Evaluating the Impact of Microwave vs. Conventional Pasteurization on NFC Apple–Peach and Apple–Chokeberry Juices: A Comparative Analysis at Industrial Scale

Abstract

The study aimed to assess the nutritional properties of fruit juices using the microwave flow pasteurization (MFP) method. The innovative spray deaeration process was also employed at two different temperatures, 25 °C and 50 °C, with three rotational speeds for the spray nozzle: 150, 450, and 750 rpm. The research focused on two not-from-concentrate (NFC) juices: apple–peach and apple–chokeberry. The innovative MFP method demonstrated significant results over 12 months of storage: no presence of Listeria monocytogenes or Salmonella spp. bacteria was detected. Polyphenol oxidases (PPOs) were inactivated, while peroxidase (POD) activity in apple–peach juice was minimal. The total polyphenol content (TPC) in the juices gradually decreased over storage time, but MFP resulted in a slower degradation of TPC than traditional pasteurization (TP). Additionally, anthocyanin and carotenoid content gradually decreased during storage time. Still, with MFP, higher concentrations of these compounds were noted up to 4 and 6 months of storage in apple–peach and apple–chokeberry juices, respectively, than with TP. The research findings indicate that MFP may be a suitable and promising technique for preserving high-quality juices with superior retention of essential nutrients. However, the recommended storage time should be at most four months.

1. Introduction

As the pace of life accelerates, the popularity of juices is on the rise due to their convenience and immediate readiness for consumption [1,2]. Despite the relatively small consumption volume, juices are a good source of nutrients, including vitamins, polyphenols, carotenoids, and anthocyanins [3,4]. Scientific research has indicated that the benefits of consuming fruit juices can be comparable to those of raw fruits [5]. This discovery holds particular significance for individuals who, for various reasons, may find it challenging to consume whole fruits. In response to consumer demands, the food industry has embraced new processing techniques to provide more sustainable nutritional value and health benefits.

From a nutritional point of view, apples deserve to be one of the main focal points in the industry. They rank among the most popular fruits in Europe [6,7,8], and their diverse varieties ensure a taste suited to most consumers’ preferences. The chemical composition of apples is contingent on their variety and the specific part of the fruit [9]. Additionally, apples are an excellent source of dietary fiber, vitamin C, and other valuable nutrients [10]. It is worth considering mixing them with different types of juices with high nutritional value to enhance nutritional value [11]. Blended juices also give an excellent opportunity to increase the consumption of less popular fruits such as peaches or chokeberries.

Fiber found in apples plays a crucial role in promoting gut health, comprising various fractions such as cellulose, pectin, and lignin, which collectively contribute to the proper functioning of the digestive system [12,13]. Meanwhile, vitamin C (L-ascorbic acid) functions as an antioxidant, neutralizing the harmful effects of free radicals and bolstering the overall immune system. Although vitamin C is abundant in fruits and vegetables, it is susceptible to degradation, especially from light, oxygen, and heat [14,15,16]. Furthermore, mixed fruit juices such as apple–peach or apple–chokeberry may increase nutritional value and palatability with better taste and consumer acceptance.

Thermal pasteurization of fruit juices is a common practice to eliminate potentially harmful microorganisms. Nevertheless, it is worth noting that it has some limitations. During thermal pasteurization, the product is passed through a heat exchanger. Typical processing parameters for fruit products are 80–90 °C for 10–60 s. As the product is heated, it develops a thin liquid layer that has a higher temperature than the rest of the liquid. This can lead to scorching and degradation of heat-sensitive compounds. Elevated temperatures can negatively affect the concentration of bioactive compounds in the food product, such as vitamins or antioxidants [17].

Additionally, high-density products such as sauces and fruit purees cannot pass through a traditional heat exchanger. The high temperature and long duration of the pasteurization process may lead to changes in the sensory quality of the product, including its color, appearance, taste, smell, and consistency [18,19]. Therefore, alternative preservation methods, including microwave pasteurization, can be implemented to mitigate the adverse effects of high temperatures and long-term treatment on the nutritional components of the product. Microwave pasteurization, a relatively new approach to food preservation, offers a faster and more effective process than traditional methods [20].

The microwave flow pasteurization (MFP) process involves heating a stream of liquid product, such as juice, under precise control to ensure the optimal temperature quickly. This destroys potentially harmful microorganisms while minimizing nutrient loss. Subsequently, the heated product is held in a unique holding tube for a specified time, followed by rapid cooling—a crucial process to prevent further nutrient decomposition [21].

Microwave heating is integral to food processing, pharmaceuticals, biochemistry, and agriculture [22,23,24]. The main advantages of microwave processing are minor changes in sensory quality, reduced enzymatic activity, shortened processing time, and the possibility of cooling the product as it flows [25,26,27,28].

In the context of our work, the research aimed to comprehensively analyze the quality of the nutritional properties of apple–peach and apple–chokeberry juices, using an innovative multifunctional device as a novel tool for juice pasteurization.

Our experiments focus on industrial-scale juice pasteurization. We utilized a specially designed prototype of a technological installation at Wosana Company. This installation was dedicated to pasteurizing juices and was crucial in production. The attempt to scale the process to an industrial level was evident in the use of this installation. Additionally, we compared two pasteurization technologies: microwave flow pasteurization and the traditional flow pasteurization process. These results make it possible to evaluate the effectiveness and efficiency of both methods on an industrial scale. The installation has practical applications in the food industry, particularly in juice production. This further confirms that our research has real industrial significance. Furthermore, comparing energy consumption between the innovative microwave pasteurization process and the traditional flow pasteurization process enables an assessment of energy optimization in an industrial context.

Numerous studies have been conducted on the application of microwave flow methods in the processing of fruit juices. One of the latest studies is the work of Seyedabie et al. [29]. This research focused on a microwave heating system for orange juice, aiming to investigate the impact of microwave treatment as an alternative to conventional methods. The experiments were conducted at various temperatures, with a power of 900 W. The analysis of the physicochemical properties of orange juice showed a decrease in pH and a slight increase in acidity. The results also indicated higher temperatures raised the browning index, turbidity, and haze value. Different treatment methods did not significantly affect the Brix content, acidity, and pH during the holding phase. However, the rise in temperature accelerated the degradation rate of vitamin C.

In another research conducted by Kernou O. et al., the authors used a hybrid technique that combines a microwave system with ultrasound [30]. The study aimed to inactivate Escherichia coli bacteria in orange juice. As part of the experiment, microwave power ranging from 300 to 900 W was applied for a variable processing time, ranging from 15 to 35 s, while the ultrasound exposure time was 10 to 30 min. Observations showed that the hybrid technique had a greater, more significant impact on the inactivation of E. coli compared to a substantial effect on the inactivation of E. coli than the separate processing of each method. These studies have shown that combining ultrasound and microwave processing can be a potential alternative to traditional methods of processing beverages based on orange juice.

It is also worth noting that a study by Jafarpour et al. showed that microwave heating technology can be an effective alternative to traditional heating methods [31]. These experiments were conducted on guava juice. In one experiment, 100 mL of guava juice was exposed to microwaves at 1450 MHz for 30 s. The results indicate that using the microwave method not only ensures the product’s microbiological safety but also saves energy and reduces processing time while preserving the nutrients in juices and allowing products to retain their natural sensory properties.

2. Materials and Methods

2.1. Juice Preparation

The experiment involved preparing apple–peach and apple–chokeberry juices from three types of not-from-concentrate (NFC) juices: apple, peach, and chokeberry, which were freshly prepared before the experiment. The juices were freshly squeezed before the pasteurization process and then transferred to the mixing tank. The next stage was traditional and innovative degassing processes.

The degassing process was conducted at Wosana company (Andrychów, Poland) using a specially designed prototype of a technological installation (patent number PL 238909) [32]. The installation featured modules for innovative and traditional degassing methods (PGI Sp. z o.o. Koluszki, Poland). Real-time monitoring of oxygen content was performed using the optical electrode, the Memosens COS81D (Endress+Hauser, Reinach, Switzerland). The innovative degassing module utilized a vacuum tank with a rotating spray nozzle at various speeds (150, 450, and 750 rpm), temperatures (25 and 50 °C), and a specially designed venting slot system. Under vacuum conditions, the traditional degassing system was carried out at 50 °C in a hermetically sealed vacuum tank. The tight vacuum tank operated at a vacuum pressure of 200 mbar or less, with a maximum pressure of 500 mbar.

Pasteurization was performed on a prototype installation designed especially for Wosana company at an industrial scale (PGI Sp. z o.o., Koluszki, Poland) using microwave and traditional flow systems. The innovative installation included a prototype microwave flow pasteurizer (MFP) connected to a traditional (TP) tubular thermal pasteurizer. The aseptic filling module was integrated into the technological line for both modules. Additionally, research was conducted on modules used for blowing and packaging PET bottles and on modules for degassing juices using both methods.

In microwave pasteurization, a frequency of 2450 MHz, a current of 63 A, and a maximum power of 18 kW were used. This frequency is one of the three most commonly employed in microwave ovens [33]. Additionally, it results in food being heated quickly, with even energy distribution during production and shallower penetration [34]. This method preserves the flavor and nutritional value of the product while enhancing bioactive compounds through greater extraction into the juice. This is made possible by the absorption of energy by cells, followed by their breakdown, allowing substances from within the cells to be released into the juice serum. These data are essential for analyzing and evaluating the efficiency of the pasteurization process on an industrial scale.

In the case of microwave pasteurization, the juice, after the degassing process using an innovative method, flowed through the process chamber. A microwave generator immediately heated the entire volume of juice. The MFP processes were heated by a microwave generator 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. TP occurred in a tubular heat exchanger, following high-temperature short-time (HTST) processes at 98 °C for 30 s. Based on the data for the prototype of the new microwave preservation technology compared to the traditionally conducted flow pasteurization process, the electricity consumption per 100 L of finished juice is as follows: for TP, 0.131 kW, and for MFP, 3.00 kW. The results were obtained based on calculations according to the formulas:

$$NP\ast M=Ec$$

(1)

$$\frac{(Ec\ast 100 L/h)}{Fi}=F$$

(2)

where NP is the nominal power, M is the maximum device power, Ec is the energy consumption for the process, 100 L/h is the flow rate of 100 L per second of finished juice, Fi is the flow intensity, and F is the final result.

For TP, the network parameter analyzer operates at 19.66 kW with a flow rate of 15,000 L/h. To produce 100 L of the finished product, we require 0.131 kW of energy. Meanwhile, for MFP, the nominal power of this process is 18 kW. Calculations were made for MFP with an average flow rate of 360 L/h at 60% power consumption. According to these calculations, 3.0 kW of energy is needed per 100 L of product. The process diagram is presented in Figure 1.

2.2. Detection of the Pathogenic Bacteria Salmonella spp. and Listeria monocytogenes

Samples were tested to exclude Salmonella spp. presence following the EN ISO 6579-1:2017-04 method [35]. Moreover, the Listeria monocytogenes were identified using the PN-EN ISO 11290-1:2017-07 method [36].

2.3. PPO and POD Activities

Following the methodology proposed by Terefe et al., 2010 [37], the activity of oxidoreductase enzymes, such as polyphenol oxidases (PPOs) and peroxidases (PODs), was assessed. The extraction process involved a mixture consisted 0.2 M sodium phosphate (pH = 6.5), 1% (v/v) Triton X-100, 4% (w/v) polyvinylpyrrolidone (PVPP), and 1 M NaCl. 4.5 mL of the extraction mixture within a 4.5 mL of juice and shaken for 1 min on a vortex (IKA, Staufen, Germany). Subsequently, centrifugation (Rotina 380R, Hettich Instruments, Tuttlingen, Germany) was carried out at 11,000× g for 30 min at 4 °C.

For PPO activity determination, 500 µL of supernatant was mixed with 3 mL of 0.05 M phosphate buffer (pH = 6.5) containing 0.07 M catechol. Using a UV–Vis spectrophotometer (6705 UV-Vis Spectrophotometer, Jenway, Nottingham, UK), measured absorbance for 10 min at 420 nm and 25 °C. A blank sample with 0.05 M phosphate buffer (pH = 6.5) instead of supernatant was used as a reference. For POD activity determination, 100 µL of supernatant was combined with 1.5 mL of 0.05 M phosphate buffer (pH = 6.5). The research commenced with the addition of 200 µL of 1% p-phenylenediamine (w/v) in 0.05 M phosphate buffer (pH = 6.5) and 200 µL of 1.5% hydrogen peroxide (v/v). Absorbance was measured for 10 min at 485 nm and 25 °C. A blank sample with 0.05 M phosphate buffer (pH = 6.5) instead of supernatant was used as a reference. PPO and POD activities were expressed as a change in absorbance per minute per milliliter of the analyzed sample.

2.4. Total Polyphenol Content (TPC)

For sample preparation, 5 mL of methanol (80% v/v) and 0.1% v/v hydrochloric acid were added to 5 mL of juice. The mixture underwent sonication for 5 min (45 kHz, 200 W, 25 °C, MKD Ultrasonic, Warsaw, Poland) and centrifugation (Rotina 380R, Hettich Instruments, Tuttlingen, Germany) at 3670× g for 5 min at 4 °C. The obtained supernatant was transferred to a 25 mL volumetric flask. The extraction process was repeated four times, following the methodology proposed by Szczepańska et al., 2020 [38]. Before analysis, the mixture was filtered using a 0.45 µm pore size filter (Macherey-Nagel, Dϋrwn, Germany). Subsequently, the total polyphenol content (TPC) was determined. A modified version of the Folin–Ciocalteu method, as outlined by Gao et al. 2000 [39], was used for TPC determination. After 1 h of incubation at room temperature (22 ± 1 °C), the absorbance of the mixture was measured at a wavelength of 765 nm using a UV-1650PC spectrophotometer (Shimadzu, Kyoto, Japan). The results were presented as milligrams of gallic acid equivalent per 100 mL of juice (mg GAE/100 mL).

2.5. Anthocyanin Content

The content of anthocyanins was determined using the HPLC method, which followed the procedure described by Oszmiański [40].

Before the determination, the samples were purified in Sep-Pak C18 columns (Waters, Milford, MA, USA). Analyses were conducted using a Waters 2695 HPLC equipped with a Waters 2996 UV-VIS detector. For anthocyanins separation, a Sunfire C18 column with dimensions of 5 µm × 250 mm × 4.6 mm, with a pre-column at 25 °C was used. A gradient [39] was applied, where eluent A was 4.5% formic acid and eluent B—acetonitrile. The results were recorded at a wavelength of λ = 520 nm, and anthocyanin was identified by comparing retention times with standards and literature data.

2.6. Antioxidant Activity (DPPH•)

The antioxidant capacity was assessed using the DPPH• (2,2-diphenyl-1-picrylhydrazyl) method, following the procedure outlined by Yen and Chen, 1995 [41], with certain modifications. In particular, 100 mL of the supernatant, prepared according to point 2.4., was mixed with 2.0 mL of 0.1 mM DPPH• solution in 80% methanol. After a 20-min incubation period at 25 °C, the absorbance was measured at a wavelength of 520 nm using a UV-vis 6705 spectrophotometer (Jenway, Nottingham, UK). The results were calculated and expressed as µM Trolox equivalents (Tx) based on the calibration curve established for various concentrations of DPPH• radicals in 80% methanol.

2.7. Carotenoid Content

The method of Mapelli-Braham et al. was used to extract carotenoids with certain modifications [42]. To 4 mL of juice, 1 mL of BHT (500 mg BHT in 1 L of hexane), 3.5 mL of hexane, and 4.5 mL of acetone were added. Subsequently, the mixture was subjected to the action of an orbital shaker (Sk-0330-Pro, DLab, Beijing, China) for 1 min at 400 rpm and then to an ultrasound batch (45 kHz, 200 W, 25 °C, MKD Ultrasonic, Warszawa, Poland) for 5 min. After this, the mixture was centrifuged (Rotina 380R, Hettich Instruments, Tuttlingen, Germany) for 5 min at 2500× g at 4 °C.

The liquid phase was extracted twice, and the organic phase was transferred to a 50 mL centrifuge tube. The first time, 3.5 mL of hexane and 2 mL of saturated sodium chloride aqueous solution were used, and then once, 4.5 mL of hexane. The organic phases were combined, and then 2 mL of the organic phase was transferred and evaporated (Rotavapor R-300, Buchi, Switzerland) at 30 °C and 160 mbar.

Finally, an HPLC analysis was carried out according to the method proposed by Melendez-Martinez et al., 2013 [43]. A photodiode array detector (Waters 2996, Waters, Milford, MA, USA) was used to determine carotenoids quantitatively. The carotenoid analysis was performed on a YMC carotenoid column (3 µm, 4.6 mm × 150 mm), maintaining the column temperature at 25 °C. The process lasted 75 min at a 1.0 mL/min flow rate. The mobile phase consisted of the following solvents: phase A—0.1% ammonium acetate with methanol; phase B—tertbutyl methyl ether. The gradients were as follows: from 0 to 44 min, 100% (A); 45–54 min, 85% (A) and 15% (B); 55–59 min, 40% (A) and 60% (B); 60–69 min, 30% (A) and 70% (B); 70–75 min, 100% (A).

2.8. Sensory Analysis

The sensory analysis of two juice components was conducted according to the PN-ISO 4121:1998 [44]. A 6-point scale was used to assess various parameters (color, appearance, smell, taste, consistency). A panel of five experts conducted sensory evaluations every two months over one year. Independent evaluations were conducted in a specialized sensory analysis laboratory, a laboratory established in compliance with PN-ISO 8589:2007 standard [45]. The sensory analysis research protocol developed complies with the requirements of the Polish Accreditation Center and ethical guidelines following the Helsinki Declaration, an international document defining ethical principles for conducting biomedical research involving humans.

2.9. Statistical Analysis

The analysis was performed using the software called STATISTICA 7.1 (StatSoft, Tulsa, OK, USA). To determine the statistical significance of differences, one-way analysis of variance (ANOVA) software (v.1.1.0) and the Tukey test at a confidence level of α = 0.05 were used.

3. Results

3.1. Pathogenic Bacteria

The presence of Salmonella spp. and Listeria monocytogenes bacteria is an essential factor that can impact the safety and quality of fruit juices during storage. It means that despite the high acidity of the juice, it can carry some dangerous microorganisms when the pasteurization process is inefficient [46]. However, neither Listeria monocytogenes nor Salmonella spp. bacteria were detected in the juices treated by MFP and TP methods during 12-month storage, which indicates that both processes are safe from a microbial point of view.

3.2. PPO and POD Activities

The study revealed that in fresh untreated juice, both peroxidase (POD) and polyphenol oxidase (PPO) enzymes were initially present in both juices. During the twelve-month storage period, PPO activity in apple–peach and apple–chokeberry juice remained undetected after MFP and TP. However, after two months of storage, POD activity was detected in the apple–peach juice treated by MFP and TP (

Table 1. POD activity in apple–peach juice during storage.

Analysis	Juice	Fresh Sample	Processing Method	Storage Time (Months)
				0	2	4	6	8	10	12
POD [%]	Appl-peach juice	100	TP 1	<LOQ	5.9 b ± 0.3	9.2 c ± 0.2	14.7 f ± 0.6	12.1 d ± 0.4	14.0 1 e,f ± 0.6	13.9 e,f ± 0.5
			MFP 2	<LOQ	4.2 a ± 0.3	6.8 b ± 0.1	12.5 d,e ± 0.5	17.2 g ± 0.5	17.0 g ± 0.4	17.1 g ± 0.3

¹ Traditional pasteurization—TP, ² Microwave processing—MFP, Mean values marked with identical letters do not show significant statistical differences, where p ≤ 0.05.

). In contrast, after the processes, POD activity was below the level of quantification (<LOQ). In both cases, a tendency toward regeneration is evident during storage time, suggesting that POD activity may increase during a long time of storage. Moreover, TP and MFP resulted in statistically significant differences in the POD activity.

3.3. Total Phenolic Content (TPC) and Antioxidant Capacity

Table 2. The impact of microwave and traditional pasteurization of the total phenolic content and DPPH• radical scavenging activity of apple–peach and apple–chokeberry juices.

Analysis	Juice	Fresh Sample	Processing Method	Storage Time (Months)
				0	2	4	6	8	10	12
Total phenolic content (mg/100 mL)	Apple–peach juice	54.15 g ± 0.21	MFP 2	65.07 c ± 1.07	60.29 e ± 0.29	57.03 f ± 0.55	54.45 g ± 0.80	46.57 h ± 0.63	47.17 h ± 0.63	47.02 h ± 0.43
			TP 1	74.48 a ± 1.07	67.35 ab ± 0.70	62.49 d ± 0.25	61.28 de ± 1.19	61.05 de ± 0.46	60.75 de ± 1.00	59.76 e ± 0.50
	Apple–chokeberry juice	127.72 e ± 2.15	MFP 2	152.60 a ± 3.86	133.63 de ± 2.82	135.61 d ± 2.55	131.97 de ± 0.78	121.04 f ± 3.07	115.13 fg ± 2.00	110.12 g ± 3.52
			TP 1	165.34 ± 1.29	143.49 bc ± 0.61	144.25 b ± 3.48	137.73 cd ± 1.31	134.85 de ± 1.52	136.06 d ± 1.74	128.63 e ± 3.13
DPPH• (µM/100 mL)	Apple–peach juice	112.76 d ± 0.60	MFP 2	183.57 ab ± 7.87	183.59 ab ± 4.22	181.80 ab ± 5.21	177.63 b ± 2.89	146.98 c ± 2.12	146.48 c ± 3.51	146.20 c ± 4.80
			TP 1	187.54 ab ± 8.97	189.06 a ± 1.88	191.29 a ± 5.07	188.25 a ± 3.09	184.60 ab ± 2.67	183.96 ab ± 3.28	181.80 ab ± 3.72
	Apple–chokeberry juice	512.20 cd ± 10.80	MFP 2	580.16 a ± 10.80	532.14 bc ± 4.02	543.71 b ± 9.60	526.30 bcd ± 3.49	480.12 e ± 1.64	468.90 e ± 6.74	431.91 f ± 1.27
			TP 1	543.65 b ± 4.76	542.45 b ± 3.67	541.52 b ± 11.75	507.75 d ± 18.23	506.85 d ± 1.99	515.54 cd ± 6.17	481.35 e ± 6.43

¹ Traditional pasteurization—TP. ² Microwave flow pasteurization—MFP. Mean values marked with identical letters do not show significant statistical differences where p ≤ 0.05.

presents the impact of MFP and TP on the TPC and antioxidant capacity measured with DPPH• radicals of both juices. Compared to a fresh sample, the TPC in apple–peach and apple–chokeberry juices significantly increased by 15% after MFP and TP processing, which indicates that high temperature with low pH may accelerate the hydrolysis of higher phenolic compounds to smaller molecules with higher affinity for Folin–Ciocalteu reagents.

The TCP of the apple–peach and apple chokecherry juices gradually decreased during storage. After 12 months of storage, the TPC of both juices treated by MFP and TP decreased by 28% and 20%, respectively.

A similar trend was observed in antioxidant capacity changes in both juices, but the degradation degree was less significant. MFP caused a 20 and 25% reduction of antioxidants in apple–peach and apple–chokeberry juices, whereas TP 3 and 11%, respectively. On the other hand, it is worth highlighting that both technologies caused significant increases in TPC and antioxidant activity due to heat treatment. However, the rise in TPC after MFP increased by 20% in both juices and over 30% after TP. Antioxidant activity increased by about 65% in apple–peach and 10% in apple–chokeberry juice, regardless of the pasteurization method used.

3.4. Anthocyanin Content

In the fresh juice, 33.84 mg of anthocyanins in L of juice were detected as three primary pigments: cyanidin-3-O-galactoside, cyanidin-3-O-glucoside, cyanidin-3-O-arabinoside whereas the most abundant compound was the galactoside (

Table 3. The total content of anthocyanins in apple–chokeberry juice [mg/L].

				Storage Time (Months)
Analysis	Juice	Fresh Sample	Processing Method	0	2	4	6	8	10	12
Total	Apple–chokeberry juice	33.84 a ± 0.10		30.55 b ± 0.05	9.16 d ± 0.07	6.24 e ± 0.07	1.49 g ± 0.06	0.57 i ± 0.05	0.32 ij ± 0.03	<LOQ
cyanidin-3-O-galactoside		23.89 a ± 0.03	MFP 2	21.78 a ± 0.03	6.45 c ± 0.05	4.41 cd ± 0.05	1.06 def ± 0.03	0.37 f ± 0.03	0.21 f ± 0.02	<LOQ
cyanidin-3-O-glucoside		1.43 a ± 0.04		0.89 c ± 0.01	0.18 d ± 0.01	0.11 e ± 0.01	0.02 g ± 0.00	0.02 g ± 0.00	<LOQ	<LOQ
cyanidin-3-O-arabinoside		8.53 a ± 0.03		7.88 b ± 0.01	2.53 d ± 0.01	1.72 e ± 0.02	0.41 h ± 0.03	0.18 jkl ± 0.03	0.11 klm ± 0.01	<LOQ
Total	Apple–chokeberry juice	33.84 a ± 0.10		29.02 c ± 0.11	3.71 f ± 0.04	1.75 g ± 0.11	1.17 h ± 0.03	1.06 h ± 0.1	0.96 h ± 0.06	0.29 ij ± 0.02
cyanidin-3-O-galactoside		23.89 a ± 0.03	TP 1	20.69 a ± 0.06	2.43 b ± 0.00	1.09 def ± 0.06	0.82 def ± 0.02	0.77 def ± 0.07	0.72 def ± 0.04	0.18 f ± 0.01
cyanidin-3-O-glucoside		1.43 a ± 0.04		0.95 b ± 0.01	0.05 f ± 0.01	0.02 g ± 0.00	0.02 g ± 0.00	0.01 gh ± 0.00	0.01 gh ± 0.00	<LOQ
cyanidin-3-O-arabinoside		8.53 a ± 0.03		7.38 c ± 0.04	1.23 f ± 0.03	0.64 g ± 0.05	0.33 hi ± 0.01	0.28 ij ± 0.03	0.23 ijk ± 0.03	0.11 lm ± 0.01

¹ Traditional processing—TP. ² Microwave flow pasteurization—MFP. Mean values marked with identical letters do not show significant statistical differences where p ≤ 0.05.

). MFP and TP caused about 10 and 15% degradation in total anthocyanins. Moreover, after two months of storage, the concentration of anthocyanins decreased significantly after both treatments. However, after MFP, 9.16 mg of anthocyanins in 1 L of juice were detected, whereas after TP, it was only 3.71 mg/L. This trend persisted until the sixth month of storage, after which the trend reversed, and higher concentrations of anthocyanins were detected in TP juices. In both processing methods, the most substantial decrease in anthocyanin content occurred between the second and fourth month of storage. After 12 months, the anthocyanin content was below the quantification level (<LOQ) for the juice processed by the MFP method, while for the juice processed by the TP method, it was 0.29 ± 0.02 mg/L.

3.5. Carotenoid Content

The initial total carotenoid content in the fresh juice was 1.10 ± 0.04 mg per L, including zeaxanthin 0.19 ± 0.01 mg/L, β-cryptoxanthin 0.56 ± 0.02 mg/L, β-carotene 0.35 ± 0.01 mg/L (

Table 4. The total content of carotenoids in apple–peach juice [mg/L].

				Storage Time (Months)
Analysis	Juice	Fresh Sample	Processing Method	0	2	4	6
Total	Apple–peach juice	1.10 b ± 0.04	MFP 2	1.77 a ± 0.04	0.65 c ± 0.04	0.45 d ± 0.03	<LOQ
zeaxanthin		0.19 a ± 0.01		0.11 b ± 0.01	0.09 b ± 0.01	0.07 b ± 0.01	<LOQ
β-cryptoxanthin		0.56 b ± 0.02		0.94 a ± 0.02	0.26 cd ± 0.02	0.19 d ± 0.01	<LOQ
β-carotene		0.35 b ± 0.01		0.72 a ± 0.01	0.30 c ± 0.01	0.19 de ± 0.01	<LOQ
Total	Apple–peach juice	1.10 b ± 0.04	TP 1	0.69 c ± 0.06	0.44 de ± 0.03	0.42 e ± 0.04	<LOQ
zeaxanthin		0.19 a ± 0.01		0.12 b ± 0.02	0.07 b ± 0.02	0.07 b ± 0.02	<LOQ
β-cryptoxanthin		0.56 b ± 0.02		0.36 c ± 0.04	0.18 d ± 0.01	0.17 d ± 0.01	<LOQ
β-carotene		0.35 b ± 0.01		0.21 d ± 0.01	0.19 de ± 0.01	0.18 f ± 0.01	<LOQ

¹ Traditional processing—TP. ² Microwave flow pasteurization—MFP. Mean values marked with identical letters do not show significant statistical differences where p ≤ 0.05.

). MFP increased the carotenoid content (1.77 ± 0.04 mg/L), whereas the TP method decreased (0.69 ± 0.06 mg/L). Both processing methods caused significant degradation of carotenoids over the storage time. Moreover, after the sixth month of storage, carotenoids were degraded (<LOQ) in all tested juices. TP and MFP did not cause statistically significant differences in zeaxanthin content during storage. However, both methods significantly influenced the β-cryptoxanthin and β-carotene concentrations.

3.6. Sensory Analysis

Sensory analysis provides a comprehensive understanding of the product’s characteristics and allows its quality to be assessed. Freshly produced apple–peach juice underwent a sensory evaluation for total quality, receiving a score of 5.9. Similarly, apple–chokeberry juice obtained a score of 5.9. Directly after production, the MFP method yielded less favorable sensory results than the TP method. However, over time and subsequent samples, juices subjected to microwave pasteurization showed minimal differences in taste assessment compared to the standard pasteurized juice.

Figure 2 shows the collected sensory data and parameter analysis for apple–peach juice, and Figure 3 shows those of apple–chokeberry juice for the MFP and TP methods.

4. Discussion

4.1. Pathogenic Bacteria Salmonella spp. and Listeria monocytogenes

Hygienic and sanitary conditions in the industrial environment play a crucial role in ensuring the safety and quality of food [47,48]. Consequently, foodborne pathogens, such as Listeria monocytogenes and Salmonella spp., are considered significant and potentially hazardous [49,50,51]. All tested juices in this study maintained microbiological stability throughout the year of storage. These results affirm that microwave pasteurization can be an effective method—for the food industry.

Research by Guo et al. (2017) [52] found that microwave sterilization can reduce the number of microorganisms in juice. In addition, increasing the temperature, time, and microwave power during pasteurization improves the effectiveness of microwave sterilization [52].

The research conducted by Mendes-Oliveira et al., 2020 [53], focusing on the microwave pasteurization of apple juice and modeling the inactivation of Escherichia coli O157:H7 and Salmonella Typhimurium at temperatures from 80 to 90 °C, aimed to study the kinetics of inactivation of these pathogens. The findings revealed that reduction increased with a rise in power, temperature, and processing time, reducing the number of microorganisms to 7 log 10 cycles. These results indicate the survival characteristics of pathogens under various non-isothermal processing conditions.

In a study conducted by Benlloch-Tinoco, M. et al., 2014, on the kinetics of inactivation of Listeria monocytogenes during microwave pasteurization and conventional thermal processing in kiwi puree, investigations were carried out at three levels of microwave power (600–1000 W) and three temperatures (50–60 °C) [54]. Under the tested conditions of microwave and conventional processing, a reduction in the count of L. monocytogenes by 5 log 10 was achieved, consistent with FDA recommendations for pasteurized products [54]. The level of microwave power applied significantly impacted the rate of L. monocytogenes inactivation, with higher power levels resulting in faster reduction. In the case of microwave heating at a power of 900 W (D 60 °C = 17.35 s) and 1000 W (D 60 °C = 17.04 s), the inactivation of L. monocytogenes occurred more rapidly than in the thermal process (D 60 °C = 37.45 s). Thus, microwave pasteurization demonstrated greater efficiency in inactivating L. monocytogenes than traditional heating. The study revealed that microwave pasteurization is an effective way of inactivating L. monocytogenes.

In a study conducted by Cañumir et al., the impact of microwave pasteurization at different power levels (from 270 to 900 W) on the microbiological quality of apple juice was examined utilizing a home microwave device with a frequency of 2450 MHz [55]. The results were compared with the effects of conventional pasteurization carried out at a temperature of 83 °C for 30 s. The study revealed that microwave pasteurization of apple juice at a power from 720 to 900 W for a period from 60 to 90 s resulted in a reduction in the population of microorganisms by 2 to 4 logarithms. Using a linear model, the deactivation time values ranged from 0.42 ± 0.03 min at 900 W power to 3.88 ± 0.26 min at 270 W power. The energy required to reduce the population of microorganisms by 90% (z-value) was 652.5 ± 2.16 W (58.5 ± 0.4 °C). These observations confirm that the inactivation of E. coli is caused by heat.

In a study by Marszałek et al., 2017 [56], microwave pasteurization was applied to strawberry puree, resulting in the complete inactivation of yeasts and molds. At two different temperatures, 90 °C and 120 °C, the total number of microorganisms was reduced to below 1 log CFU/mL.

Meanwhile, a study by Monroy et al., 2018 [57] observed a complete reduction of coliform bacteria in tamarind drinks. This result was attributed to the phenomenon associated with more excellent energy dispersion during microwave pasteurization than conventional pasteurization. This research highlights the potential of microwave pasteurization.

Selective heating is a critical mechanism that effectively allows microwaves to inactivate microorganisms [58]. Microwaves, a form of electromagnetic radiation, are absorbed by microorganisms, leading to faster heating than the surrounding fluid. This phenomenon is particularly beneficial in pasteurization processes, where rapid and effective inactivation of microorganisms is crucial for ensuring food safety [59]. Electroporation is a process that induces the formation of pores in the cell walls of microorganisms. These pores allow substances to flow through the cell membrane, leading to cell content leakage. Consequently, cells undergo lysis or decay. This mechanism is critical in many technologies, such as microwave pasteurization, effectively inactivates microorganisms [60].

4.2. PPO and POD Activity Levels

Our results showed that both pasteurization methods led to the deactivation of PPO and a significant reduction in POD activity below the limit of quantification. Similar results were obtained by Radoiu et al., 2021 [61]. Observations showed that POD activity reached over 90% deactivation within 5 min at set temperatures of 90 °C and 100 °C [61]. These studies were conducted on yellow peas using microwave heating at a frequency of 2450 MHz.

Research by R. Dhar et al., 2024, showed that at 90 °C, the continuous microwave flow method achieved more than 90% inactivation of PPO, while the conventional thermal method required a temperature of 95 °C for 5 min [62]. In addition, the continuous microwave flow method showed faster enzyme inactivation and better retention of TPC and AOC.

It is worth noting the study by Marszałek et al. in 2015 conducted studies on strawberry puree and showed that regardless of the temperature (90 °C and 120 °C), PPO activity was reduced by about 80% in just 10 s [28]. They also observed that shortening the microwave processing time to 7 s at a temperature of 90 °C reduced PPO activity to about 62%, highlighting the significant impact of processing time on enzyme activity. Moreover, the study emphasized that PPO shows better temperature resistance than peroxidase (POD) [63].

During the studies of Arjmandi et al., 2017, it was shown that higher power and shorter working time in a continuous, semi-industrial microwave oven allowed for obtaining cocktails of higher quality with more significant enzyme reduction than conventional thermal processing [25]. Our studies observed higher quality with more significant enzyme reduction until the sixth month of storage. Additionally, studies by Arjmandi et al. in 2017 showed that microwave heating, especially at higher power and shorter times, such as 1600 W/206 s and 3600 W/93 s, reduces the residual enzymatic activity of peroxidase, pectin methyl esterase, and polygalacturonase [25].

The research results indicate that both pasteurization methods (MFP and TP) effectively deactivated polyphenol oxidase (PPO) and significantly reduced peroxidase (POD) activity below the detectable limit in both juices. This means that both MFP and TP methods can effectively control enzymatic activity in fruit juice, which is essential for its quality and shelf life.

4.3. Total Polyphenol Content (TPC) and Antioxidant Capacity

Antioxidants in juice, such as polyphenols, are crucial in neutralizing free radicals such as DPPH• by donating a hydrogen atom. This reaction leads to the formation of a reduced form of DPPH•, causing a change in the initial coloration of the solution [64]. The observed decrease in absorbance is directly proportional to the content of the oxidized form of DPPH• remaining in the solution. Hence, the higher the concentration of antioxidants in the juice, the faster the reduction of cationic radical’s DPPH• over time [65].

Chlorogenic acid, epicatechin, and phloridzin are typical polyphenolic compounds found in apple juice [66]. Many studies have observed that as the content of polyphenolic ingredients decreases, the antioxidant activity decreases [67,68,69]. On the other hand, the increase in polyphenol content and the associated increase in antioxidant activity induced by microwave radiation on fruit tissue may lead to tissue damage and better extraction of components into juice [52].

Research conducted by various scientists has shown that both conventional heating and microwaving significantly impact enzyme activity and antioxidant content in juices. Our research study emphasized the effect of temperatures at low pH, which can accelerate the hydrolysis of higher phenolic compounds into smaller molecules. Temperature also had an impact during the studies by Cavalcante et al., 2021 [70].

A study by Cavalcante et al., 2021 compared conventional and microwave heating in the temperature range from 50 to 90 °C [70]. The studies adopted three kinetic models to reduce polyphenol oxidase (PPO) activity in buffer and coconut water. The results underscored the significant impact of temperature, time, and heating methods on polyphenol oxidase activity. The study indicates that the Weibull model is suitable for describing the inactivation of the PPO enzyme. It was chosen to simulate PPO inactivation curves and compare traditional heating with microwave heating. The predicted inactivation of PPO using focused microwave heating was higher than that of conventional heating at temperatures above 70 °C.

A study by Kumar et al., 2017 showed that applying MFP to pomelo juice positively affected the concentration of phenolic compounds, surpassing the effects achieved through TP [71]. The total phenolic content of fresh pomelo juice was observed to be 710 mg GAE/L. After processing, it decreased to 690.5 mg GAE/L and 705.3 mg GAE/L for conventional and microwave pasteurization. These results confirm the potential use of the microwave method to improve the concentration of bioactive compounds in pomelo juice. Similar effects were observed in the case of apple–peach and apple–chokeberry juices. Compared to the fresh sample, the TPC in these juices significantly increased by 15% after MFP and TP processing.

The research conducted by Siguemoto et al., 2019 [72] aimed to examine the impact of phenol content under twelve processing conditions using microwave heating and conventional heating. The research findings indicated that changes in phenol content in pasteurized samples were independent of the processing technology. However, it was noted that the content of polyphenols increased during thermal processing, resulting in better extraction from the juice tissues. Moreover, apple juices treated by microwave pasteurization retained their nutritional values better than those pasteurized using conventional heating, with a similar storage time. The conclusions from the study conducted by Siguemoto et al., 2019 [72] indicate that microwave processing technology has promise in thermal preservation. These results may indicate shorter heating time and can help maintain product quality.

Proper storage conditions depend on enzymes’ activity and antioxidants’ stability. Temperature control is crucial for maintaining fruit juices’ quality and nutritional value, such as apple–peach or apple–chokeberry juice. In our research, the TPC in apple–peach and apple–chokeberry juices gradually decreased during storage, as seen in other studies.

4.4. Anthocyanin Content in Apple–Chokeberry Juice

Anthocyanins, with their unique redox properties, serve as significant antioxidants. Assessing the content of oxidized and reduced anthocyanin pigments in fruit products is crucial for shaping color and determining their antioxidant capabilities [73,74,75].

In NFC apple juice, anthocyanins are generally not present with significant concentration, whereas the chokeberries are an essential source of these pigments. The content of anthocyanins in chokeberries varies and ranges from 150 mg [76] to 1790 mg [77] per 100 g of fresh weight (FW) of this fruit. However, it is essential to note that anthocyanins are generally sensitive to processing conditions, including the presence of oxygen, exposure to light, pH changes, etc. [78,79,80,81]. In our tests, a decrease in anthocyanin content can be observed during storage. Additionally, research by Jaiswal et al. shows that the anthocyanin content decreases over time [82]. Research by Jaiswal et al. shows that anthocyanins remain stable at high temperatures without oxygen but quickly degrade in their presence [82]. Moreover, Kim et al.‘s research indicates that anaerobic heating conditions may reduce anthocyanin oxidation [83]. This result confirms our choice of an innovative venting method during pasteurization. Anthocyanins are particularly susceptible to the action of heat and changes in the physicochemical parameters of the environment, as confirmed by Vegara in his studies [84]. In the study conducted by Deylami et al. [85], examining the effect of blanching on the stability of anthocyanins in mangosteen fruits, it was determined that the loss of anthocyanin content ranged from 69% to 89% at a temperature of 60–100 °C during a 12-min cooking process. Higher amounts of anthocyanins affect the product’s maintenance of color and nutritional values. Additionally, in apple–chokeberry juice, a color change can be observed. The sensory results are presented in Section 3.6.

All these studies confirm the downward trend in the content of anthocyanins in fruit juices during storage. In our research, in both processing methods, the most significant decrease in anthocyanin content occurred between the second and fourth month of storage, while after 12 months, the content of anthocyanins in the juice processed by the MFP method was below the level of detectability. Research also points to the need to apply appropriate processing and storage techniques to optimize the content and stability of anthocyanins. Therefore, further research and innovations in processing technology can improve the quality of fruit juices, enhancing their nutritional value and health benefits for consumers.

4.5. Total Content of Carotenoids

The presence of carotenoids in apple–peach juices can also affect their color and appearance and enhance the juice’s nutritional value and antioxidant activity, as confirmed by Lu et al. [86]. The studies identified and quantified 19 free carotenoids, including 11 xanthophylls in ‘Cara Cara’ fruits.

However, over time, carotenoid levels may begin to decline. In our studies, a decrease in the level of carotenoids was observed during 12 months of storage. This phenomenon may be related to thermal processing and the juice’s pH, as demonstrated in a study by Lu (2018) [87]. It showed how epoxy-carotenoid esters were sensitive to the low pH of Cara juice, and esterified β-cryptoxanthin was the only ester group that survived thermal processing [87].

Fratianni et al., 2010, demonstrated pasteurization temperature’s effect on reducing the carotenoid content in orange juice (Citrus sinensis Osbeck) [88]. They also found that increasing the microwave pasteurization temperature from 70 to 85 °C for 60 s causes significant degradation of carotenoids, a decrease of about 50% [88].

Thermal food processing usually leads to the degradation of carotenoids, primarily through oxidation reactions. Some plant enzymes, such as lipoxygenase, also accelerate this degradation [89,90]. During pasteurization, in both the TP and MFP methods, the heat generated by the process affects the degradation of carotenoids. Additionally, carotenoids are compounds sensitive to oxidation, and during pasteurization, access to oxygen is inevitable. Our study noted that the innovative venting method during MFP had a favorable effect on carotenoid content. Generally, a higher proportion of carotenoids is evident in MFP-processed juice than in TP.

4.6. Sensory Analysis

Evaluating the juices’ overall taste, appearance, aroma, and texture is essential. In the case of apple–peach and apple–chokeberry juices, sensory analysis plays a crucial role in determining consumer acceptance. The juices’ color, clarity, and visual appeal are essential to consumer perception. Additionally, the aroma and taste profile are key attributes that influence the overall sensory experience. Consumers highly value the appearance and taste of juices, paying particular attention to the external characteristics of food products [91,92,93,94]. However, according to the literature, when the total color difference (ΔE) between a treated and untreated sample is less than 1.5, it can be judged virtually imperceptible to the human eye [95].

The sensory analysis revealed a well-balanced combination of apple and peach flavors and a pleasing aroma of apple–peach juice. The natural sweetness from the fruits and the absence of any off-flavors contributed to a positive sensory perception. The overall appearance of the juice, including its color and clarity, was also found to be appealing, which is essential for consumer preference. Similarly, in the case of apple–chokeberry juice, the sensory evaluation highlighted the distinctive deep red color and rich aroma, contributing to a visually and aromatically appealing product. The taste profile exhibited a harmonious blend of the apple’s mild sweetness and the unique tartness of chokeberries, which was well-received during sensory analysis. The highest differences were observed in the color evaluation during the overall assessment of the juices. Changes in color in the juices may result from the activity of tissue enzymes (PPO and POD) and non-enzymatic Maillard browning reactions [96]. Additionally, research conducted by Cao et al., 2011 showed that the products of enzymatic oxidation of phenolic compounds can oxidize anthocyanins, leading to the precipitation of polymeric compounds that sedimentation of the juices [97].

Overall, the sensory analysis indicated that of apple–peach and apple–chokeberry juices were well-received in appearance, aroma, and taste, suggesting that these juices have the potential for positive consumer acceptance. This positive sensory evaluation is crucial for the success of these products in the market and underscores the importance of sensory analysis in product development and quality assessment.

5. Conclusions

In conclusion, the research findings indicate that microwave flow pasteurization (MFP) may be a highly suitable and promising technique for preserving high-quality fruit juices while retaining essential nutrients. The innovative MFP method showed significant results over a 12-month storage period, with no presence of Listeria monocytogenes or Salmonella spp. bacteria being detected. Moreover, the inactivation of polyphenol oxidases (PPO) and minimal peroxidase (POD) activity in apple–peach juice demonstrate the effectiveness of MFP in maintaining the physicochemical and sensory quality of the juices.

Furthermore, the gradual decrease in total polyphenol content (TPC) and other bioactive compounds during storage was slower with MFP than traditional pasteurization (TP), indicating better retention of essential nutrients. Additionally, higher concentrations of anthocyanins and carotenoids were noted for up to 4 and 6 months of storage in apple–peach and apple–chokeberry juices, respectively, when using MFP.

These findings suggest that MFP has the potential to offer a more sustainable and effective preservation method for fruit juices, contributing to better nutritional value and overall quality. However, the recommended storage time for MFP-treated juices should not exceed four months. Further research and development in this area can help optimize MFP techniques for commercial-scale applications and improve the shelf life of high-quality fruit juices.

6. Patents

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