1. Introduction
Currant fruit belongs to the genus
Ribes cultivated in the temperate regions of Europe and North America, part of South America, Asia and Northwest Africa [
1]. Currants are woody, perineal shrubs with almost 150 varying species. Currant berries are known for their attractive colors like black, red, and white (yellow) with unique flavors of sweet and tart. Berries are important sources of dietary fibers, micronutrients and phytochemicals [
2]. The total global production of currants was around 650,000 tons for the last 10 years [
3]. Currant fruits are consumed as eagerly as fresh ones, but are also found in the process of making juices, nectars, jams, jellies or syrups [
4]. Raw fruits are very much susceptible to spoilage and might quickly undergo microbial deterioration. Therefore, juice production could be a preferable method, as well as a quick and an easy way to preserve the nutritional values of currant for a prolonged time.
One of the most common, predominant and commercially used species in fruit production is black currant (
Ribes nigrum L.) [
5]. This currant fruit has a color ranging from dark purple to black with a glossy skin and contains a calyx persisting at the apex part of the fruit. Black currant fruits are rich in bioactive substances, mainly phenolics and vitamin C. The amount of vitamin C in black currant ranges from 40 to even 310 mg/100 g of fresh fruit. Anthocyanins are the pigments responsible for the dark color of black currant fruit. There are four main anthocyanins found: delphinidin-3-
O-glucoside, delphinidin-3-
O-rutinoside, cyanidin-3-
O-glucoside and cyanidin-3-
O-rutinoside. Their content could reach up to 250 mg/100 g of fresh fruit which constitutes nearly 80% of the flavonoids in black currant [
2,
6,
7,
8].
The next most appreciated species is red currant (
Ribes rubrum L.). Red currant berries are mainly consumed in an untreated form as a refreshing sour summer fruit. Also, they can be used for cooked dishes like tarts and for processing into juice, jam and jelly. The red color of the fruit is also caused by anthocyanins, but its content is lower compared to that of black currant and does not exceed 60 mg/100 g of fresh weight. The main anthocyanins identified in red currant are cyanidin-3-
O-rutinoside, cyanidin-3-
O-xylosylrutinoside and cyanidin-3-
O-glucoside [
9,
10]. Also, vitamin C content is relatively low compared to that of black currant, whereas the content in the range from 20 to 70 mg/100 g is still appreciable from the nutritional point of view [
2,
7,
9,
11].
White currant (
Ribes sativum) is the least popular species of currant. This fruit does not contain any anthocyanins, but is still considered as a valuable crop. White currant berries contain a high amount of hydroxybenzoic acid derivatives and proanthocyanidins [
12]. White currant fruit can be considered as a source of ascorbic acid. The content of this compound could vary from 21 to 39 mg/100 g of fresh weight [
13,
14].
The advancement in science and technology over the years have enabled its development in the utilization of an innovative and versatile technology like ultrasound for the purpose of food processing, process control and preservation [
15]. Some examples of applications are non-destructive examination of food composition, texture and other physicochemical features. During processing, ultrasound could generate emulsions or inactivate microbials in products or sterilize surfaces [
16]. A few other studies have suggested that ultrasound is very useful for enhanced extraction of valuable compounds from plant materials [
17,
18]. A potentially promising application of ultrasound is in the field of juice processing, as it could improve nutrients content and facilitate preservation [
19]. Sonication treatment of juice could enhance the amount of phenolic compounds in the case of some fruits, like pitch, grapefruit or strawberry [
20,
21,
22].
However, the influence of ultrasound applied on mash before juice pressing on obtained juice parameters and bioactive compounds contents has not been well investigated and results obtained are not consistent. Some authors suggest that treating fruit tissue before juice extraction by ultrasound could disrupt the cell wall and might lead to increased juice yield, and could improve extraction of water-soluble compounds [
21]. Positive results of such treatment in terms of yield, physicochemical features and antioxidant compounds in the case of mulberry were observed [
23]. On the other hand, Radziejewska-Kubzdela et al. [
24] in the case of barberry juice did not observe an increase of yield and ascorbic acid content in the juice obtained. Furthermore, the antioxidant capacity of the juices made from the mash subjected to sonication decreased by about 55% compared to raw material. To the best of our knowledge the studies about the influence of enzymatic combined with ultrasound treatment of the mash before juice production, especially in the case of different colored currant fruits, are limited. Also, enzymatic maceration combined with sonication has not been well examined yet. Therefore, the aim of this study was to evaluate the effect of ultrasound treatment of black, red and white currant fruits mash prior to juice pressing on bioactive compounds content, antioxidant capacity, yield and some quality parameters of the obtained juice.
3. Materials and Methods
3.1. Currant Fruit
The three kinds of currant fruits: black currant [BC] (cv. Öjebyn), red currant [RC] (cv. Jonkheer van Tets) and white currant [WC] (cv. Weißea us Jüterbog in English: White from Juterbog) were used. Plants were cultivated on a private farm in central Poland (Ziemia Łódzka district, Sieradz county) in 2018. Ripe currant fruits were manually harvested in June, destemmed, packed in plastic bags in portions of 500 g each, and frozen at −18 °C. Fruit were stored in a freezer before being used for juice preparation.
3.2. Enzymatic and Ultrasound Mash Treatment
Enzymatic combined with ultrasound treatment of mash was conducted in the ultrasound water bath SW3H (Sonoswiss AG, Ramsen, Switzerland). This equipment operated at a frequency of 37 kHz with ultrasonic effective power of 80 W. The currant fruits were defrost at room temperature. Completely thawed fruits were crushed in a Thermomix TM 3 (Vorwerk SE and Co. KG, Wuppertal, Germany) mixer at 50 °C for 8 min at speed level 5. After crushing, portions of the mashed fruits were weighed at about 500 g (exact mass of portion was noted), transferred and evenly distributed into the stainless-steel chamber of the ultrasound water bath. Enzyme Fructozym EC color (Erbslöh Geisenheim GmbH, Geisenheim, Germany) was used for enzymatic mash treatment. Mash maceration is an important step in currant juice production. It cause improved press and avoid gelatinization of the juice. Enzymatic maceration conditions, dosage and time were set according to enzyme producer guidelines. To the crushed currant, the enzyme was added (0.3 mL/kg of mash), and this was mixed and then placed in a water bath at 50 °C without ultrasound in order to allow the enzyme to work. After a specific time, the ultrasound bath was set to maximum ultrasound power and mashes were sonicated for the durations shown in
Figure 1. Total time of processing of each samples was 75 min.
3.3. Juice Pressing
Mash after treatment was transferred to the cage of the hydraulic press Para-Press (Paul Arauner GmbH, Kitzingen, Germany). The pressing pressure was produced by tap water (3.0 ± 0.1 bar) by means of a rubber bladder for 5 min. Juice was collected in a plastic bucket and weighed. Then, the juice was transferred to a plastic bottle and frozen at −50°C then stored in that condition until analysis.
3.4. Evaluation of Juice Yield
Mashes before the juice production and obtained juice were weighed on a laboratory scale. The juice yields were calculated from the formula as follows:
3.5. Determination of pH, Titratable Acidity and Soluble Solids
Analysis of pH was conducted with pH-meter Orion model 710A (Thermo Fisher Scientific, Waltham, MA, USA). Titratable acidity (TA) determination of the juice samples was done using titration method with 0.1 M NaOH. The total acidity was expressed in g citric acid/L. The total soluble solids (TSS) were determined in an optical refractometer, model HI96801 (Hanna Instruments, Smithfield, RI, USA) and expressed as percentage of sucrose (°Bx).
3.6. Total Phenolic Content (TPC)
The total phenolic content was assessed using the Folin–Ciocalteu method [
45]. Prior to the analysis, the juice samples were centrifuged at 5000×
g for 10 min. The results were expressed as mg gallic acid equivalents/100 mL (GAE).
3.7. Determination of Phenolic Compounds by HPLC
Determination of phenolic compounds was carried out on an Agilent Technologies 1200 Rapid Resolution system (Agilent, Santa Clara, CA, USA) equipped with degasser, binary pump, autosampler holder, column holder and diode array detector (DAD). For chromatographic resolution, Zorbax SB C-18, 5 µm, 4.6 × 150 nm column was used. The mobile phases were (A) 6% acetic acid in 2 mM sodium acetate (v/v) and (B) acetonitrile. The elution gradient was linear as follows: 0–15 min: 0–5% B; 15–25 min: 5–20% B; 25–30 min: 20–30% B; 30–35 min: 30–50% B; 35–40 min: 100% B. Juice samples were centrifuged at 5000× g for 10 min, appropriately diluted with distilled water, and filtered through 0.45 µm PTFE syringe filters. Gallic acid was quantified at 280 nm and other phenolics at 320 nm. Gallic acid and chlorogenic acid were used as external standards.
3.8. Determination of Anthocyanins by HPLC
The anthocyanin content of juices was determined using HPLC (Agilent, Santa Clara, CA, USA) as described by Oszmiański and Sapis [
46]. Juices before analysis were centrifuged at 5000×
g for 10 min and diluted with distilled water. The same HPLC system as for phenolic compounds determination was used. The mobile phases were: (A) 10% formic acid in water (
v/
v) and (B) formic acid/acetonitrile/water (1/3/6;
v/
v/
v). The elution gradient was linear as follows: from 0 to 15 min, the solvent B increased from 20 to 50%, from 16 to 20 min, it increased up 100%, from 20 to 21 it remained at 100% and from 21 to 23 min it decreased to 20%. The detector was set for scanning in the range of 400 to 700 nm. Quantification was undertaken at 520 nm and calculated as cyanidin-3-
O-glucoside.
3.9. Color Measurement
The color parameters of the currant fruit juice samples were measured using a spectrophotometer Konica-Minolta CM-3600 d (Konica Minolta Inc., Tokyo, Japan). The color parameters were expressed in terms of the CIE L*a*b* system. Measurement parameters were as follows: measurement in transmittance, the observer angle 10°, illuminant D65 as light source. Measurements were conducted in a glass cuvette with optical length of 10 mm.
3.10. Determination of Ascorbic Acid Content by HPLC
Ascorbic acid content determination was performed according to [
47]. Currant juices were mixed with 1% meta-phosphoric acid. To reduce dehydroascorbic acid into ascorbic acid form before chromatographic separation, dithiothreitol was used. The HPLC system was the same as described in
Section 3.7. The mobile phase was 1 mM potassium dihydrogen phosphate in water and methanol. The detector was set for scanning in the range of 200 to 380 nm. Quantification was undertaken at 245 nm.
3.11. Antioxidant Capacity Determination (TEAC)
The antioxidant capacity was determined using ABTS
●+ assay following the procedure of Re et al. [
48]. The results were expressed as micromoles of Trolox equivalent antioxidant capacity per mL of juice (TEAC).
3.12. Statistical Analysis
The analysis of variance (ANOVA) was used to determine the significance of the main effects. Tukey’s post hoc test was used to determine differences between the mean values of multiple groups. Correlations were analyzed with Pearson’s test. Statistical significance was set at p < 0.05. The Statistica 13.1 software (TIBCO Software Inc., Palo Alto, CA, USA) and Excel 2010 (Microsoft Corporation, Redmond, WA, USA) was used for the analyses.