Application of an Electromagnetic Field for Extending the Shelf-Life of Not from Concentrate (NFC) Apple Juice

Abstract

The research material consisted of not from concentrate (NFC) apple juice, which underwent innovative processes of spray deaeration and flow microwave pasteurization. Traditional commercially available pasteurization was the control. Deaeration was conducted at two different temperatures (25 °C and 50 °C) and three rotational speeds of the spray disc (150 rpm, 450 rpm, 750 rpm). Flow microwave pasteurization was carried out at 90 °C with a flow rate of 5.8 L/min. Deaeration at a temperature of 25 °C and a rotation speed of 150 rpm resulted in a significant reduction in oxygen levels in the juice to 0.9 mg/L. Microwave pasteurization led to an almost 100% reduction in oxidoreductases activity (PPO, POD). Immediately after the process, microwave-pasteurized juice exhibited better stability of the total polyphenol content, with a TPC of 73.8 mg/100 mL. Microwave pasteurization caused a threefold increase in antioxidant capacity (176.8 µM/100 mL) compared to fresh juice. The color of microwave-pasteurized juice was more appealing than that of traditionally pasteurized juice.

1. Introduction

Pasteurization is a thermal method of food preservation used to reduce microorganisms and inactivate enzymes that directly affect food quality. Additionally, it impacts the sensory attributes and nutritional value of the treated juice [1,2,3]. The use of high temperatures during pasteurization has an adverse effect on the concentration of bioactive compounds in the food product [4]. Nowadays, there is a growing interest in consuming high-quality products that contain bioactive compounds with beneficial health effects, such as polyphenols with anti-cancer and anti-inflammatory properties [5,6,7]. Due to this trend, recent research has been conducted to explore the possibility of using alternative preservation methods, such as microwave flow pasteurization (MFP) [8].

Microwaves are a form of electromagnetic energy ranging from 300 MHz to 300 GHz [1]. The International Electrotechnical Commission in Geneva established that only microwaves with frequencies of 2450 MHz, 915 MHz and 896 MHz (the band used in the USA) may be used for industrial purposes [9]. The lower frequency radiation causes deeper penetration of the product, while the higher frequency ensures a more even distribution of energy with shallower penetration [10]. Microwaves affect foods in two ways. The first involves acting on molecules with a dipolar nature, primarily water molecules, causing them to reorient by 180° and simultaneously generating heat through molecular friction. The second way is based on microwaves affecting ions present in food, causing them to move according to the electromagnetic field. The collision of ions with other molecules results in the dispersion of heat inside the product [11,12].

During MFP, the stream of the liquid product is heated, held in a holding tube for a limited time and cooled in the final stage [13]. MFP has been utilized to preserve various fruit and vegetable products, such as citruses [14], oranges [15], vegetable smoothies [8] and strawberry purees [16]. The advantages of microwave heating include the following: a reduction in processing time [17], minimal changes in sensory quality [8], decreased enzyme activity [4] and microbial reduction [18], as well as the absence of overheating and improved energy efficiency [19,20]. Some authors have suggested the possibility of a non-thermal effect on enzyme and microorganism activities due to the electromagnetic field [21,22]. Another benefit of microwave heating is an increased extraction efficiency of bioactive compounds into the juice serum. The energy absorbed by cells causes their lysis by releasing intracellular fluids [23].

Consuming fruits and fruit juices is an integral part of maintaining a healthy diet and offers numerous health benefits [24]. Apples are particularly noteworthy for their high levels of nutrients, with phenolic compounds deserving special attention [25]. Apple juice, known for its nutritional richness, bio-functional properties and well-balanced flavor profile [26], ranks as the second most produced and consumed fruit juice globally [27]. The annual production of apple juice in the European Union (EU) exceeds 2 billion liters (2015–2017) [28]. Preserving the highest concentration of polyphenols in apple juice while maintaining sensory quality presents a significant technological challenge [29]. Therefore, the objective of this study was to investigate, for the first time, the impact of industrial-scale flow microwave pasteurization on the quality of apple juice, including sensory evaluation, physicochemical attributes and microbiological safety during storage.

2. Materials and Methods

2.1. Apple Juice Preparation

Fresh, unpasteurized juice was acquired directly before the experiment from a local producer. The juices were freshly squeezed from the respective type of fruit, and they were naturally cloudy. After delivery, the juices were transferred to a blending tank and then directed to the subsequent stage, which involved innovative and traditional degassing processes.

2.2. Deaeration of Juice

The degassing process was conducted using a prototype technology specifically designed and constructed under industrial conditions at the Wosana Company, located in the Andrychów, Poland. This installation comprised modules for both innovative and traditional degassing methods (PGI Sp. z o.o., Koluszki, Poland). Furthermore, the installation was equipped with a real-time oxygen measurement device, and the measurements were conducted using a Memosens COS81D optical electrode. The innovative juice degassing module was built based on the solutions patented by our research team (Patent no. PL 238909) [30]. The innovative processing consisted of a vacuum tank with a rotating spray nozzle (at 150, 450 and 750 rpm at 25 and 50 °C) and a specially designed arrangement of slots for deaeration. The oxygen dissolved in the juice treated at various rotational speeds of the spray nozzle and at different juice temperatures was measured. Additionally, traditional degassing at 50 °C was performed in a hermetically sealed vacuum tank operating at a vacuum pressure of 200 mbar or lower, with a maximum limit of 500 mbar.

2.3. Pasteurization Process

Technological tests using both microwave and traditional methods were conducted on a prototype installation designed for industrial scales (PGI Sp. z o.o., Koluszki, Poland). The innovative installation consisted of a prototype flow microwave pasteurizer coupled with a traditional tubular thermal pasteurizer. Both modules were integrated into a technological line with an aseptic bottling module. The modules for the PET bottles blowing and packaging, as well as modules for juice degassing for both methods, were also investigated. In the case of microwave pasteurization, the juice, after degassing with the innovative method, flowed through the process chamber. The entire volume of the juice was immediately heated due to the influence of the microwave generator. Microwave pasteurization was carried out at a temperature of 90 °C, with a flow rate of 5.8 L/min, and the bottling temperature was maintained at 20–25 °C. Additionally, traditional pasteurization occurred in a tubular heat exchanger, with the flow and temperature parameters set in line and the high-temperature-short-time (HTST) process set at 98 °C for 30 s.

2.4. Microbiology

Analysis of the total number of microorganisms, including yeasts and molds, was carried out in accordance with PN-EN ISO 4833-1:2013-12 [31] and PN-ISO 21527-1:2009 [32].

2.5. Enzyme Activities

The activity of oxidoreductive enzymes, namely polyphenoloxidase (PPO) and peroxidase (POD), was determined following the methodology proposed by Terefe et al. (2010) [33].

For the extraction, the mixture was composed of 0.2 M sodium phosphate (pH = 6.5), with 1% (v/v) triton X-100, 4% (w/v) polyvinylpyrrolidone (PVPP) and 1 M NaCl. Apple juice and the extraction mixture in a 4.5 mL: 4.5 mL (v/v) ratio were shaken for 1 min on a vortexer (IKA, Staufen, Germany). Subsequently, centrifugation was carried out (Rotina 380R, Hettich Instruments, Tuttlingen, Germany) at 14,000 rpm for 30 min at 4 °C.

For PPO determination, 500 µL of the supernatant was taken and combined with 3 mL of 0.05 M phosphate buffer (pH = 6.5), containing 0.07 M catechol. Absorbance was recorded for 10 min at 420 nm and 25 °C using a UV-Vis spectrophotometer (6705 UV-Vis spectrophotometer, Jenway, Nottingham, UK). A blank sample containing 0.05 M phosphate buffer (pH = 6.5) instead of the supernatant served as the reference. PPO activity was expressed as the change in absorbance per minute per milliliter (mL) of the sample analyzed.

For POD determination, 100 µL of supernatant was used and mixed with 1.5 mL of 0.05 M phosphate buffer (pH = 6.5). To initiate the racemization process, 200 μL of a 1% p-phenylenediamine (w/v) in 0.05 M phosphate buffer (pH = 6.5) and 200 μL of 1.5% hydrogen peroxide (v/v) were added. Absorbance was recorded for 10 min at 485 nm and 25 °C. A blank sample containing 0.05 M phosphate buffer (pH = 6.5) instead of the supernatant was used as the reference. POD activity was expressed as the change in absorbance per minute per milliliter (mL) of the sample analyzed.

The residual activity of the tested enzymes (PPO and POD) was calculated using the formula:

$$RA=\frac{A}{A0}·100$$

(1)

where A is the activity of the treated juice, and A0 is the activity of fresh juice.

2.6. Polyphenol Content (TPC and Poyphenols Profile)

2.6.1. Extraction

To prepare the sample for analysis, methanol (5 mL, 80% v/v) and hydrochloric acid (0.1% v/v) were added to 5 mL of apple juice. The mixture was then sonicated for 5 min (45 kHz, 200 W, 25 °C, MKD Ultrasonic, Warsaw, Poland) and subsequently centrifuged (Rotina 380R, Hettich Instruments, Tuttlingen, Germany) at 3670× g for 5 min at 4 °C. The supernatant from this process was transferred to a 25 mL volumetric flask. Extraction was repeated four times following the methodology proposed by Szczepańska et al. (2020) [34]. Before analysis, the mixture was filtered using a filter with a pore size of 0.45 μm (Macherey-Nagel, Düren, Germany).

2.6.2. Total Polyphenol Content (TPC)

The supernatant obtained, as described in Section 2.6.1, was utilized for the determination of the TPC (total polyphenol content). The TPC was determined using a modified version of the Folin–Ciocalteu method, as outlined by Gao et al. (2000) [35]. The absorbance of the mixture was measured after 1 h of incubation at ambient temperature (22 ± 1 °C) at a wavelength of 765 nm on a UV-1650PC spectrophotometer (Shimadzu, Nakagyo-ku, Japan). The results were expressed in milligrams of gallic acid equivalent per 100 mL of juice (mg GAE/100 mL).

2.6.3. HPLC Analysis of the Polyphenols

The supernatant, prepared following the procedure outlined in Section 2.6.1, was used for determining the polyphenol profile, as described by Tsao et al. (2003) [36]. An analytic column, Sunfire C18 (5 μm, 4.6 mm × 250 mm), paired with a Sunfire C18 Sentry (5 μm, 4.6 mm × 20 mm) protective cartridge, both from Waters, was employed for the analysis. Compounds were quantified using a photodiode detector (Waters 2996, Waters, Milford, MA, USA). The samples were eluted using an HPLC gradient consisting of 6% (v/v) acetic acid (solvent A) and acetonitrile (solvent B) following this profile: 0 to 45 min, 100% (A); 45 to 60 min, 85% (A) and 15% (B); 60 to 65 min, 70% (A) and 30% (B); 65 to 70 min, 50% (A) and 50% (B); 70 to 73 min, 100% (B); and finally 73 to 75 min, 100% (A). The separation of 10 μL samples was carried out over a 75 min period at a flow rate of 1.0 mL/min with a column temperature of 25 °C.

2.7. Antioxidant Activity (DPPH•)

The measurement of antioxidant capacity was conducted utilizing the DPPH• (2,2-diphenyl-1-picrylhydrazyl) method described by Yen and Chen (1995) [37], with some modifications. Specifically, 100 microliters of the supernatant, prepared according to Section 2.6.1, was mixed with 2.0 mL of a 0.1 mM 80% methanolic solution of DPPH. The absorbance was measured at a wavelength of 520 nm and a temperature of 25 °C after a 20 min incubation period using a 6705 UV–vis spectrophotometer (Jenway, Nottingham, UK). The results were calculated based on a calibration curve established with different concentrations of DPPH• radicals in 80% methanol and expressed as μM Trolox equivalents (Tx).

2.8. Color Parameters

The measurement was carried out using a Color Quest XE colorimeter (HunterLab 166, HunterLab, Reston, VA, USA) equipped with a xenon flash lamp. Glass cuvettes with an optical path of 1 cm were employed for the measurements. The results were reported in accordance with the CIE L*a*b* system, utilizing illuminant D65 and 10° observer. The data were expressed using the Hunter scale parameters: L* (lightness/darkness), a* (redness/greenness) and b* (yellowness/blueness) values. The total color difference (ΔE) was calculated using Equation (2), where L*, a* and b* represent the values of the control sample:

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

(2)

2.9. Sensory Analysis

The sensory analysis of apple juices was performed in accordance with PN-ISO 4121:1998 [38]. A 6-point scale was used to assess various attributes, including color, appearance, consistency, smell, taste and overall quality assessment. The sensory evaluation was carried out by a panel of 5 trained individuals at monthly intervals, spanning a period of one year. All samples were assessed independently in a sensory analysis laboratory established in compliance with PN-ISO 8589:2007 [39].

The research protocol for the sensory analysis was performed according to our institutional, as well as the Polish Centre of Accreditation, requirements with respect to all ethical guidelines. This study was conducted in accordance with the Declaration of Helsinki and other relevant ethical standards for research involving human participants.

2.10. Statistical Analysis

The statistical significance of the differences in the mean values was determined using STATISTICA 7.1 software (StatSoft, Tulsa, OK, USA), with one-way analysis of the variance (ANOVA) using Tukey’s test at a confidence level of α = 0.05.

3. Results and Discussion

3.1. Deaeration of Juice

Oxygen, present in the air, fills the spaces between fruit cells and saturates the juice during the disintegration of plant tissues and extraction of the juice, initiating oxidation reactions that can result in enzymatic browning, changes in aroma and a loss in nutritional value [16,18]. To mitigate the oxidative degradation of juice components, juices undergo a deaeration process prior to preservation [40]. This process leads to a reduction in dissolved oxygen concentration in the juice [41]. Figure 1 illustrates the relationship between the oxygen content in apple juices and the speed of the rotating disc at 25 °C and 50 °C, respectively.

As anticipated, low-temperature deaeration demonstrated superior efficiency in oxygen removal compared to the traditional deaeration method (4.6 mg oxygen/L). The most promising result was achieved when deaeration was carried out at 25 °C with a rotation speed of 150 rpm. This could be a result of increased oxygen solubility in the juice at lower temperatures and a potentially higher efficiency in the gas removal process. Moreover, this process resulted in an 80.4% reduction in oxygen levels compared to the traditional industry deaeration method and an impressive 86.4% reduction compared to high-temperature deaeration (150 rpm/50 °C). Certainly, the sample deaerated at a high temperature with a rotation speed of 150 rpm, reaching 6.6 mg oxygen/L of juice. Unexpectedly, high-temperature deaeration did not yield the desired results and the oxygen concentration was even higher compared to conventionally deaerated juice. It is worth noting that the effectiveness of high-temperature deaeration can be influenced by the juice’s inherent composition. Different juices may possess varying levels of dissolved oxygen, and certain juices might contain compounds that react with oxygen, thereby increasing their concentration during the deaeration process. During the deaeration process at higher temperatures, molecules in the juice acquire higher kinetic energy due to increased thermal activity. This increased kinetic energy may make the oxygen removal process more challenging, as oxygen molecules bound to juice molecules may be less likely to be released. Furthermore, factors such as pH, temperature and processing conditions can significantly impact the solubility of oxygen in the juice [40]. Understanding these variables is essential for optimizing the deaeration process and achieving consistent and desirable results.

3.2. Microbiology and Enzymatic Activity

The presence of microorganisms (bacteria, yeasts, molds) is a critical factor that influences the shelf-life of juices, leading to undesirable changes, such as fermentation processes, including alcoholic and lactic fermentation [42]. MFP and traditional pasteurization were effective in the inactivation of microorganisms at 90 °C from 1.83 × 10², 4 × 10² and 1.1 × 10³ log CFU/mL, respectively, for bacteria, molds and yeasts to below the limit of quantification. All juices were microbiologically stable during the whole 12-month storage time, which confirmed that MFP can be a useful technology for the industry. The study conducted by Tajchakavit et al. (1998) [43] demonstrated that microwave pasteurization effectively inactivates Saccharomyces cerevisiae and Lactobacillus plantarum in apple juice. The study showed that these microorganisms were inactivated at 60 °C and 65 °C, respectively. Moreover, Nikdel et al. (1992) [44] illustrated nearly 100% inactivation of L. plantarum using microwave flow pasteurization on orange juice with beneficial effects observed after 20 s at 70 °C. In another study by Marszałek et al. (2017) [45], microwave pasteurization of strawberry puree resulted in complete inactivation of yeasts and molds, with a total microbial count < 1 log CFU/mL when two different temperatures 90 and 120 °C were used. Gonzalez-Monroy et al. (2018) [46] observed almost complete inactivation of aerobic bacteria, molds, yeasts and coliforms in tamarind beverages. The result was attributed to the phenomenon associated with a wider distribution of energy in microwave pasteurization compared to traditional pasteurization. According to Guo et al. (2017) [47], microwaves affect microorganisms through different mechanisms: selective heating, electroporation and magnetic field coupling. The selective heating effect can lead to faster heating of microorganisms that absorb microwave energy compared to the surrounding liquid, thereby reaching pasteurization temperatures [48]. Electroporation induces the formation of pores in the cell walls of microorganisms, resulting in the leakage of cell contents and, consequently, cell lysis [49]. Microbial inactivation can also occur due to the coupling of electromagnetic energy with critical particles inside the cell, such as proteins or DNA. Microwaves impact the cell’s internal components, leading to their demise [50].

Another factor contributing to changes in juices is the influence of enzymes, which can lead to phenomena such as browning [21]. Our study revealed that both microwave pasteurization and traditional pasteurization resulted in the inactivation of PPO and a significant reduction in POD activity (almost 99.8%). Throughout almost the entire storage time, no PPO activity was detected. However, the study demonstrated the ability of POD to regenerate during the storage time. After 12 months of storage, a minimal level of POD activity (5%) was observed. In the early 1950s, Guyer and Holmquist [51] demonstrated the regenerative activity of POD with 99.9% inactivation of the enzyme. In a study conducted by Arjamndi et al. (2016) [8], it was shown that higher temperature and microwave pasteurization power (3600 W/93 s) resulted in the complete inactivation of peroxidases in vegetable smoothies. Similar results were obtained by Matsui et al. (2007) [52] in a study on coconut water where the authors found that microwave pasteurization was more efficient compared to traditional pasteurization. Marszałek et al. (2015) [16], in their study on strawberry puree, noted an approximately 80% reduction in PPO activity within 10 s regardless of the temperature applied (90 °C and 120 °C). Reducing the microwave processing time to 7 s at 90 °C resulted in a reduction in PPO activity to approximately 62%. The authors also demonstrated that PPO exhibited greater temperature resistance relative to POD. According to de Ancos et al. (1999) [53], the stability of peroxidase mainly depends on the fruit species.

3.3. Polyphenols Content and Antioxidant Activity

The fresh apple juice initially contained 32.5 mg GAE/100 mL (

Table 1. Total polyphenol content and antioxidant capacity (DPPH•) in FJ, TP and MFP-preserved apple juices.

	TPC [mg GAE /100 mL]		Antioxidant Capacity (DPPH•) [µM/100 mL]
FJ	32.5 ± 0.9 h		51.9 ± 1.4 d
Storage Time (Month)	TP	MFP	TP	MFP
0	38.8 ± 1.2 ef	73.8 ± 2.3 a	65.9 ± 2.3 c	176.8 ± 2.7 a
2	38.1 ± 0.4 ef	67.3 ± 0.6 b	66.9 ± 3.9 c	172.3 ± 3.2 a
4	36.1 ± 1.1 fg	59.2 ± 2.1 c	64.6 ± 2.0 c	168.7 ± 6.1 a
6	34.6 ± 0.5 gh	43.2 ± 1.3 d	63.2 ± 1.6 c	143.1 ± 0.7 b
8	33.1 ± 0.4 h	45.0 ± 1.7 d	62.1 ± 3.0 c	146.2 ± 2.6 b
10	33.7 ± 0.6 gh	43.5 ± 0.7 d	63.4 ± 1.6 c	143.4 ± 0.7 b
12	32.0 ± 0.6 h	32.0 ± 0.6 h	63.1 ± 0.8 c	141.5 ± 3.1 b

GAE—gallic acid equivalent, FJ—fresh juice, TP—thermal pasteurization, MFP—microvave flow pasteurization. Mean values marked with the same letters within the parameter do not differ statistically significantly in the same factor, p ≤ 0.05.

). As a result of the MFP preservation process, the measured TPC increased by 127%. In the apple juice treated by traditional pasteurization, the TPC slightly increased in comparison to the fresh juice, reaching only 38.8 mg GAE/100 mL. The MFP was found to be a more effective method regarding the release of this bioactive compound from higher molecular structures. A similar phenomenon was not observed in the case of TP juice. Throughout the entire storage period, the MFP-treated juice maintained a higher polyphenol concentration compared to the traditionally pasteurized juice and the fresh juice. However, after the 6th month of storage, a sharp decrease in the TPC was observed in the MFP-treated juice. In contrast, in the traditionally pasteurized juice, the concentration of total polyphenols did not change significantly after 12 months of storage. In a study conducted by Kumar et al. (2017) [54], a more favorable effect on the concentration of phenolic compounds in pomelo juice treated with MFP (705.3 mg GAE/100 L), compared to TP (690.5 mg GAE/100 L), was observed.

To confirm that the increase in total polyphenol content in the MFP-preserved apple juices was caused by the release of lower molecular weight phenolic compounds from bigger structures, the content of individual phenolic compounds was determined (

Table 2. Concentration of individual polyphenols in FJ, TP and MFP-preserved apple juices.

	Floridzin (mg/L)		Epicatechin (mg/L)		Chlorogenic Acid (mg/L)		Caffeic Acid (mg/L)
FJ	10.29 ± 0.15 a		2.67 ± 0.04 d		27.89 ± 0.05 d		0.76 ± 0.10 a
Storage Time (Month)	TP	MFP	TP	MFP	TP	MFP	TP	MFP
0	10.67 ± 0.22 a	10.70 ± 0.11 a	2.57 ± 0.08 d	27.47 ± 0.18 a	27.50 ± 0.15 d	150.89 ± 0.02 a	0.81 ± 0.02 a	0.60 ± 0.01 b
2	7.01 ± 0.12 c	7.55 ± 0.17 b	1.35 ± 0.36 e	6.42 ± 0.28 b	22.09 ± 0.11 e	131.37 ± 0.17 b	<LOQ	<LOQ
4	5.19 ± 0.21 de	5.44 ± 0.05 d	<LOQ	5.61 ± 0.14 c	21.49 ± 0.03 e	121.97 ± 1.15 c	<LOQ	<LOQ
6	4.98 ± 0.12 e	3.43 ± 0.08 g	<LOQ	<LOQ	21.95 ± 0.48 e	5.46 ± 0.03 g	<LOQ	<LOQ
8	4.48 ± 0.05 f	3.36 ± 0.04 g	<LOQ	<LOQ	8.51 ± 0.32 f	<LOQ	<LOQ	<LOQ

Mean values marked with the same letters within the parameter do not differ statistically significantly in the same factor, p ≤ 0.05.

). Primary polyphenolic compounds typically found in apple juice include chlorogenic acid, epicatechin and phloridzin [55]. Our research aligns with these findings. Traditional pasteurization (TP) did not result in statistically significant differences in the content of polyphenolic components in the juice compared to fresh juice (FJ). However, the study showed that microwave pasteurization led to a statistically significant increase in the content of polyphenolic components, namely epicatechin and chlorogenic acid. After 4 months of storage, there was a noticeable degradation of these tested compounds, ultimately resulting in their content falling below the limit of quantification after 8 months of storage.

The antioxidant activity of fresh juice was measured at 51.9 µM/100 mL [Table 1] and after MFP treatment, it significantly increased to 176.8 µM/100 mL. Similar to the polyphenol content, the antioxidant activity remained at a higher level throughout the storage period compared to TP juice. This phenomenon can be justified by the presence of various compounds, including simple individual polyphenolic compounds, which was confirmed in Table 2 and other studies [56]. Numerous studies have confirmed a positive correlation between antioxidant activity and the content of polyphenols [57,58,59,60], which was also observed in our study. The calculated Pearson’s linear correlation coefficient for these two parameters was 0.840 (α = 0.05). The increase in polyphenol content and the associated increase in antioxidant activity, induced by microwave radiation on the juice tissue, can result in tissue damage and greater extraction of components into the juice serum [47]. A similar effect was observed in the study of tomato puree fixed with microwave energy [61]. Comapa et al. (2019) [62] demonstrated that the microwave power had an impact on the antioxidant activity measured with the DPPH free radical assay in camu camu juice. In the tested conditions, a certain dependence was observed, with lower power resulting in higher antioxidant activity. The study by Perez-Grijalva et al. (2018) [63] showed that the use of microwave treatment before pressing had a positive effect, not only on pressing efficiency but also on increasing the polyphenol content and antioxidant activity in blackberry juice compared to traditionally pressed juice.

3.4. Color Change and Sensory Evaluation

The impact of both pasteurization methods on the color change and sensory quality of the juices is depicted in Figure 2 and Figure 3. Microwave pasteurization exhibits a lesser effect on the initial color of the product compared to TP. According to the literature, when the total color difference is less than 1.5, it indicates changes that are imperceptible to the human eye [64]. In our study, this threshold was exceeded by more than four times after MFP and even more significantly after TP, suggesting that both techniques caused significant changes in apple juice color. However, the changes induced by MFP were less visually apparent compared to TP-treated samples. In a study of camu camu juice by Comapa et al. (2019) [62], the use of higher power and longer microwave time (900 W, 45 s) resulted in lower changes in ΔE. The authors attributed this effect to the greater extraction of bioactive compounds, which contributed to maintaining the color at a high level.

Color changes were also observed during sensory evaluation (Figure 3). The storage period of the juice was associated with a decline in its sensory quality, which was reflected in its overall evaluation. Throughout the storage period, the juice treated with MFP consistently received higher scores during sensory evaluation compared to TP-treated juice. Cinquanta et al. (2010) [65] showed that the color change after microwave preservation of orange juice is mainly related to the temperature used rather than the treatment time. These observations suggest that while both pasteurization methods lead to noticeable color changes, MFP appears to have a milder impact on the initial color of the juice when compared to TP. The findings align with previous research indicating that adjusting the microwave power and time parameters can influence the color stability by enhancing the extraction of bioactive compounds that contribute to color preservation, as well as lower heat dose due to faster energy transfer by the electromagnetic field in comparison to heat conduction.

4. Conclusions

The most recent findings in our ongoing research continue to support the effectiveness of the innovative deaeration method for minimizing the dissolved oxygen concentration, thereby enhancing the stability of the bioactive compounds during storage.

Additionally, our investigations have revealed that flow microwave pasteurization remains a highly promising alternative to traditional thermal pasteurization for the preservation of NFC apple juice. This method ensures the production of a safe and stable product, achieving nearly 100% inactivation of oxidoreductive enzymes. Notably, microwave pasteurization in a continuous flow system consistently leads to NFC apple juice with improved physicochemical and sensorial properties, including higher polyphenol content and antioxidant activity when compared to traditional pasteurization. This enhanced quality of the juice persists throughout the entire storage period under typical conditions, although certain phenolic components may exhibit variations.

The latest conclusion from current research underscores the significant advantages of the flow microwave pasteurization technique as a valuable alternative to traditional thermal pasteurization. This technology enables the retention of higher levels of bioactive ingredients, contributing to the overall maintenance of the superior sensory quality in preserved NFC apple juice. The developed deaeration technology can also contribute to energy savings in fruit juice production. The lower the dissolved oxygen in the product, the better the thermal conductivity, leading to more efficient attainment of the desired pasteurization temperature.

5. Patents

Marszałek, K. Method of producing juices and fruit beverages. Patent No. 238909, 2021.