Optimization of Ohmic Heating Pasteurization for Passion Fruit Juice and Comparison with Conventional Thermal Treatment

Abstract

Ohmic heating pasteurization (OHP) is a volumetric thermal processing technology that enables rapid and uniform heat generation within liquid foods. This study aimed to optimize the OHP conditions for passion fruit juice processing by maximizing thermal efficiency and preserving quality attributes. A Box–Behnken design combined with response surface methodology was applied to evaluate the effects of pasteurization temperature (75–95 °C), holding time (15–45 s), and voltage gradient (10–30 V/cm) on the system performance coefficient (SPC), total color difference (ΔE), and vitamin C retention. Multi-response optimization was conducted to simultaneously enhance SPC and vitamin C content while minimizing color change. The optimal OHP conditions were identified as a temperature of 82.5 °C, a holding time of 25 s, and a voltage gradient of 18.5 V/cm. Under these conditions, the processed juice exhibited a high SPC value (0.80 ± 0.05), moderate vitamin C retention (20.07 ± 3.18 mg/100 mL), and low total color difference (ΔE = 7.64 ± 1.08). Enzymatic assays further demonstrated substantial inactivation of quality-related enzymes, with residual activities of peroxidase and polyphenol oxidase reduced to 8.07 ± 1.74% and 12.6 ± 2.1%, respectively, compared with untreated juice. These results indicate that optimized ohmic heating pasteurization can achieve efficient thermal processing while maintaining the physicochemical and nutritional quality of high-acid fruit juices.

1. Introduction

Pasteurization is a widely employed thermal technique in fruit juice processing, playing a crucial role in ensuring food safety, extending shelf life, and enhancing product stability. However, conventional pasteurization methods, such as conduction and convection, have notable drawbacks, including non-uniform heat distribution, extended processing times, and degradation of heat-sensitive nutrients. These limitations are particularly problematic in fruit juices, where excessive thermal exposure can lead to significant losses in vitamin C and adversely affect sensory attributes such as taste, color, and aroma, ultimately impacting consumer acceptance [1,2,3]. Consequently, increasing attention has been directed toward alternative heating technologies that can achieve rapid and uniform heat transfer while better preserving product quality [4].

There are many types of pasteurization processes, and the high-temperature short-duration (HTST) method is commonly used in processing juice as it heats quickly with minimal heat damage. The high temperature and short time exposure of HTST make it possible to reduce the heat damage to heat-sensitive compounds such as vitamins, pigments, flavors, etc., which can occur with lower-temperature processing for longer times. Yet despite these benefits, HTST is still an indirect heat transfer process and can lead to uneven heat distribution and local hot-spotting, especially in viscous or particulate-containing fruit juices [5]. Due to these drawbacks, there has been growing interest in alternative heating technologies that can offer rapid and uniform heating without compromising product quality. In this regard, ohmic heating has received attention as an alternative method of generating volumetric heat, which makes the generation and transfer of heat in the product through electrical resistance possible, thus possibly having better control over thermal treatment compared to conventional HTST systems [6].

Ohmic heating is an advanced thermal processing method that uses electric current to generate internal heat uniformly throughout the food matrix. It offers several advantages over conventional systems, including volumetric heating, minimal surface degradation, and improved retention of color, flavor, and antioxidant content [7]. Extensively applied in liquid and semi-liquid food systems, ohmic heating shows considerable promise for high-quality fruit juice pasteurization [8]. A key factor influencing the efficiency of ohmic heating is the electrical conductivity (EC) of the food, which is affected by temperature, ionic concentration, viscosity, and composition [9]. Generally, EC increases with temperature due to enhanced ion mobility and lower viscosity. Its relationship with total soluble solids (TSS) is nonlinear, as high TSS levels may reduce EC due to increased viscosity and limited free water.

The Box–Behnken design (BBD) has emerged as a robust statistical tool for optimizing processing parameters in fruit juice production. By employing three levels for each factor, BBD minimizes the number of experimental runs required while enabling comprehensive analysis of response surfaces [10]. Its effectiveness has been demonstrated in optimizing various food processes, such as pectin extraction from fruit peels [11] and improving fermentation conditions in functional beverages [12]. The interpretability and efficiency of BBD make it a valuable approach for identifying optimal processing conditions that enhance product quality and sustainability in the juice industry [13].

In addition, recent advancements in electrode plate manufacturing, low-cost optic fiber temperature sensors, and IoT-based control systems have revitalized interest in ohmic heating as a viable pasteurization method for fruit juices and liquid foods. According to Phonchan et al. [14], the efficiency of ohmic pasteurization in maintaining the quality of passion fruit juice is mainly affected by pasteurization temperature, holding time, and voltage gradient. This agrees with the observation reported by Priyadarshini et al. [13]. While different fruit juices, including madan juice [15], carrot juice [16], and mandarin juice [17] have been studied for ohmic heating application under some pathogen periods, there is a scarcity of data on passion fruit juice, especially with respect to underlying enzyme inactivation mechanisms and quality preservation under optimized pasteurization conditions. These constitute the gaps to address to make ohmic heating a dependable and cost-effective pasteurization technology for tropical-based high-acid beverages.

Thus, the present study was designed to explore the optimal conditions for pasteurization temperature (75, 85, and 95 °C), holding time (15, 30, and 45 s), and voltage gradient (10, 20, and 30 V/cm). According to Phonchan et al. [14], the effectiveness of ohmic pasteurization in preserving the quality of passion fruit juice is primarily influenced by pasteurization temperature, holding time, and voltage gradient. Although ohmic heating has several advantages, limited information is available regarding its impact on fruit juice quality. Assawarachan and Tantikul [18] investigated the electrical conductivity of passion fruit juice and reported that the highest thermal efficiency during pasteurization was achieved at a voltage gradient of 10–30 V/cm. Many RSM-based studies on ohmic heating pasteurization (OHP) have mainly focused on parameter screening or single-response optimization, with limited attention given to statistically validating optimized conditions using comparative quality indices. Consequently, quality assessment has often been conducted separately from multi-response optimization involving systematic validation. As a result, there are relatively few modeling-based studies addressing multi-response optimization of key OHP variables combined with a comprehensive evaluation of physicochemical properties, enzyme inactivation behavior, color stability, and system performance. Addressing this gap is essential for developing a reliable processing window predicted through simulation-based optimization. In this context, the present work employs a Box–Behnken design to optimize OHP conditions for passion fruit juice and to assess representative quality indices under optimized conditions in comparison with conventional thermal pasteurization.

2. Materials and Methods

2.1. Materials

The passion fruit used in this study was developed by the Royal Project Development Center in Mae Hae, Mae Hae District, Chiang Mai Province, Thailand. The fruits were harvested at the commercial ripening stage during the 2025 harvesting season (March–April). The juice was filtered and extracted using thoroughly washed fruits. Washed passion fruits were manually opened, and the pulp was separated for juice extraction. The juice was extracted with a laboratory-scale mechanical press developed by the Smart Farm Engineering and Agricultural Innovation Program, School of Renewable Energy, Maejo University, Thailand. The mechanical pressure press system accomplishes juice extraction, minimizing heat-induced quality degradation. Juice was collected and filtered on a coarse filter to separate solids prior to analysis and pasteurization. A digital refractometer (Hanna Instruments, HI 96801, Woonsocket, RI, USA) was used to measure total soluble solids (TSS), which were first registered at 11.5 °Brix.

2.2. Batch Ohmic Heating Unit for Laboratory Pasteurization

2.2.1. System Description

A laboratory-scale ohmic heating pasteurization (OHP) system was developed under the Smart Farm Engineering and Agricultural Innovation Program at the School of Renewable Energy, Maejo University, Thailand, for batch pasteurization of passion fruit juice. The system comprises a 10 L mixer tank equipped with a mechanical stirrer, a solenoid valve for flow control, and an acrylic OHP chamber (25 mm inner diameter, 10 mm length, 3 mm wall thickness). Three K-type thermocouples are strategically positioned within the chamber to monitor temperature distribution during the ohmic heating process. A programmable logic controller (Haiwell model D7-G, Xiamen, China) records real-time, current, and temperature data at 5 s intervals. After pasteurization, the treated juice flows through a cooling coil, reducing the temperature to 10 °C before being directed into the filling chamber equipped with a UV-C sterilization system. A schematic diagram of the laboratory-scale ohmic heating pasteurization system used in this study is shown in Figure 1.

2.2.2. Experimental Operation Procedure

Before each experiment, all parts of the ohmic heating system that came into contact with food, such as the ohmic cell, pipelines, and juice containers, were sterilized in a steam autoclave (STE-18-D/E, Icanclave, Dongguan, China) at 121 °C for 15 min. This was done to ensure the system was clean and to keep experiments from mixing. The juice itself was not put through an autoclave to kill bacteria; instead, the ohmic heating process did that. The experimental operation of the laboratory-scale ohmic heating pasteurization (OHP) system was performed as follows. The passion fruit juice was added to the mixer tank and stirred mechanically to achieve a homogeneous mixture at an initial temperature of approximately 25 °C. Valve 1 was opened thereafter so that a 200 mL portion of the passion fruit juice could flow into the ohmic heating chamber (ohmic cell). The juice was then heated with the required volume up to the closed-valve level, and the juice was heated under the specified ohmic heating conditions. After heating, the valve system switched automatically to the opening position and allowed treated juice to flow through the cooling unit, where the temperature was cooled down quickly to below 10 °C. Water-ice sludge was also formed inside the bottom part of this cooler. Then, chilled juice exited from valve 3 and entered the glass bottles through the filling unit. A UV-C sterilization lamp (TUV 8W T5, Philips Lighting, Eindhoven, The Netherlands) was installed in the filling part to reduce microbial contamination of airborne particles during the packaging. All OHP lines and the process chamber were steam sterilized prior to each run to achieve sterile operation while avoiding carryover from experiment to experiment. Filled juice samples were then refrigerated at 10 °C until further physicochemical analysis. Microbial analysis was not conducted in this study and is therefore addressed as a limitation.

The ohmic heating pasteurization (OHP) principle is based on the generation of internal heat from the conversion of electrical energy, which occurs as an alternating electric current flows through the electrically conductive food matrix. The heat is produced volumetrically by Joule’s law (Q = I²Rt, where I is the electric current, R is the electrical resistance of the product, and t is the heating time). OHP, because the heating is uniform over the entire volume, leads to lower temperature gradients and a reduced thermal inertia compared with conventional surface heating techniques. This volume heating mode can efficiently heat the product and cook it thoroughly in terms of both thermal efficiency and quality, without overcooking the surface, thereby preserving rich nutrients remaining in the bulk. In this study, to achieve uniform energy dissipation and suppress electrochemical corrosion, a low-frequency AC (50 Hz) with stainless steel electrodes was used [13,14,15,18].

2.2.3. Conventional Pasteurization (Control)

Conventional pasteurization was performed as a control treatment via a batch water bath system (Memmert GmbH + Co. KG, Schwabach, Germany). Glasses bottles were filled with passion fruit juice samples and pasteurized through immersion in a thermostatically controlled water bath at 85 ± 1 °C for 25 s, which matches the time–temperature conditions used during ohmic heating pasteurization. The juice temperature was continuously controlled with a calibrated thermometer to maintain accurate thermal input. Following pasteurization, bottles were immediately placed in an ice-water bath and quickly cooled to less than 10 °C to limit additional thermal degradation. The pasteurized samples were then stored under a refrigeration temperature (8–10 °C) before physicochemical and enzymatic analysis.

2.3. System Performance Coefficient

To assess the efficiency of the ohmic heating system, the system performance coefficient (SPC) was determined as the ratio of the sensible heat absorbed by the juice to the energy supplied to the system (1). Assuming a constant specific heat within the examined temperature range, SPC was calculated using juice mass (m), specific heat (C_p), voltage (V), current (I), processing time (t), and temperature difference (T _initial to T _final).

$$SPC=\hspace{1em}{\displaystyle \frac{Hea{t}_{abs}}{Engerg{y}_{del}}}=\hspace{1em}{\displaystyle \frac{m{C}_P(T_{final}-T_{initial})}{{\displaystyle \sum VIt}}}$$

(1)

The specific heat capacity (C_p) of high-moisture foods’ juice above the freezing point can be determined using the Seibel empirical formula, where X_m denotes the moisture content of the juice [16,18].

$$C_p=\hspace{1em}3.35X_m+0.837$$

(2)

where m is the mass of juice (kg), Cₚ is the specific heat capacity (J·kg⁻¹·°C⁻¹), T₁ and T_f are the initial and final temperatures (°C), V is the applied voltage (V), I is the electric current (A), and t is the processing time (s). The SPC is dimensionless and represents the ratio between the sensible heat absorbed by the juice and the total electrical energy supplied to the system. The moisture content (X_m) of passion fruit juice was approximately 0.88% w.b.

2.4. Optical Properties

Color was measured in reflectance mode using a Chroma Meter (CR-400, Konica Minolta Co., Ltd., Osaka, Japan). Juice samples were placed in a standard sample cup, and color values (CIE-L*a*b*) were recorded under D₆₅ illumination. Reflectance measurement was selected due to the turbid nature of passion fruit juice, which may interfere with transmittance-based color analysis.

The L*, a*, and b* values were measured at three different positions for each sample and averaged. L* represents lightness, a* indicates redness to greenness, and b* denotes yellowness to blueness. All measurements were performed in triplicate. The total color difference (ΔE) between untreated and treated samples was calculated as follows:

$$\Delta E\hspace{1em}= \sqrt{{\left(L*-L_0^{*}\right)}^2+{\left(a*-a_0^{*}\right)}^2+{\left(b*-b_0^{*}\right)}^2}$$

(3)

where L₀*, a₀*, and b₀* refer to the values of the control or untreated sample.

2.5. Determination of Vitamin C Content

The ascorbic acid content was determined using a titrimetric method, employing 2,6-dichlorophenol-indophenol (DCPIP) as the oxidizing agent. The juice sample was titrated against a standardized DCPIP solution prepared from analytical-grade 2,6-dichlorophenol-indophenol (Merck, Darmstadt, Germany) until a persistent pink endpoint was observed. The ascorbic acid concentration was quantified by comparing the titration results with a calibration curve constructed using L-ascorbic acid as the standard [13,15,17].

2.6. Enzyme Activity

Peroxidase (POD) and polyphenol oxidase (PPO) activity was measured spectrophotometrically to assess the stability of the enzymes post-pasteurization. Juice samples were initially centrifuged (5424 R, Eppendorf, Hamburg, Germany) at 10,000× g for 10 min at 4 °C, and the supernatant was used as a crude enzyme extract. The catalase activity was assayed by the change in absorbance at 470 nm due to oxidation of guaiacol in the presence of hydrogen peroxide. The reaction mixture contained phosphate buffer (pH 6.5), guaiacol solution, hydrogen peroxide, and enzyme extract. One unit of POD activity was the amount of enzyme resulting in an increase in absorbance at 470 nm per minute under assay conditions. PPO activity was expressed as the increase in absorbance at 420 because of the oxidation of catechol as the substrate. The reaction mixture consisted of phosphate buffer (pH 6.8), catechol solution, and enzyme extract. The increase in absorbance of 0.001 per minute was designated as one PPO unit. The protein content of the extract was measured by standard protein assay, and enzyme activities were reported as U·min⁻¹·mg⁻¹ protein. The remaining enzyme activity was calculated by comparison with the untreated juice, which was set at 100% [19].

2.7. Statistical Evaluation

Design-Expert 13.0 (Stat-Ease Inc., Minneapolis, MN, USA) was used to improve the response surface by applying the Box–Behnken Design (BBD) with three factors: temperature (75, 85, 95 °C), holding time (15, 30, 45 s), and voltage gradient (10, 20, 30 V/cm), leading to a total of 15 experiments. One-way ANOVA with Tukey’s test (p < 0.05) was used to compare ohmic and conventional pasteurization. Analyses were performed using SPSS 20.0 (IBM Corp., Armonk, NY, USA) under a Maejo University license. All experiments were conducted in triplicate, and results are expressed as mean ± standard deviation.

3. Results

3.1. Effect of Input Variables on the Response Surface

The experimental processes were based on Box–Behnken Design (BBD) with three variables: temperature, holding time, and voltage gradient at 3 levels. BBD needs 15 experimental runs, including at least three center points, compared with the 27 experiments in a full factorial design for a three-level system. This design permits efficient estimation of linear, quadratic, and interaction effects without the use of extremely high experimental levels. The BBD method has been extensively used in food process optimization and is especially suitable for thermal processing investigations [20,21]. The influences of the treatment temperature (75–95 °C), holding time (15–45 s), and voltage gradient (10–30 V/cm) on the system performance coefficient (SPC), total color difference (∆E), and vitamin C retention of BBD in this study were investigated systematically, as given in

Table 1. Effect of OHP parameters on the physicochemical properties of passion fruit.

DOE	Temperature (°C)	Holding Time (s)	Voltage Gradient (V/cm)	SPC	ΔE	Vitamin C (mg/100 mL)
1	95	30	30	0.66	10.27	11.34
2	75	15	20	0.82	10.46	17.94
3	75	30	30	0.61	10.14	16.42
4	85	15	30	0.76	7.88	18.85
5	75	45	20	0.77	10.27	15.82
6	85	30	20	0.86	6.21	20.91
7	85	15	10	0.72	7.41	19.11
8	95	45	20	0.74	14.65	10.37
9	85	45	30	0.75	9.63	17.87
10	75	30	10	0.63	9.27	16.82
11	95	15	20	0.71	12.24	11.34
12	85	30	20	0.91	7.27	22.62
13	85	30	20	0.88	6.58	21.49
14	95	30	10	0.63	11.71	12.28
15	85	45	10	0.73	8.42	18.05

Note: Table 1 presents the experimental design matrix generated by the Box–Behnken design. Values represent set experimental conditions rather than measured responses.

. The influence of these factors on selected responses was analyzed by RSM. The system performance coefficient (SPC) is used as an indicator of thermal efficiency in OHP. Small voltage angles caused the come-up time to be long, so there was a large energy loss, and small SPC values were noted [22]. Heat transfer efficiency decreased with a rise in electrical conductivity, associated with increased evaporation and structural modifications in the liquid phase, as well as electro-generation of oxygen gases onto electrode surfaces [13,18]. This effect of reduced contact area between the juice and the electrodes, in turn, worsened energy losses. Fuzzy factor analysis of the holding time data revealed that the influence of tensile holding time on SPC seems to be next in importance, following temperature and voltage gradient, as depicted by Figure 2.

Response surface plots were used to study the impacts of OHP processing variables on the total color difference (ΔE) and retention of vitamin C. Higher values of ΔE and color degradation were observed at higher processing temperature and holding time conditions, while lower temperatures and holding times resulted in better color retention (Figure 3). Vitamin C retention was predominantly affected by temperature and holding time; greater thermal exposure led to more ascorbic acid degradation (Figure 4). As to ΔE and vitamin C retention, the voltage gradient had little effect in this range. These results are in agreement with those reported for fruit juices, where temperature–time combinations were found to be the main factors controlling the nutrient degradation and color changes during ohmic heating [17,23].

3.2. Analysis of Optimal Conditions for Ohmic Pasteurization

Identifying optimal processing conditions is essential for enhancing efficiency while maintaining the nutritional and sensory attributes of fruit juices. This section identifies the optimal parameters for ohmic pasteurization that concurrently improve vitamin C retention, reduce total color difference (ΔE), and maximize the system performance coefficient (SPC), following the assessment of input variable effects through response surface methodology (Section 3.1).

A multi-objective optimization utilizing the desirability function approach was performed to attain the optimal compromise among the three responses. Optimal conditions were obtained from predictive models created by quadratic regression analysis and confirmed through experimental trials. The optimized parameters were selected to achieve effective thermal treatment while preserving key quality attributes of the product. This section delineates the numerical optimization outcomes, encompassing the amalgamation of temperature, holding duration, and voltage gradient that produced the highest overall desirability index. The anticipated values of the three primary responses under these conditions are juxtaposed with actual experimental values to validate model adequacy and practical feasibility.

Selecting an appropriate experimental design is crucial for constructing a mathematical model to analyze responses during process optimization using Response Surface Methodology (RSM). For systems exhibiting non-linear behavior or curvature, a second-order model incorporating linear, quadratic, and interaction terms is necessary to accurately describe the system’s response.

The Box–Behnken design was chosen for this study due to its efficiency in reducing experimental runs while providing sufficient data to estimate the parameters of the second-order model effectively. This design facilitates the evaluation of main effects, quadratic effects, and two-factor interactions, as represented in Equation (4), enabling the identification of critical points and accurate prediction of outcomes under various experimental conditions [24].

$$y\hspace{1em}=\hspace{1em}{\beta }_0+{\displaystyle {\sum }_{i=1}^k{\beta }_i{X}_i+{\displaystyle {\sum }_{i=1}^k{\beta }_{ii}{X}_i^2}}+{\displaystyle {\sum }_{1\le i\le j}^k{\beta }_{ij}{X}_i{X}_j+\epsilon }$$

(4)

where β₀ is the constant term, β_ii represents the coefficients of the quadratic parameter, X_i and X_j represent the variables, and ε is the residual associated with the experiments.

Identifying critical points in nonlinear systems requires a second-order polynomial equation derived from experimental data. Setting the first derivative of the predictive model to zero yields the independent variable values that result in maximum or minimum response levels. The critical point’s nature (maximum, minimum, or saddle point) is evaluated using matrix techniques and eigenvalue analysis for accurate classification.

Second-order polynomial equations were developed to predict key response variables: system performance coefficient (SPC), vitamin C content, and total color difference (ΔE). Optimization goals were defined to maximize SPC and vitamin C while minimizing ΔE, reflecting optimal juice quality preservation. Equations (5)–(7) present the resulting predictive models and optimal conditions.

$$\mathrm{SPC}=-3.8336+0.1088X_1-0.0057X_2+0.0256X_3+0.00007X_1{X}_2$$

(5)

$$(\mathrm{R}2=0.9633)+0.00008X_1{X}_3-0.00001X_2{X}_3-0.0006X_1^2-(7.46x{10}^{-6})X_2^2-0.0008X_3^2$$

$$\Delta \mathrm{E}=265.66-6.0515X_1-0.7789X_2+0.4489X_3$$

(6)

$$(\mathrm{R}2=0.9017)+0043X_1{X}_3+0.0362X_1^2+0.0071X_2^2+0.0004X_3^2$$

$$\mathrm{Vitamin} \mathrm{C}=-0.5708+1.4403X_1+0.0456X_2+0.1149X_3$$

(7)

$$\begin{gathered} (\mathrm{mg}/100 \mathrm{mL})-0.0008X_1{X}_3-0.0085X_1^2-0.0008X_2^2-0.0011X_3^2 \\ (\mathrm{R}2=0.9488) \end{gathered}$$

X₁ represents temperature (°C), X₂ denotes holding time (seconds), and X₃ indicates voltage gradient (V/cm).

Numerical analysis was performed utilizing Design-Expert^® version 13.0 to assess the impact of key ohmic heating process (OHP) parameters—temperature, holding time, and voltage gradient—on system performance and thermal treatment efficacy. The optimization objectives, as outlined in

Table 2. Optimized ohmic heating conditions based on numerical analysis of SPC, ΔE, and vitamin C retention.

	Optimization	Numerical Results
OHP Parameters	Goals	Temperature (°C)	Holding Time (s)	Voltage Gradient (V/cm)
SPC	maximize	81.14	19.76	19.78
ΔE	minimize	83.35	28.09	14.82
Vitamin C (mg/100 mL)	maximize	83.29	26.53	21.09

, focused on achieving effective thermal treatment while minimizing quality-related changes. The analysis determined optimal process conditions within these ranges: temperature between 81.14 and 83.29 °C, holding time between 19.76 and 28.09 s, and voltage gradient between 14.82 and 21.09 V/cm. These ranges reflect operating conditions that balance process performance with the preservation of key quality attributes, particularly nutrient retention and color stability.

To ensure practical applicability under real-world processing conditions, the mean values of the optimal ranges were selected and slightly adjusted to account for expected thermal inertia and equipment limitations in pilot-scale operations. Consequently, the chosen parameters for the experimental validation of the ohmic heating pasteurization (OHP) system were a temperature of 82.5 °C, a holding time of 25 s, and a voltage gradient of 18.5 V/cm. These settings were proposed to achieve effective pasteurization while minimizing thermal degradation, thus ensuring effective thermal treatment while preserving product quality.

Table 3. Comparison of the predicted OHP parameters from mathematical models with actual experimental results.

OHP Parameters	Mathematical Models Prediction	Actual Experimental
SPC	0.82	0.80 ± 0.05 NS
ΔE	6.56	7.64 ± 1.08 NS
Vitamin C (mg/100 mL)	22.09	20.07 ± 3.18 NS

^NS: not significant differences at the 95% confidence level (p > 0.05).

compares the predicted values from the mathematical models with the actual experimental results for the OHP parameters. No statistically significant differences were observed (p > 0.05), verifying the predictive accuracy of the developed models. The validated mathematical model allowed for the precise determination of optimal conditions for the pasteurization process.

3.3. Comparison of Ohmic and Conventional Pasteurization on Quality Attribute

A comparative study was conducted to evaluate the quality indices of passion fruit juice subjected to ohmic heating pasteurization (OHP) under optimal conditions compared with the juice treated with conventional thermal methods. The comparison of physicochemical indexes, such as pH, total soluble solids, color stability, vitamin C content, enzyme activities, and system performance coefficient (SPC), was conducted between the two systems. The SPC value reported for fresh juice represents the baseline electrical–thermal behavior of the system prior to pasteurization and is therefore used as a reference for comparison with ohmic-heated and conventionally pasteurized samples. As summarized in

Table 4. Physicochemical properties of passion fruit juice subjected to various pasteurization.

Physicochemical Properties	Fresh Juice	Conventional Pasteurization	Ohmic Heating Pasteurization
%TSS	11.74 ± 0.98 a	12.64 ± 0.84 a	13.42 ± 0.51 a
pH	3.72 ± 0.12 a	3.54 ± 0.08 a	3.32 ± 0.12 b
CIE-L*a*b*
L*-value	46.21 ± 3.41 a	34.82 ± 6.31 b	41.39 ± 4.61 ab
a*-value	16.12 ± 2.82 a	19.11 ± 5.73 a	14.82 ± 1.18 a
b*-value	60.42 ± 4.11 a	48.29 ± 3.67 b	54.59 ± 4.36 ab
ΔE	0.00 ± 0.00 a	16.91 ± 4.37 c	7.68 ± 3.35 b
Vitamin C (mg/100 mL)	27.63 ± 4.37 a	11.34 ± 5.62 b	20.21 ± 4.72 a
Peroxidase (POD)	100 ± 0.00 a	32.5 ± 3.1 b	8.07 ± 1.74 c
Polyphenol oxidase (PPO)	100 ± 0.00 a	48.7 ± 2.8 b	12.6 ± 2.1 c
SPC	0.00 ± 0.00 a	0.54 ± 0.08 b	0.85 ± 0.07 c

Values are expressed as mean ± SD (n = 3). Different lowercase letters in the same row indicate significant differences (p < 0.05).

, the physicochemical properties, enzyme activities, and system performance coefficients (SPC) of passion fruit juice processed by different pasteurization methods are compared.

Table 4 indicates that the pH of passion fruit juice decreased following both pasteurization processes. The pulp exhibited a pH of 6.44 ± 0.11 and was not further analyzed due to its high viscosity, which could have influenced the measurement accuracy. The pH of fresh juice was measured at 3.72 ± 0.12, which decreased to 3.54 ± 0.08 following conventional pasteurization and further declined to 3.32 ± 0.12 during the OHP [25]. The reduction in the OHP-treated juice was statistically significant (p < 0.05). The rise in %TSS values may be attributed to moisture loss during heating, as observed in fruit juices subjected to thermal processing [26]. The color difference was evaluated using ΔE. The ΔE value for traditional pasteurized juice was higher (16.91 ± 4.37) while the OHP-treated juice exhibited a lower ΔE value (7.68 ± 3.35), indicating reduced color change in ohmic heating treatments. This suggests that the rapid and uniform heating of OHP minimizes local overheating, thereby reducing pigment denaturation. The vitamin C content decreased in the treated samples relative to fresh samples (27.63 ± 4.37 mg/100 mL) for both pasteurization methods. The conventionally pasteurized juice contained 11.34 ± 5.62 mg/100 mL of vitamin C, whereas the OHP-treated juice exhibited a higher concentration of 20.21 ± 4.72 mg/100 mL. The improved retention of vitamin C in OHP-processed juice is likely due to reduced thermal exposure time and enhanced heating methods, which protect against the oxidation of ascorbic acid to dehydroascorbic acid during processing. Significant reductions in the activities of both enzymes were noted following pasteurization. The residual peroxidase (POD) activity declined from 100% in fresh juice to 32.5 ± 3.1% following conventional pasteurization and to 8.07 ± 1.74% after OHP treatment. Polyphenol oxidase (PPO) activity decreased to 48.7 ± 2.8% and 12.6 ± 2.1% for conventional and OHP treatments, respectively. The increase in inactivation of POD and PPO under OHP conditions may result from the synergistic effects of rapid volumetric heating and electric field-induced conformational changes in enzyme molecules, which expedite protein denaturation. The System Performance Coefficient (SPC) of OHP-treated juice (0.85 ± 0.07) was significantly higher than that of fresh juice (0.54 ± 0.08), as presented in Table 4.

4. Discussion

The study found that OHP and conventional pasteurization affect passion fruit juice quality differently. Physicochemical characteristics, enzyme activities, color stability, and vitamin C retention between conventional pasteurization and alternative processing methods vary due to different mechanisms of heat treatment. It is interesting to note the lower pH in OHP-treated juice when compared with pasteurization. The pH reduction could be due to the physicochemical phenomenon of Ohmic heating, which may increase structural mobility of ions and/or modify dissociation equilibria under the influence of an electric field. Similar pH changes have been observed in other ohmic heating studies, showing trends consistent with the process used instead of formulation differences. Both pasteurization processes caused a slight increase in TSS, but this increment was not statistically different. This gain is known to be the result of water loss under heating, a phenomenon observed also in other thermally treated fruit juices. Little deviation from the average confirms that ohmic heating did not affect soluble solids content when compared with conventional pasteurization under present conditions. Color maintenance is of importance for fruit juices, since it decays through pigment breakdown reactions and non-enzymatic browning. Lower ΔE for OHP-treated juice reflects a lower extent of color change compared to heat-processed juice. Advantages of ohmic heating include rapid and uniform bulk heating that may mitigate local overheating and thermal impairment to thermosensitive pigments.

Vitamin C, a volatile compound sensitive to heat and oxidation, is a common nutritional indicator in fruit juices. Experiments found that OHP-treated juice preserved more vitamin C than conventional pasteurization due to its shorter heating time. As both forms of ascorbic acid are stable, the time and temperature required for thermal oxidation must be reduced. Fast target temperatures are employed to reduce exposure times at elevated temperatures, which would otherwise increase the generator formation of dehydroascorbic acid by thermal oxidation of ascorbic acid. Lower oxidative enzyme activity may also contribute to enhanced vitamin C protection in OHP. Peroxidase (POD) and polyphenol oxidase (PPO) activity, measured by the POD-POD-TMB assay, and the reaction rate constant revealed that OHP significantly inactivated these enzymes compared to thermal pasteurization. POD, a thermally stable heme-protein enzyme model, denatures by breaking H-bonds and altering tertiary structure at high temperatures. Rapid volumetric heating and electric fields can accelerate structural changes during ohmic heating, leading to increased enzyme inactivation. Chemical stress has been suggested for PPO, which is also affected by thermal and electrical stress. The system performance coefficient (SPC) of OHP-treated juice was higher than that for fresh juices, showing a change in efficiency produced with ohmic heating. The conversion of electric energy into thermal energy in the product matrix lowers heat-transfer resistance and increases processing efficiency. In general, quality parameters, including color, vitamin C content, and enzyme stability of passion fruit juice, are well maintained during ohmic pasteurization compared with conventional thermal treatment. These results indicate that ohmic-heating technology can be considered as an alternative pasteurization method for fruit juice to ensure its quality.

5. Limitations

This research was performed in a controlled environment using a laboratory-scale batch ohmic heating pasteurization system. As such, the data presented may not entirely reflect performance in continuous or industrialized systems where key features such as flow dynamics, electrode design, and heat losses may vary. Furthermore, the optimization and quality assessment were only conducted for a single high-acid fruit juice (passion fruit juice) within a nominated range of total soluble solids, potentially limiting direct extrapolation to other juice matrices with different compositional or viscosity characteristics. In addition, our study mainly concentrated on the physico-chemical properties, enzyme deactivation, and system performance coefficient. However, the kinetics of microbial inactivation and its shelf-life stability during longer periods of storage have not been studied, which will ultimately better characterize the industrial potential for improved ohmic heating pasteurization conditions.

6. Conclusions

This study examined the effects of ohmic heating pasteurization (OHP) on the quality attributes of passion fruit juice in comparison with conventional thermal pasteurization. Results showed that OHP caused slightly better retention of important quality attributes compared to conventional heating, with comparable physicochemical changes. Moreover, juice treated with OHP had a lower total color difference (ΔE) and higher retention of vitamin C, which means it maintained color and nutritional value better. OHP also caused more inactivation of oxidative enzymes, peroxidase (POD) and polyphenol oxidase (PPO), which are related to the deterioration of the quality of fruit juices. These responses are associated with the fast and homogeneous volumetric heating mechanism of ohmic heating, causing a lower effective thermal exposure and non-localized overheating. The pH and total soluble solids (TSS) were not significantly different between treatments, indicating that OHP does not cause physical damage of considerable severity to cause adverse changes in pH and TSS. Generally, the results in this study indicate that ohmic heating pasteurization is an alternative thermal processing method for processing passion fruits.