Introduction

Blackcurrant (Ribes nigrum L.) is one of the most widely grown and economically significant berry crops in many countries [1, 2]. Its popularity has significantly increased in recent years due to its potential health-beneficial compounds, such as vitamin C, anthocyanins, and phenolic acids among others [3,4,5,6,7]. Blackcurrant can be consumed fresh, but it is often, due to its perishable nature, used to produce products such as fruit juice, nectar, syrup, concentrate, jam, and jelly. Juice production involves different processing steps such as crushing, enzyme treatment, juice separation, clarification, and preservation. Crushing can be done in different ways using crushers, mills, and shredders. Application of pectinolytic enzymes, such as [polygalacturonase, pectin lyase, and pectin methylesterase, is often involved during juice production to disrupt the cell wall and increase the juice yield, as well as to increase the release of anthocyanins and other valuable compounds from the cells [8,9,10,11]. Juices produced in this way usually exhibit a reduction in viscosity due to the degradation of pectin, and some changes in physicochemical properties, such as the content of total soluble solids (TSS), pH value, and turbidity [11,12,13]. To extend the shelf-life of fresh produce, typically a pasteurization as a cost-efficient and established juice preservation method is applied. However, it is recognized in the literature that high temperature treatment can have negative effects on certain quality attributes of juice products, such as freshness and volatile profile, color, nutritional value, and concentration of anthocyanins among others [14,15,16,17]. These changes are attributed to rapid oxidation and degradation processes occurring, not only during the pasteurization, but also along the production course, e.g., during crushing, solid–liquid separation, clarification, and other relevant processes, which in turn can have a significant impact on the final product quality [10, 16, 18,19,20].

In the quest to minimize the quality losses of fruit juices, other extraction and preservation technologies have been widely investigated over the past few decades. Unlike alternative technologies for traditional pasteurization, where pulsed electric fields (PEF) and high hydrostatic pressure (HHP) are probably among the most intensively investigated [21, 22], the spiral filter was introduced only around 10 years ago as an alternative to traditional juice pressing systems[23, 24]. Studies related to this technology can be considered relatively new and still further information related to the technology, especially when combined with other processing technologies is missing. Studies including the spiral filter generally report improved product quality and color of the product, as well as reduced oxidation of valuable compounds due to the limited exposure of the fruit mash to oxygen [25,26,27,28,29,30]. Also, environmental benefits of combined PEF and spiral filter press in a mobile processing unit have been reported, when compared to traditional small-scale counterparts, with benefits mostly related to juice yield and energy use for juice processing [31].

While there are numerous comparison studies between PEF, HPP, and thermal processing [32,33,34,35,36,37,38,39], still different research outcomes can be found, and no clear recommendation can be made to the industry interested in these technologies. These differences are mostly related to different raw materials and juices investigated, ways and scale of how the juice was obtained and processed, and the storage and what type of storage was investigated. In particular, a very limited number of studies combined all three approaches: (1) spiral filter for juice extraction and (2) PEF and HPP compared to the traditional thermal preservation of the juice, (3) while considering the changes during storage. This is especially of interest to know the limitations of PEF and HPP in the inactivation of enzymes, where certain quality benefits obtained immediately after the processing might be lost during storage [36, 40,41,42,43,44,45]. Needless to say, that large number of studies were performed on a lab scale and self-made equipment, which makes the interpretation and comparison of the results very challenging.

Hence, the focus of this study is to compare large-scale advanced preservation technologies (PEF and HPP) with traditional thermal treatment for the preservation of blackcurrant juice produced with an innovative spiral filter press and investigate selected physical and chemical parameters of the juice immediately after the treatment and after refrigerated storage.

Materials and methods

Extraction of blackcurrant juice

Individually quickly frozen (IQF) blackcurrant berries (Ribes nigrum L.) were purchased locally (Elo-Frost GmbH & Co. KG Vechta, Germany) and kept at −18 °C. Before the usage, the berries were allowed to thaw at 4 ± 1 °C for 2 days. For improving the processability and the release of valuable compounds, pectolytic enzyme preparation (pectinesterase, pectin lyase, and endo-polygalacturonate, Fructozym® P and Fructozym® Colour, Erbslöh Geisenheim GmbH, Germany) were diluted with tap water in a ratio of 1:20 and added 8 g of preparation per kilogram of berries. The berries with the enzymes were allowed to rest overnight [8].

For the preparation of the juice, the blackcurrants were crushed using a MultiCrush system (GEA Westfalia Separator, Oelde, Germany) operating under nitrogen to evacuate oxygen and minimize oxidation of the fruit mash. Subsequently, the mash was continuously transported into the spiral-filter (VaculiQ 1000–300 GEA Westfalia Separator, Oelde, Germany), which uses a vacuum to separate the juice through a sieve located around the spiral. A compression spiral with four juicing channels and a shaft inclination of 25 to 38° and a sieve with a pore size of 100 µm were used for the extraction process. It has been reported previously that the technology has the potential not only to increase the juice yield but also to minimize oxidation and result in a juice with improved quality compared to conventional pressing systems [23, 28]. Processing conditions for the juice extraction are described in Table 1. The extraction was done in technical duplicate on the same day, and further analyses were done for individual batches. The extracted juice was directly subjected to different treatments. Untreated extracted juice at day zero was considered as a control sample.

Table 1 Processing conditions for the vacuum spiral filter
Full size table

Preservation treatments of the blackcurrant juice

Preliminary studies were carried out to determine the processing conditions for the selected preservation technologies (data not shown). For this purpose, blackcurrant juice was purchased from a local store and inoculated with indicator microorganisms Lactobacillus plantarum, Listeria innocua, and Escherichia coli. The inoculated juices were treated with three different technologies according to a central composite design (CCD) DoE, aiming to achieve a 5-log reduction of the microorganisms. The experiments were performed in duplicate, and the runs of the experiments were randomized. The fitting of the models was done for second-order models and the evaluation was done with a two-sided 95% confidence interval. During the analysis of the models, backward elimination was applied with p < 0.05 to remove non-significant terms of the equation to improve the fit. Data analysis was performed using Minitab. The conditions for each of the treatments and post-treatment sample handling are described in the following subsections.

Thermal treatment

To perform the continuous thermal treatment of the juices, a tube-in-tube heat exchanger (DIL e.V., self-construction, Quakenbrück, Germany) coupled to a spiral pump (BCSB 025-12, Seepex GmbH, Bottrop, Germany) was used. Thermal pasteurization was carried out at 74 °C with a holding time of 3 s and a total processing time of about 15 s [46]. The samples were collected and cooled immediately in an ice bath.

PEF treatment

The PEF treatment was carried out on a continuous, pilot scale unit (PEF Advantage P10 Elea®, Elea Vertriebs- und Vermarktungsgesellschaft mbH, Quakenbrueck, Germany). A buffer tank was connected to the PEF unit with a spiral pump (BCSB 025-12, Seepex GmbH, Bottrop, Germany) providing a flow rate of 70 l/h. The PEF unit had a maximum output voltage of 25 kV and a rated power of 10 kW. It consists of two co-linear chambers with a 10 mm diameter and an electrode gap of 10 mm. The electrodes are made of titanium and are separated by ceramic insulators. The total residence time in both chambers was calculated to be 1.31 s. Bipolar rectangular pulses with 6 µs width were applied. Considering the synergistic effect of mild temperature before the PEF treatment (typically 30–40 °C) for microbial inactivation [47], the juice was preheated up to 35 °C in the heat exchanger (Sect. 2.2.1). The product outlet temperature was measured inside the pipe during the process, with a built-in thermocouple, and externally outside of the pipe just after the treatment chambers. The treated juice was collected and immediately cooled down in an ice bath. The experiments were performed at an electric field strength of 15 kV/cm and a specific energy of 90 kJ/l. The outlet temperature did not exceed 54 °C during the treatment.

HHP treatment

For the HHP treatment, the juice was filled in 300 ml PET bottles directly after the extraction, followed by the pressure treatment performed on an industrial scale (Wave 6000–55, Hiperbaric, Burgos, Spain). The system has a 55-L high-pressure vessel and operates in batch mode at room temperature. The pressure build-up rate is 150 MPa/min and an almost immediate pressure release. Before the treatment, the temperature of the samples was room temperature around 18 °C. The blackcurrant juice samples were treated at a pressure of 400 MPa for 1 min.

Post-treatment sample handling and shelf-life study

Treated and untreated samples were filled in transparent 300 ml sterile PET bottles with oxygen scavenger lining (kindly provided by Valensina GmbH, Vechta, Germany). Due to the limitations in the research setup to perform aseptic filling, the bottles were filled in the vicinity of a Bunsen burner and under the laminar flow cabinet to minimize the risk of contamination after the preservation treatment and during the filling. One part of the samples was analyzed immediately after the treatments, and another part was deep-frozen and stored until further analysis. The samples intended for the shelf-life study were stored refrigerated at 4 ± 1 °C for 8 weeks, where the samples were taken at weeks 1, 2, 6, and 8. On each sampling day, 2 bottles (1 for each replicate) of juice samples (control and treated) were selected randomly and transferred to a deep freezer at −18 °C, where they were stored until the analysis.

Microbiological analysis: total viable counts, yeasts, and molds

The microbial load was examined immediately after the treatments and during the storage and is expressed in the numbers of total viable count (TVC), yeasts, and molds. The microbial counts were carried out in duplicates (n = 4). The total plate count of mesophilic bacteria was evaluated according to the standardized microbiological method. The test sample was cultivated on a non-selective solid agar medium plate count agar (PCA) and post-spatula rub-out method. The prepared plates were incubated at 30 °C for 72 h [48]. The total yeast and mold counts were determined as per the official collection of methods and analysis of the German Food and Feed Code. The sample was cultivated on yeast extract-glucose-chloramphenicol (YGC) agar at 25 °C for 4–5 days. The colonies were counted and expressed as the number of colonies forming units (CFU) per ml of the sample [49].

Sugar content and sugar profile

The total sugar content and sugar profile were determined using high-performance liquid chromatography (HPLC) coupled with a differential refractometer (RI). The method was performed according to the procedure described by the Association of Official Agricultural Chemists [50] with minor modifications. For sample preparation, distilled water was added to the juice and shaken vigorously in a tempered water bath at 70 °C for 15 min. After the solution had reached the temperature of 25 °C, Carrez I and II reagents were added. Upon this, the solution was filtered (0.45 µm), and injected into the refractometer. The apparatus consisted of an HPLC (Alliance Waters Separations Modul 2690) with a 250/4 Nucleosil 100–3 NH2 column under isocratic conditions and a mobile phase of water and acetonitrile 30:70 (v/v). Sugars were identified and quantified by an 8-point calibration with standard solutions of glucose monohydrate, fructose, and sucrose (R > 0.998). Results are expressed as g of sugar per 100 g of juice.

Titratable acidity and organic acids analysis

Titratable acids were analyzed using a potentiometric method, according to the Official collection of methods for sampling and examining food following §64 of the German Food and Feed Code (LFGB) with titration of a 25 ml sample with 0.1 M NaOH solution to a pH value of 8.1. The results are expressed in millimoles of H+ concentration present per liter of the sample and further converted into conventional acidity by multiplying them with a factor of 0.074 for tartaric acid [51].

An internal HPLC method was used for the determination of organic acids (citric acid, lactic acid, and malic acid). For the measurement, the samples were diluted 1 to 10 in 0.2% phosphoric acid. This was followed by a membrane filtration (0.45 µm). Afterward, the samples were analyzed with an HPLC (HPLC System Alliance Separations Module 2695, Waters Corp., Milford, MA, USA) in connection with a 220 nm diode array detector (photodiode array detector 2996, Waters Corp., Milford, MA, USA). As a separation column, a Nucleodur C18 Gravity-SB (5 µm, 250 × 4 mm, Macherey–Nagel & Co.GmbH, Düren, Germany) was used. The measurement was done with an isocratic elution, as eluent aqueous phosphate buffer (pH 2.7, 0.4 ml/min) was used. The amount of titratable acid was expressed as g of tartaric acid equivalent/l of juice, while organic acids were expressed as g/100 g of juice.

Vitamin C concentration (ascorbic acid/dehydroascorbic acid)

The quantification of vitamin C was done based on a slightly modified method described previously [52]. The juice samples were extracted with a 3% phosphoric acid-acetic acid solution and incubated for 16 h in a dark environment. Before injection into the HPLC (HPLC System Alliance Separations Module 2695, Waters Corp., Milford, MA, USA), the samples were filtered through a 0.45 µm membrane filter. A LiChrospher 100 RP 18 analytical HPLC column was used with a methanol dest. water solution buffered with acetate/acetic acid as the mobile phase (45:55 v/v, isocratic, 0.8 ml/min). As a detector, a fluorescent detector (fluorescence detector 2475 multi lambda, Waters Corp., Milford, MA, USA) with an excitation wavelength of 350 nm and an emission wavelength of 430 nm was used. The results are expressed as mg of ascorbic acid per 100 g of juice.

Residual enzyme activity measurement

Residual activity of polyphenol oxidase (PPO) and peroxidase (POD) was measured spectrophotometrically according to Galeazzi et al. [53] and Gui et al. [54], with slight modifications according to Bi et al. [55]. Methylcatechol was used as a substrate for PPO and guaiacol for POD, both substrates were dissolved in a phosphate buffer with a pH value of 6.5. For both assays, the absorbance was measured at 470 nm and the enzyme activity was expressed in µmol*l−1 *min−1.

Oxygen radical absorbance capacity (ORAC)—antioxidant capacity

The Oxygen Radical Absorbance Capacity (ORAC) assay measures the peroxyl radical scavenging capacity and serves as an indication of the antioxidant capacity of the sample. At 37 °C, the thermo-decomposition of 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH) produces the peroxyl radical. The assay was conducted according to Huang [56] with slight modifications as described by Stübler [57] and is based on the fluorescence decay measurement due to the fluorescein’s peroxyl radical oxidation. At first, the samples were centrifuged at 11,000 × g for 20 min and the supernatants were diluted with phosphate buffer (75 mM, pH 7.4). Subsequently, 25 μl of this dilution was added to 150 μl fluorescein (8 nM), prepared in a phosphate buffer (75 mM), and placed into a black 96-well microplate. The sample was incubated for 30 min at 37 °C. After the incubation, 25 μl AAPH (75 mM) solution in phosphate buffer was added to the solution, and the absorbance was measured every minute for 1 h at an excitation wavelength of 485 nm and an emission wavelength of 528 nm using a microplate reader (Synergy H1, BioTek Instruments Inc., USA). The results were expressed as mM Trolox equivalent (TE) per liter.

Total phenolic content (TPC)

Total phenolic content was measured using the Folin-Ciocalteu method [58]. Samples of blackcurrant juice were mixed with Folin and Ciocalteu’s reagent in equal volumes (0.1 ml), followed by the addition of 1.0 ml sodium carbonate solution. The mixtures were incubated for 2 h at 25 °C and the absorbances were recorded at 760 nm. Gallic acid standard solutions were used for calibration and the results were expressed as g of gallic acid equivalent (GAE) per liter of juice (g GAE L−1).

Total monomeric anthocyanin content (TAC) and composition of anthocyanins

The total anthocyanin content was determined using the pH differential method [59]. The composition of anthocyanins was determined using an HPLC with diode-array detection (DAD) method for the separation and quantification of anthocyanins from bilberry [60]. Both methods are briefly described below.

pH differential method for total monomeric anthocyanin content

The sample was first centrifuged at 24,900 × g for 20 min, and the supernatant was diluted tenfold with 0.025 M potassium chloride buffer at pH 1 or 0.4 M sodium acetate buffer at pH 4.5. The absorbance of the sample was measured at 520 nm and 700 nm with a microplate reader (Synergy H1, BioTek Instruments Inc., Winooski, USA). The TAC is expressed in milligrams of cyanidin-3O-rutinoside (C3R) per liter of juice and calculated using Eq. (1):

$$TAC=\frac{({\left({A}_{520}-{A}_{700}\right)}_{pH1}-{\left({A}_{520}-{A}_{700}\right)}_{pH\mathrm{4,5}}\ . MW\ .DF\ .{10}^{3}}{\varepsilon \ . l}$$
(1)

where A is the absorbance at the specific wavelength and the specific pH value, MW is the molecular weight of C3R (595.4 g/mol), DF is the dilution factor, l is the path length and ε is the molar absorptivity (29,600 l*mol−1*cm−1) [61].

HPLC–DAD method for composition of anthocyanins

Analyses were performed on a Nexera-i LC-2040C 3D Plus (Shimadzu Corporation, Kyoto, Japan) equipped with a UV/Vis detector and a 3 μm Luna® C18 (2) column (4,6 mm × 250 mm) (Phenomenex, Torrance, CA, USA). To determine anthocyanins in the aqueous phase, samples were centrifuged at 15,000 g for 30 min at 4 °C and filtered using a 0.45 µm before injection.

Degradation kinetics of vitamin c and anthocyanins

To obtain insights into the effects of storage and different treatments on vitamin C in the treated blackcurrant juice, a zero-order kinetic model was used to model the changes in the vitamin C concentration in the blackcurrant juice:

$${\text{C}}(t)={{\text{C}}}_{0}-{\text{kt}}$$
(2)

where \(C(t)\) is the concentration at a specific storage time \(t\) (weeks), \({C}_{0}\) is the initial concentration and \(k\) represents the slope, which indicates the decay rate. The half-life (\({t}_{1/2}\)) of vitamin C was calculated using Eq. (3):

$${{\text{t}}}_{1/2}= \frac{{{\text{C}}}_{0}}{2{\text{k}}}$$
(3)

The degradation of anthocyanins in the juice was modeled using a first-order rate equation:

$${\text{C}}(t)= {{\text{C}}}_{0}{{\text{e}}}^{-{\text{kt}}}$$
(4)

Half-life estimation, in this case, is independent of initial concentration and is determined using the following equation:

$${{\text{t}}}_{1/2}= \frac{\mathit{ln}2}{{\text{k}}}$$
(5)

The appropriate kinetic model was identified by examining the changes in response during the storage, by calculating the \({R}^{2}\) along with a visual inspection of the parity and residual plot. The kinetic parameter was performed using linear and non-linear regression functions in Sigmaplot (Version 14.0 SYSTAT, Point Richmond, CA, USA).

Statistical analysis

All the analyses were performed in triplicates, while some were done in duplicate for each technically duplicated juice sample. The results were expressed as the arithmetic means with the standard deviation of all the replications. To identify the outlier and improve the normality of the dataset, the interquartile range (IQR) test was used in Microsoft Excel. One-way analyses of variance (ANOVA) with Tukey’s posthoc test for individual differences were used to compare the different quality parameters and specific treatments over time. The significance level of the results was set at p < 0.05. The correlation between enzymatic activity and storage period for the different treatments in the juice was evaluated by regression analysis. All plots and statistical analyses were performed in Sigmaplot 14.0 (Systat Software Inc., San Jose, USA).

Results and discussion

Microbiological analysis

The results of the microbiological counts in treated juice and during storage are presented in Fig. 1. The initial counts in the untreated blackcurrant juice were around 4 ± 0,09 log CFU/ml, with a significant (p = 0.013) increase of roughly 0,6 log CFU/ml during storage up to 4.6 ± 0,15 log CFU/ml after week 4. After that point, the microbial counts were significantly (p < 0.001) reduced to a value of 3.7 ± 0,4 log CFU/ml at the end of the 8-week storage period, indicating the potential dying of part of the microbial population. In the comparison of the initial count and the count at the end of the storage period, no significant (p = 0.630) differences were noted. All treatments caused a significant decrease (< 0,001) in the microbial load. Up to two log reduction was achieved in the treated juice, being the highest after the thermal treatment (2-log reduction), while PEF and HHP resulted in a 1-log and a 1.5-log reduction of total microflora, respectively. During storage, the measured values of the treated samples fluctuated slightly, but this was not significant and rather indicates a natural fluctuation. The inactivation is comparable to other studies for example for PEF-treated (35 kV/cm, 72 µs, 0.6 log CFU/ml reduction) pomegranate juice [62], thermally (90 °C, 60 s, 2 log CFU/ml reduction) and high-pressure treated (300 MPa, 3 min, 1 log CFU/ml reduction) white-grape juice [63], and PEF (38 kV/cm, 219.3 kJ/l, 5 log CFU/ml reduction) and thermally (96 °C, 5 min, 3 log CFU/ml reduction) treated mango nectar [64]. However, it should be noted that compared to our current study, the treatment intensities in the mentioned studies were slightly higher. During the storage period, the number of microorganisms did not change significantly, which is probably related to refrigerated storage and the low pH value of the blackcurrant juice (close to 2.8).

Fig. 1
figure 1

Effect of different treatments on total viable plate count (TVC), yeast, and mold content in blackcurrant juice during the storage period of 8 weeks. The presented bars are the mean, while the error bars represent the standard deviation of the mean (n = 4)

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The initial number of yeasts in untreated blackcurrant juice was counted at 4.1 ± 0,19 1og CFU/ml. Like the TVC the yeast count starts to increase significantly (p < 0.001) until week 2 up to a value of 4.6 ± 0.15 log CFU/ml. During the remaining storage time, the number of yeasts decreased significantly (p < 0.001) to 3.3 ± 0,51 log CFU/ml. The decrease could be related to the facultative anaerobic nature of the yeast in combination with the low pH of the juice, mentioned above [65]. Each of the investigated preservation technologies reduced the number of yeast colonies below the detection limit (< 1 log CFU/ml), and no subsequent growth was observed for these samples throughout the investigated storage period.

Also, the initial mold content of 4.3 log CFU/ml was successfully reduced to below the detection limit by each of the treatments. Presumably, due to the low oxygen content in the bottle in combination with their high oxygen barrier properties, and the strict aerobic nature of molds, the untreated sample showed a decrease in the mold number during the storage. These obtained results are consistent with results previously reported in the literature for orange juice, fruit smoothies, and aronia berry puree [32, 45, 66].

Sugar profile

The effect of HHP, PEF, and thermal treatment, as well as the impact of storage on the sugar profile of blackcurrant juice, is presented in Fig. 2. The total sugar content in the untreated blackcurrant juice sample was 10.97 g/100 g, which corresponds with the concentration values found in the literature, 7.4 to 11.0 g/100 g for enzymatically or non-enzymatically processed blackcurrant juice [1, 67]. From Fig. 2, it can be seen that the untreated sample had a relatively high content of fructose and glucose but a very low sucrose concentration. This could be explained by the metabolization of glucose at the late-ripening stages of the fruit [68]. During storage, the sucrose content decreased in all samples. This is presumably due to the degradation of sucrose to fructose and glucose in the acidic conditions of the blackcurrant juice [36, 44]. After the sucrose degradation, the fructose and glucose content, during the refrigerated storage, remained relatively stable in all samples. Comparable findings were reported for differently processed apple juice stored for 3 weeks at 4 °C. Thermal (72 °C, 15 s, and 85 °C, 30 s), PEF (12.5 kV/cm, 76.4 kJ/L and 12.3 kV/cm, 132.5 kJ/L), and HHP (400 MPa, 3 min, and 600 MPa, 3 min) treatments influenced the sugar composition. The authors noted that during a 3-week storage period, the sucrose content decreased significantly, while the glucose and fructose content increased not significantly [36]. In the current study, the sugar profile in treated blackcurrant juice was affected to a lesser extent compared to the findings in the literature, which could be due to lower processing intensity (PEF and HHP) compared to other studies as well as to the low concentration of sucrose in the pressed juice.

Fig. 2
figure 2

Sugar profile of differently treated blackcurrant juice samples at week 0 (W0), 1 (W1), 4 (W4), and 8 (W8) of storage. Each point represents the mean, while the error bars represent the standard deviation of this mean (n = 4)

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Acid content

Acids are the second largest constituent in most fruits and are important sensory contributors, considering that the sugar-acid ratio determines the characteristic flavor and astringency of the fruit and the product thereof. Citric acid is considered to be the most important acid in blackcurrant and accounts for about 88% of the total acids, followed by malic acid which contributes about 11%. These values are also found in the literature [69]. In the current study, the concentration of citric acid in the untreated blackcurrant juice sample was 3.44 g/100 g, and 0.36 g/100 g of malic acid. It could be found that the value of titratable acidity calculated as tartaric acid equivalent was 40.06 g/L (Fig. 3). These results are in agreement with the reported results on blackcurrants in other studies [70,71,72]. During the storage of blackcurrant juice, titratable and citric acid concentrations slightly increased over time, while the concentration of malic acid slightly decreased. These changes can be potentially attributed to microbial metabolic by-products in the samples. A comparable increase in titratable acids was observed for pomegranate juice treated with high pressure at 450 and 550 MPa for 30–150 s [73]. Our results are in agreement with previous research findings regarding pressure-treated fruit juices, such as cucumber and white grape juice [42, 63].

Fig. 3
figure 3

Acid profile of differently treated blackcurrant juice samples at weeks 0, 1, 4, and 8. Each point represents the mean, while the error bars represent the standard deviation of this mean (n = 4)

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Vitamin C concentration

In the current study, the untreated blackcurrant juice sample contained 206 mg/100 g of vitamin C (Table 2), which is significantly higher compared to the concentration reported previously for blackcurrant juice (2.54–45.02 mg/100 g) [2]. However, the measured value is comparable to the vitamin C content of fresh fruits, which is reported to be between 150 and 285 mg/100 g [74, 75]. This higher concentration of vitamin C could be attributed to two effects. The first one may be related to the enzymatic pre-treatment of berries that breaks down the cell membranes and allows for improved extraction of the vitamin. Secondly, the spiral filter used in this study, where exposure of the fruit mash to oxygen is limited due to the addition of nitrogen, could have potentially resulted in lower oxidation and higher concentration of vitamin C [76].

Table 2 Effect of different treatments on vitamin C concentration in blackcurrant juice at week 0 (W0) and 8 (W8) of storage and zero-order kinetic modeling of the degradation
Full size table

Comparing PEF and thermally treated samples to untreated samples, no significant differences in vitamin C concentration could be observed. Only a slightly higher concentration was detected for the high-pressure treated sample. Even though a slight reduction of vitamin C was observed for all samples after 8 weeks of refrigerated storage, most of the initial concentration (around 93%) could be retained. It is reported in the literature that the high anthocyanin content of blackcurrants has a positive effect on the stability of vitamin C. Iversen [18] concluded that the main antioxidant in black currant nectar is Vitamin C, but anthocyanins were able to regenerate the formed ascorbic acid oxide radical by oxidizing them back to Vitamin C. This was shown by a higher degeneration rate for the anthocyanins in comparison with the rate of Vitamin C (1.1*10–3 days for Vitamin C; 3.2*10–3 for total Anthocyanins). While the thermal sensitivity of ascorbic acid has often been studied and reported in different fruit matrices [77], different results can be found regarding vitamin C retention after PEF and HHP treatments [78, 79]. It is believed that the PEF and HHP treatments have a less diminishing impact on vitamins in general, mostly due to the fundamental principles of the treatment and lower thermal load. The degradation of vitamin C content in our study was described using a zero-order equation, which is consistent with previously reported work on strawberry juice with a correlation factor between 0.994 and 0.996 [80], for a model fruit juice with a correlation factor of 0.89 to 0.98 for an oxygen content in the bottle between 0.03 and 4.84% [81]. The concentration of vitamin C in all the treatments exhibited similar degradation rates (between 1.37 and 1.52 weeks−1, Table 2), and the half-life was calculated to be between 70 and 76 weeks. No significant difference was observed between the decay rate of the control and the differently treated blackcurrant samples (p = 0,287). Even a lower decay rate of 1.1 × 10–3 days−1 was reported for enzymatically treated blackcurrant nectar thermally treated at 80 °C for 27 s and stored at 20 °C [18]. These differences primarily arise from the type of blackcurrant cultivar and the processing parameters, such as temperature during treatment and storage conditions.

Enzyme activity measurement

Almost no enzyme activity was measured for all the investigated samples, including the untreated sample (data not shown). The activity values for polyphenol oxidase (PPO) ranged between 0.08 and 0.1 µmol/l*min and for peroxidase (POD) ranged between 0.003 and 0.01 µmol/l*min.

Oxygen radical absorbance capacity (ORAC)—antioxidant capacity

The oxygen radical absorbance capacity (ORAC) assay is often considered advantageous over other antioxidant assays because it uses a single biologically relevant free-radical source, i.e., peroxyl radical for antioxidant determination [82]. The results of the assay for each treatment and investigated storage day are presented in Fig. 4. The untreated sample had ORAC values of 56.29 mM TE/l (Fig. 4), which is more than 3 times higher compared to values reported in the literature for blackcurrant extracts or drinks [83, 84]. The measured ORAC values indicate that they are closer to the values found in the fresh blackcurrant berries, typically around 36.9 to 93.1 mM TE/kg [85]. These higher ORAC values in the blackcurrant juice could be attributed either to the ripening state of the berries or to the beneficial extraction and preservation conditions used in this study. Comparing the treated samples each other, thermal and PEF-treated samples showed 7% and 9% higher ORAC values compared to the untreated samples, respectively. This increase could be related to the potential formation of Maillard reaction products in response to heat which can exhibit peroxyl radical scavenging activity [86]. During storage, all samples exhibited a reduction in antioxidant capacity, namely untreated, thermally, and PEF-treated samples around 35% reduction, whereas HHP-treated samples showed around 20% reduction of the initial antioxidant capacity. The loss of antioxidant capacity correlated with the observed degradation of ascorbic acid (Sect. 3.4), phenols (Sect. 3.7), and anthocyanins (Sect. 3.9). With their loss, it can be expected that the antioxidant activity will reduce accordingly. Similar observations on ORAC reduction were reported in a study on strawberry puree stored over 16 weeks at 6 °C and 22 °C. The ORAC values were reduced by 9% and 18% for samples stored at 6 and 22 °C, respectively [87], clearly indicating their stability dependence on storage temperature. On the contrary, no significant change in ORAC values was observed for thermally treated (95 °C, 3 min) blueberry juice and puree stored over six months at 25 °C [15].

Fig. 4
figure 4

Oxygen radical scavenging of the differently treated samples of the storage period of 8 weeks. Each point represents the mean, while the error bars represent the standard deviation of this mean (n = 6)

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Total phenolic content (TPC)

Blackcurrants are considered a good source of polyphenols. Depending on the cultivar, the total phenolic content (TPC) in fresh berries can vary from 5.5 to 13.4 g GAE/kg [85], whereas in juices and extracts, this value is reported to be lower around 2.5 g GAE/kg [84]. Our results show that the TPC with a value of 5.32 g GAE/kg (Fig. 5) was significantly higher in the extracted, untreated juice. Presumably, the combination of enzymatic pre-treatment and vacuum spiral filter for juice extraction could have resulted in a juice with a higher TPC, closer to the one in fresh fruit. By looking at the processing, it can be seen that thermal treatment resulted in a significant increase (6,7%) in the TPC compared to the untreated juice sample (Fig. 5). Contrary to that, the HHP treatment resulted in a significant reduction in TPC by 8% compared to the initial value in untreated juice. For the PEF treatment, no significant changes, in comparison with the untreated samples, could be observed. These findings are in line with other studies where the reduction of TPC in the range of 6% and 22% was reported for the HHP treatment of tomato juice (600 MPa, 1 min) and apple puree (400 MPa, 5 min), respectively [88, 89]. Although the exact mechanism behind this increase and reduction in TPC is not fully understood, these discrepancies in the results may be attributed to the possible higher release of phenolic compounds or potential interactions between polyphenols and proteins, which in turn might give different analytical results [90, 91].

Fig. 5
figure 5

Total phenolic content of the differently treated samples of the storage period of 8 weeks. Each point represents the mean, while the error bars represent the standard deviation of this mean (n = 4)

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Secondly, they can be attributed to different working principles and thermal loads for the three technologies, and with that the level of impact on phenolic-binding proteins in juices [57, 92]. As for the PEF treatment, different findings are reported in the literature concerning the TPC. As expected, low-intensity PEF treatment (1–5 kV/cm, 10 kJ/kg) of whole blueberry fruit resulted in an increase of TPC by 43% [93]. On the other hand, high-intensity PEF treatment (36 kV/cm, 100 µs, energy input missing) of blueberry juice resulted in no significant change in the TPC [94]. Additionally, no significant reduction was observed in the PEF (35 kV/cm, 200 Hz, energy input missing) treated fruit juice-soymilk beverage compared to the untreated sample [95]. During storage, the TPC for the investigated samples remained relatively stable, with no huge differences between week 0 and week 8. Other studies reported a slight increase in TPC over storage, for example, in eight pasteurized commercial fruit beverages studied for 20 days of storage at temperatures of 4 °C, 8 °C, and 11 °C. The study reported an increase in TPC for all beverages and for each storage temperature, with samples stored at 4 °C exhibiting the highest increase [96]. This increase could be attributed to an increment in polyphenolic compounds due to microbial growth or the reactions between oxidized polyphenols [43, 97]. The formation of new compounds with an antioxidative character could affect the measurements since the Folin-Ciocalteu reagent reacts with reducing substances to form chromogens that can also be detected spectrophotometrically [98]. Therefore, it is important to consider the potential interference of these new compounds when using the Folin-Ciocalteu reagent, as these potential interferences can affect the accuracy of the antioxidative measurements.

Total monomeric anthocyanin content (TAC)

Total monomeric anthocyanin content (TAC) can vary between 1.3 and 4.1 mg/kg among fresh blackcurrant berries, depending on the variety, which can be a cultivar or a native one [85]. Major anthocyanins in blackcurrant are cyanidin 3-O-rutinoside, delphinidin 3-O-rutinoside, cyanidin 3-O-glucoside, and delphinidin 3-O-glucoside. These anthocyanins are also confirmed in our study (Table 4). The untreated juice sample had a total anthocyanin content of 970 mg/l (Table 3), which is in the range of the TAC levels of commercially available ready-to-drink blackcurrant juice products [2]. HHP treatment had no significant impact on the TAC, and it was comparable to the control sample. Thermal and PEF treatments resulted in a comparable increase of TAC by approximately 10%. In other studies, the high-pressure treatment of bayberry juice at 400 – 600 MPa for 10 min resulted in no significant changes in the TAC, implying the pressure stability of anthocyanins [99]. Strawberry juice treated by PEF (5 kV/cm for 1 ms, 7 µs pulse width and frequency of 150 Hz), showed 2 to 4% higher anthocyanin content compared to the control sample [100].

Table 3 Total anthocyanin content in differently treated blackcurrant juice at week 0 (W0) and 8 (W8) of storage, measured with the pH differential method and first-order kinetic modeling of the degradation
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Only a limited number of studies are available on the stability of anthocyanin in complex food systems, their possible interactions with other compounds (e.g., proteins), and their stability against different preservation treatments (e.g., thermal, PEF, or HHP) [101]. The anthocyanin degradation and half-life estimation in the blackcurrant juice of our study were evaluated using the first-order kinetic equation for both methods used: pH differential (Table 3) and chromatography method (Table 4). All treated samples analyzed using the pH differential method exhibited a good fit to the model, validating the accuracy of the correlation between storage time and anthocyanin content (Table 3). However, chromatography measurements indicated higher degradation of the TAC occurred for PEF-treated samples compared to thermal and HPP samples. This trend was observed for all the analyzed anthocyanins (D3R, D3G, C3R, and C3G) (Table 4). This increased degradation in the PEF-treated sample could be attributed to either the increased reaction of anthocyanins with other organic compounds or the cleavage of the anthocyanins during the storage period [102, 103]. Similar findings on the stability of monomeric anthocyanins in blackcurrant juice at different storage temperatures revealed different degradation rates [104]. The k value decreased with increasing storage temperature over the 60-h study period and the low half-life of 180 h at 4 °C. This variation in the results can be explained by the instability and susceptibility of anthocyanins during processing and storage, which are largely influenced by pH value, high temperature, oxygen presence, light, and the presence of other substances such as ascorbic acid, sugars, sulfites, and metal ions [105].

Table 4 The content of the main anthocyanins in differentially treated blackcurrant juice at week 0 (W0) and 8 (W8) of storage, measured with the HPLC – DAD method and first-order kinetic modeling of the degradation
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Conclusion

This study investigated the effects of an innovative spiral filter press, a vacuum pressing system, in combination with selected gentle preservation technologies on the physiochemical properties of blackcurrant juice. The juice was studied immediately after the processing, as well as during the refrigerated storage. The results showed that the physicochemical properties of the blackcurrant juice obtained with the spiral filter press are positively influenced, such as a high release of vitamin C, or the total phenol content compared to conventionally produced juices. Although the benefits have been seen after the extraction of the juice, only slight improvements have been observed for the alternative preservation techniques HPP and PEF compared to the thermal treatment. All three technologies resulted in a microbially stable juice over the 8 weeks of shelf-life, and other parameters were differently affected. For example, the HPP resulted in the highest vitamin C content, but the antioxidant capacity and total phenolic content were the highest for the PEF and thermally treated juice. This makes it challenging to make a statement on which technology is the most suitable for industrial application not only for the blackcurrant juice, but also for other juices. In that sense, taking into account other parameters relevant to the industrial production, such as volume of production, preferred packaging, investment, and operational costs as well as other aspects, are relevant and along with the quality parameters should be considered when choosing the most appropriate technology for preservation of juices.