ready-to-eat products which, without processing, would not be an option.

4. Conclusions

This study aimed to assess the feasibility of Pulsed Electric Fields (PEF), High Pressure (HP), and Osmotic Dehydration (OD) for enhancing the processing of cherry tomatoes with a focus on extending shelf life and improving quality attributes. After evaluating various parameters, the most suitable conditions were determined as following: PEF: 1.8 kV/cm, 100 pulses; HP: 600 MPa, 5 min; OD: 60% glycerol, 35 °C, 60 min. PEF and HP pre-treatments accelerated tomato dehydration by up to 90 min through electroporation of plant cells while preserving most of the measured quality attributes (color, texture, sensory acceptance) by affecting the inactivation of texture degrading Polygalacturonase (PG) (in the case of HP). All treatments demonstrated their ability to extend the shelf life of tomatoes by up to four-fold or, equivalently, by significantly raising the required storage temperature to achieve the same shelf life. Treated tomatoes can serve as convenient chilled snacks or versatile ingredients in various food formulations, such as ready to eat salads. This research presents opportunities to enhance tomato processing techniques and diversify their culinary uses, aligning with consumer preferences for high-quality, nutritious options with reduced energy consumption.