Effect of Pre-Treatment on the Pressing Yield and Quality of Grape Juice Obtained from Grapes Grown in Poland

Abstract

Gradual climate warming is favoring viticulture in Poland. At the same time, there is a lack of information about the suitability of grape varieties grown in Poland for processing. The primary aim of the study was to determine the effect of pre-treatment on the pressing yield of grape juice and its qualitative assessment. The study applied pre-treatment of raw material, involving either enzymatic liquefaction of the pulp in the first case or freezing and thawing of the pulp prior to pressing in the second case. There was considerable variation among the grape varieties studied in terms of the characteristics under analysis. The varietal characteristics had a significant effect on the pressing yield and the quality of the juice obtained. Pre-treatment had different effects on the pressing yield of the individual grape varieties and the quality of the obtained juices. The research carried out may improve the efficiency and quality of agricultural production with the rational use of locally grown grape hybrids.

1. Introduction

The grapevine is one of the most economically important orchard crops. In recent years, growing interest in vine cultivation in Polish soil and in climatic conditions has been noted. This is linked, among other things, to the warming of the climate in Poland. Poland has a moderate, warm, transitional climate with an average annual air temperature of approximately 9.5 °C (2020). Varieties that have suitable functional qualities are being sought, especially in terms of the production of juice, wine and health-promoting properties. The development of viticulture in Poland is a result of the increasing availability of varieties adapted to the climatic conditions prevailing in Poland. At the same time, the cultivation of local grape varieties and their on-site processing contribute to the development of sustainable agriculture. Grapes and products derived from them are characterized by a high content of bioactive components [1,2]. The chemical composition of fresh grapes includes sugars, pectins, tannins, vitamins A, B1, B2, and C, as well as polyphenolic compounds, with the dark grapes additionally containing anthocyanins. Grapes are consumed unprocessed as fresh fruit and in the form of processed products, such as raisins, juices, nectars, isotonic drinks, and wines [3,4]. Many studies point to the beneficial effects of grapes and their products on human health [3,5]. Particularly important are phenolic substances that reduce the incidence of coronary heart disease, atherosclerosis, and certain types of neoplasms [6]. In addition, grapes are a source of simple carbohydrates, namely, glucose and fructose. Grape juice also contains valuable organic acids as well as iron, potassium, phosphorus, calcium, cobalt, and magnesium salts. The literature provides few studies concerning the assessment of the individual grape varieties cultivated under Poland’s climate and soil conditions in terms of the pressing yield as well as the physical properties and the chemical composition of the obtained juice.

From the producer’s perspective, one of the major parameters of the grape juice extraction process is the pressing yield, which is affected by the process conditions and the varietal characteristics of the grapes [7]. For consumers, on the other hand, it is the quality of the juice obtained that is important, particularly its sensory qualities and health-promoting characteristics. Therefore, the technological process should be conducted in such a manner that both conditions mentioned above are met at the same time. In Poland, the most commonly cultivated grape varieties are multispecies hybrid grapes, and there is limited information on the effectiveness of pre-treatment, e.g., for Vitis vinifera hybrids grown in a climate typical of Poland. Grape pressing is a crucial phase of the production process, as the extraction of chemical compounds has a significant effect on the composition of the juice [8]. The process of fruit juice pressing commonly involves enzymatic treatment of the pulp prior to pressing. In recent years, there have been increasing efforts to use physical pulp processing methods to enhance the yield of fruit and vegetable juice pressing. One of the methods proposed to increase the yield of fruit and vegetable juice pressing and to improve the quality of the juice obtained is thermal treatment involving freezing and thawing the pulp before pressing [9,10,11]. It is a method that enables, in the case of certain raw materials, an increase in process efficiency and an improvement in juice quality without applying enzymatic treatment. This is important because some studies have indicated that the enzymes used can affect the quality of the juice obtained, as they cause a certain degree of deglycosylation of anthocyanins and other polyphenols, which results in their destabilization [12,13]. In contrast, freezing has virtually no effect on the degradation of biologically active ingredients contained in fruits and vegetables [14].

The primary aim of the study was to determine the effect of pre-treatment on the pressing yield of grape juice and its qualitative assessment. The scope of the study included the determination of the quality parameters of the juice, such as the extract content, the pH value, density, and the sugar, total polyphenol, and ash contents. The study applied pre-treatment of raw material, involving either enzymatic liquefaction of the pulp in the first case or freezing and thawing of the pulp prior to pressing in the second case. Knowledge about the technological and health-promoting properties of juice obtained from grapes grown in Poland will support the diversification of production and the development of local vineyards.

2. Materials and Methods

The study was carried out on eight varieties of white and red grapes from grape varieties intended for cultivation in Poland for the purpose of obtaining grapes for the production of grape wines, as indicated in the Regulation of the Minister of Agriculture and Rural Development of 14 January 2005. The fruit was sourced from vines cultivated on the Experimental Farm of the University of Life Science in Lublin, Poland and was harvested from young (5-year-old) vines of the following varieties:

Hibernal (cross of Chancellor x Riesling), an interspecific hybrid, origin: Germany, light green clusters,

Jutrzenka (cross of Seyve Villard 12-375 x Pinot Blanc), an interspecific hybrid, origin: Poland, yellow and green clusters,

Muscaris (cross of Solaris x Muskat Blanc), an interspecific hybrid, origin: Germany, yellow and green clusters,

Seyval Blanc (cross of Seibel 4995 x Seibel 4986), an interspecific hybrid, origin: France, yellow and green clusters,

Marechal Foch (cross of Gold Riesling x (vitis riparia x vitis rupestris)), an interspecific hybrid, origin: France, dark blue-black clusters,

Merlot (cross of Madeleine Noire des Charentes x Cabernet Franc), species: Vitis vinifera, origin: France, black clusters,

Regent (cross of (Silvaner x Mueller Thurgau) x Chambourcin), an interspecific hybrid, origin: France, dark blue clusters,

Zweigelt (cross of Lemberger x Saint Laurent), species: Vitis vinifera, origin: Austria, dark blue clusters.

These are relatively unpopular vine varieties in the world but adapted to the climate in Poland. They are possible to grow on small orchard farms. Fruit without visible signs of disease and not mechanically damaged was accepted for the study after reaching harvest maturity. After harvesting, the fruit was stored for 24 h under refrigeration (t = 8 °C, humidity of 60%). Each fruit variety was divided into three groups: the control sample (C), the sample subjected to the enzymatic (E) treatment, and the sample subjected to the thermal treatment involving freezing and thawing (F). Before testing, the fruit was washed in cold water, gently dried with tissue paper, and crushed. The enzymatic treatment involved treating the pre-crushed fruit with a pectolytic enzyme preparation (Pektoenzym, Biowin sp. z o.o., Łódź, Poland) in the amount of 0.2 mL per 1 kg. Enzymes were applied to the samples by spraying. After the addition of the enzymatic preparation, the samples were placed in an incubator (SUP-4, Wamed, Warsaw, Poland) at a temperature of 25 ± 1 °C for 4 h. The thermal treatment involved the freezing of the fruit at a temperature of −20 ± 1 °C, followed by storage in a refrigerator (F6243W, Gorenje Group, Velenje, Slovenia) for 24 h, and thawing at 20 ± 1 °C in an incubator (SUP-4, Wamed, Warsaw, Poland). This is an innovative technology not previously used in grape processing.

The pressing process was carried out using a self-assembled test stand comprising a cylindrical basket press with a capacity of 10 dm³, a hydraulic pressing system (UHJG 20/C/2, Hydrotech, Lublin, Poland) and a measuring system consisting of a strain gauge (EMS50, WObit, Poznań, Poland) interacting with a digital recorder (MG-TAE1, WObit, Poznań, Poland) [15]. Portions of fruit weighing 300 g were placed in a canvas bag, then placed into the press chamber and pressed until a pressure of 0.06 MN was achieved, after which the process was stopped. The press piston moved at a constant speed of 0.5 mm·s⁻¹. The juice was poured into a container, and the weight and volume of the juice obtained, density, acidity and the extract content were then determined.

The pressing yield was determined according to the following formula:

W = M_j/M_s × 100%,

(1)

where W denotes the pressing yield, %; M_j denotes the weight of juice after pressing, in kg; and M_s denotes the weight of the sample subjected to pressing, in kg.

The juice was poured into bottles, capped, and stored in a refrigerator until chemical testing. Acidity was measured using a CP-411 pH meter (Elmetron, Zabrze, Poland), and the results were expressed on the pH scale [16]. The extract content was determined in accordance with Polish Standard [17] using an ATAGO PAL-3 refractometer (Atago, Tokyo, Japan) and expressed in °Bx. The juice density was determined using a pycnometer and was expressed in kg·m⁻³ [18]. The total polyphenol content was determined using the Folin–Ciocalteu colorimetric method, and the results were expressed as an equivalent of caffeic acid (mg/100 mL), whereas the sugar content was determined by the HPLC method and expressed in g/100 g [19]. In addition, the ash content was determined and expressed in %.

The study results were subjected to a multivariate analysis of variance using Statistica 12 software [20]. The significance of the differences between the mean values was determined using Tukey’s test and an LSD test. The study results are presented in tables and graphs. The graphs present the mean values and whiskers representing standard deviations, whereas the tables present the mean values and standard deviation values. The mean values shown in graphs and tables, marked with the same letters, are not statistically significantly different (p ≤ 0.05).

3. Results and Discussion

3.1. Effect of the Varietal Characteristics of the Grapes on the Pressing Yield and Quality of the Obtained Juice

The average pressing yield for the individual grape varieties ranges from 48.8 to 62.9%, with these differences being statistically significant (Figure 1). The highest average pressing yield, at a level of approximately 60%, was obtained for the varieties Marechal Foch, Seyval Blanc, and Zweigelt Regent, and the lowest was for the variety Muscaris. The average pressing yield for the juice from the variety Muscaris was 22.4% as compared to the variety Marechal Foch. Dumas et al. [7] showed that the pressing yield was markedly influenced by the grapes’ varietal characteristics and the pressing method employed (up to 76%). According to Aerny et al. [21], an increase in the pressing yield from 55% to 85% results in an increase in density, the pH value, and the sugar content of the juice. Ferreira-Lima et al. [22] also reported that the pressing conditions significantly affect the process efficiency and the characteristics of the obtained juice.

The effect of the varietal characteristics of the grapes on the extract content of the juice obtained was statistically significant (Figure 2). A high extract content, e.g., exceeding 14 °Bx, is indicative of the suitability of the grapes for processing [23]. The average extract content of the juices studied ranged from 18.3 to 22.7 °Bx. The lowest extract content of the juice was exhibited by the varieties Seyval Blanc and Zweigelt, whereas the highest content, exceeding 22 °Bx, was shown by two varieties, i.e., Muscaris and Marechal Foch. According to Krośniak et al. [24], the varieties Seyval Blanc and Jutrzenka are characterized by a high extract content. Silva et al. [23] reported that the high extract content of grapes is determined by their varietal characteristics and the year of fruit harvest. Nardello et al. [25] found that the extract content is, to a large extent, determined by the variety and the juice pressing technique and ranges from 12.8 to 20.47 °Bx. Certain varieties are characterized by a higher extract content exceeding 25 °Bx [26].

The average acidity (pH) of the juice of the studied grape varieties varied, with the differences being statistically significant (Figure 3). The lowest average acidity (pH) at a level of 3.18 was shown by the juice from the variety Seyval Blanc, whereas the highest acidity, at a level of 3.82, was shown by the juices from the varieties Marechal Foch and Zweigelt. An analysis of the graphs showed that, in most cases, the juices obtained from white grape varieties had lower average acidity (pH). On the other hand, according to Krośniak et al. [24], a higher acidity (pH) was shown by the variety Seyval Blanc, and a lower acidity was shown by the varieties Jutrzenka and Marechal Foch. Silva et al. [23] found that the pH of grape juice may vary from 3.09 to 3.69, depending on the variety and the year of harvest. However, Nardello et al. [25] obtained pH values ranging from 3.06 to 3.84. pH values ranging from 3.2 to 3.4 are optimal in terms of grape processing, as they ensure the stability of anthocyanins and the intense color of grape juice [27]. The application of high pressure during pressing results in a decrease in acidity and an increase in the pH of the juice [8]. Differences in the pH values of juice obtained from individual grape varieties compared to the results of other authors may be caused by the terroir and year of harvest.

The study showed no effect of the varietal characteristics of the grapes on the average density of the juice obtained. The average juice density was found to be approximately 1.038 g/cm³ and was lower than the results obtained by other authors [4]. According to Fongaro et al. [28], grape juice density ranges from 1.060 to 1.068 g/cm³ and shows no statistically significant differences between white and red grapes harvested over five consecutive years (2012–2016).

Sugars represent the main component of fresh fruit and grape juice [29] and their content is determined by the ripeness of the fruit, the region of origin, the climate, the variety and processing conditions. The main sugars in grape juice include glucose and fructose, followed by sucrose [1]. The juice from the grape varieties studied varied in terms of the monosaccharide content, with these differences being statistically significant (Figure 4 and Figure 5). The average fructose content of the juice from the individual grape varieties ranged from 8.3 to 13.1 g/100 g and was slightly higher than the average glucose content, which ranged from 7.3 to 10.5 g/100 g. The lowest average fructose and glucose contents were noted for the juices obtained from the varieties Seyval Blanc and Zweigelt, whereas the highest contents were noted for the varieties Marechal Foch, Jutrzenka, and Muscaris. As for the average monosaccharide content, the differences between the individual varieties reached 50%. The grape varieties studied by de Souza Prado [30] showed lower fructose (77.2–78.2 g/L) and glucose (87.9–88.1 g/L) contents.

An analysis of the graphs (Figure 4, Figure 5 and Figure 6) indicates that the average monosaccharide content of the juices studied was several times higher than the average sucrose content. The juices obtained from the grape varieties studied varied greatly regarding the sucrose content. As can be seen from the graph (Figure 6), the lowest sucrose content was noted for the juice from the varieties Merlot (0.50 g/100 g) and Muscaris (0.92 g/100 g). In contrast, the highest sucrose content was noted for the juice from the varieties Zweigelt (3.37 g/100 g), Seyval Blanc (2.33 g/100 g) and Regent (2.3 g/100 g).

The juice obtained from the grape varieties studied varied significantly in terms of the average polyphenol content, which ranged from 14.4 mg/100 mL to 41.12 mg/100 mL (Figure 7). The highest polyphenol content was found in the juices obtained from the Muscaris and Merlot varieties. The average polyphenol content of the Merlot variety juice was nearly 2.5 times higher than that of the Jutrzenka variety juice. Overall, juices from white grape varieties had a lower polyphenol content than those obtained from red varieties. The exception is the white variety Muscaris, which had a higher polyphenol content than red varieties such as Marechal Foch, Regent, and Zweigelt. For example, the polyphenol content of the pulp from the red grape variety Gołubok amounted to approximately 19 g·GAE L⁻¹ [31]. On the other hand, Krośniak et al. [24] showed that the variety Marechal Foch was characterized by a high polyphenol content (36.8 g·GAE L⁻¹), and, for the white grape varieties, the variety Jutrzenka exhibited the highest polyphenol accumulation capacity (26.8 g GAE L⁻¹). According to Nardello et al. [25], the varietal characteristics and the pressing method significantly affect the polyphenol content of grape juice.

The ash content is indicative of the mineral content of the juices. The highest average ash content was obtained for the variety Marechal Foch (0.46%), and the lowest for the variety Jutrzenka (0.33%). The values obtained were similar to those obtained by Mota et al. [32].

The average yield of pressing juice from red grapes was higher by approximately 6%, as compared to the average yield of pressing juice from white grapes, with this difference being statistically significant (

Table 1. The pressing yield and quality of juices from white and red grapes.

	White Grapes	Red Grapes
Yield, %	56.07 ± 6.48 a	59.45 ± 4.31 b
Extract, °Bx	20.66 ± 1.72 a	19.93 ± 1.65 b
pH	3,41 ± 0.18 a	3.73 ± 0.17 b
Density, g/cm3	1.035 ± 0.023 a	1.041 ± 0.028 a
Fructose, g/100 g	11.51 ± 1.64 a	10.05 ± 1.38 b
Glucose, g/100 g	9.19 ± 1.41 a	8.81 ± 0.92 b
Sucrose, g/100 g	1.68 ± 1.13 a	1.92 ± 1.39 a
Polyphenol, mg/100 mL	23.45 ± 9.34 a	30.75 ± 7.42 b
Ash, %	0.39 ± 0.07 a	0.41 ± 0.08 a

a, b—average values in a row marked with the same letter are not statistically significantly different (p < 0.05).

). The juice obtained from white grapes showed that the average extract content was statistically significantly higher by 3.7%, and the average acidity, i.e., the pH value, was lower by 8.5%. However, the average density of the grape juice and the average ash content were similar and exhibited no statistically significant differences. The juices from white and red grapes differed statistically significantly regarding the average sugar content. The monosaccharide, i.e., fructose and glucose contents of the juices studied were high, as compared to the sucrose content. The juice from red grapes showed an average fructose content lower by 12.7% and an average glucose content lower by 4.1%, whereas the average sucrose content was 14.3% higher. The juice from red grapes was characterized by a high average polyphenol content of 31.1% compared to that from white grapes.

3.2. Effect of Pre-Treatment of Grapes on the Pressing Yield and Quality of the Obtained Juice

The application of the enzymatic (E) or thermal (F) treatment of the grape pulp prior to pressing affects the pressing yield of the juice and its quality parameters. Previous studies conducted by the authors on apples, chokeberries, rhubarb, and root vegetables indicated that the thermal treatment by freezing and thawing the pulp prior to pressing increases the process efficiency and improves the quality characteristics of the obtained juices [9,10,11].

A detailed analysis of the study results shows that pre-treatment affects the pressing yield of individual grape varieties differently. For the white grape varieties, a slight effect of the enzymatic (E) treatment was noted at a level of several percent on the increase in the yield of pressing juice from the varieties Jutrzenka and Seyval Blanc, and of the thermal (F) treatment on the increase in the yield of pressing juice from the variety Hibernal (

Table 2. Effect of pre-treatment on the pressing yield (%) of the grape varieties studied.

Color	Variety	Control (C)	Enzymatic Pre-Treatment (E)	Thermal Pre-Treatment (F)
White grapes	Jutrzenka	58.99 ± 0.76 a	63.81 ± 0.01 b	47.84 ± 0.86 c
	Hibernal	54.74 ± 0.32 a	57.07 ± 0.68 a	59.46 ± 1.16 b
	Muscaris	49.53 ± 2.75 ab	51.79 ± 0.91 a	45.03 ± 1.39 b
	Seyval Blanc	58.48 ± 1.16 a	67.58 ± 1.15 b	58.58 ± 1.12 c
Red grapes	Marechal Foch	63.39 ± 0.75 a	66.54 ± 1.04 a	58.87 ± 1.99 b
	Merlot	54.31 ± 1.69 a	56.53 ± 1.45 ab	52.92 ± 0.38 b
	Regent	54.39 ± 1.81 a	60.82 ± 1.09 b	62.34 ± 1.07 b
	Zweigelt	58.24 ± 2.26 a	62.62 ± 0.64 b	62.38 ± 1.55 b

a, b, and c—average values in a row marked with the same letter are not statistically significantly different (p < 0.05).

). For the Jutrzenka and Seyval Blanc varieties, thermal treatment (F) resulted in a slight decrease in the process efficiency compared to the control sample (C). In addition, a decrease in the pressing yield after thermal (F) processing was observed for the red grape varieties Marechal Foch and Merlot. In contrast, for the varieties Regent and Zweigelt, the application of enzymatic (E) or thermal (F) treatment increased the yield at the same level compared to the control sample (C). Haight and Gump [33] found that the application of enzymatic preparations under industrial conditions can increase the pressing yield for hard-to-press grape varieties by up to 8.5%. In contrast, slow-freezing the fruit pulp before pressing leads to a gradual increase in the size of ice crystals, resulting in damage to the tissue structure [34]. This damage causes loss of cell turgor after thawing and significant drip of cell juice.

The current study showed that (enzymatic or thermal) pre-treatment has no effect on the extract content of the juice for the following grape varieties: Hibernal, Muscaris, and Marechal Foch (

Table 3. Effect of pre-treatment on the extract content (°Bx) of the juice of the grape varieties studied.

Color	Variety	Control (C)	Enzymatic Pre-Treatment (E)	Thermal Pre-Treatment (F)
White grapes	Jutrzenka	20.80 ± 0.43 a	20.77 ± 0.06 a	22.80 ± 0.10 b
	Hibernal	19.63 ± 0.23 a	19.57 ± 0.06 a	19.60 ± 0.27 a
	Muscaris	22.77 ± 1.96 a	22.37 ± 0.06 a	23.10 ± 0.27 a
	Seyval Blanc	18.43 ± 0.35 a	19.17 ± 0.25 b	18.90 ± 0.20 ab
Red grapes	Marechal Foch	22.13 ± 0.41 a	22.63 ± 0.21 a	22.01 ± 0.27 a
	Merlot	20.01 ± 0.36 a	19.73 ± 0.15 b	19.27 ± 0.23 ab
	Regent	18.97 ± 0.61 a	20.01 ± 0.20 b	19.51 ± 0.10 ab
	Zweigelt	19.23 ± 0.15 a	19.27 ± 0.15 a	16.47 ± 0.25 b

a, b—average values in a row marked with the same letter are not statistically significantly different (p < 0.05).

). In the other cases, a slight increase was observed in the extract content of the juice after the enzymatic (E) or thermal (F) treatment, with the exception of the Zweigelt variety, for which a decrease of several percent was noted in the extract content after the thermal (F) processing.

In general, the changes in the juice pH after the enzymatic (E) or thermal (F) treatment were either statistically insignificant or small, i.e., at the level of a few percent (

Table 4. Effect of pre-treatment on the pH value of the grape varieties studied.

Color	Variety	Control (C)	Enzymatic Pre-Treatment (E)	Thermal Pre-Treatment (F)
White grapes	Jutrzenka	3.46 ± 0.03 ab	3.29 ± 0.02 a	3.48 ± 0.04 b
	Hibernal	3.45 ± 0.03 a	3.29 ± 0.02 b	3.40 ± 0.04 a
	Muscaris	3.66 ± 0.03 a	3.51 ± 0.03 b	3.72 ± 0.01 a
	Seyval Blanc	3.11 ± 0.02 a	3.13 ± 0.01 a	3.31 ± 0.02 b
Red grapes	Marechal Foch	3.87 ± 0.04 a	3.83 ± 0.01 a	3.75 ± 0.01 b
	Merlot	3.46 ± 0.01 a	3.43 ± 0.02 b	3.56 ± 0.01 c
	Regent	3.68 ± 0.02 a	3.81 ± 0.02 b	3.85 ± 0.01 b
	Zweigelt	3.74 ± 0.01 a	3.71 ± 0.03 a	4.01 ± 0.04 b

a, b, and c—average values in a row marked with the same letter are not statistically significantly different (p < 0.05).

). The study showed a slight, but statistically significant, decrease in the pH value of the juice after the enzymatic (E) treatment for the varieties Hibernal, Muscaris, and Merlot and a slight increase in the pH value of the juice after the thermal (F) treatment for the varieties Seyval Blanc, Merlot, and Zweigelt.

The application of pulp pre-treatment prior to pressing statistically significantly affected the density of the juice obtained (

Table 5. Effect of pre-treatment on the density (g/cm³) of the juice of the grape varieties studied.

Color	Variety	Control (C)	Enzymatic Pre-Treatment (E)	Thermal Pre-Treatment (F)
White grapes	Jutrzenka	1.073 ± 0.009 a	1.026 ± 0.005 b	1.027 ± 0.007 b
	Hibernal	1.062 ± 0.006 a	1.007 ± 1.003 b	1.021 ± 0.016 b
	Muscaris	1.055 ± 0.008 a	1.044 ± 0.006 a	1.008 ± 0.004 b
	Seyval Blanc	1.054 ± 0.004 a	1.043 ± 0.006 a	1.006 ± 0.004 b
Red grapes	Marechal Foch	1.077 ± 0.001 a	1.043 ± 0.008 b	1.007 ± 0.002 c
	Merlot	1.062 ± 0.003 a	1.021 ± 0.005 b	1.006 ± 0.003 c
	Regent	1.051 ± 0.012 a	1.087 ± 0.007 b	1.015 ± 0.006 c
	Zweigelt	1.059 ± 0.014 a	1.046 ± 0.007 a	1.014 ± 0.011 b

a, b, and c—average values in a row marked with the same letter are not statistically significantly different (p < 0.05).

). According to Zuritz et al. [35], the density of grape juice is approximately 1.097 g/cm³, and this value can vary slightly depending on factors such as temperature and the concentration of soluble solids (sugars) in the juice. The control sample juices (C), with the exception of the variety Regent, were characterized by a higher density than the juices obtained after the enzymatic (E) treatment of the pulp. However, in all cases, the density of the juices after the thermal (F) treatment was clearly lower compared to the density of the control sample juices. For example, the density after the enzymatic (E) or thermal treatment (F) of the juices from the varieties Jutrzenka and Hibernal was lower by approximately 4% compared to the density of the control sample juices.

The (enzymatic or thermal) pre-treatment applied had either no effect or a slight effect on the fructose content of the juice of the grape varieties studied (

Table 6. Effect of pre-treatment on the fructose content (g/100 g) in the juice of the grape varieties studied.

Color	Variety	Control (C)	Enzymatic Pre-Treatment (E)	Thermal Pre-Treatment (F)
White grapes	Jutrzenka	13.72 ± 0.27 a	11.81 ± 0.35 b	12.81 ± 0.38 c
	Hibernal	11.01 ± 0.22 a	10.21 ± 0.20 b	10.71 ± 0.21 a
	Muscaris	12.81 ± 0.26 a	12.62 ± 0.38 a	14.01 ± 0.42 b
	Seyval Blanc	9.65 ± 0.29 a	9.71 ± 0.19 a	9.11 ± 0.77 a
Red grapes	Marechal Foch	11.01 ± 0.33 a	12.12 ± 0.36 b	11.7 ± 0.35 ab
	Merlot	9.99 ± 0.21 a	9.97 ± 0.20 a	9.85 ± 0.20 a
	Regent	9.94 ± 0.30 a	10.91 ± 0.33 b	10.31 ± 0.31 ab
	Zweigelt	6.76 ± 0.14 a	9.33 ± 0.19 b	8.81 ± 0.18 c

a, b, and c—average values in a row marked with the same letter are not statistically significantly different (p < 0.05).

). Only for the juice from the variety Zweigelt was a marked increase observed in the fructose content, i.e., by 38% after the enzymatic (E) treatment and 30% after the thermal (F) treatment. As for the other varieties, the changes in the fructose content compared to the control sample amounted to several percent.

An increase was observed in the glucose content after the enzymatic (E) treatment for the juice obtained from the varieties Marechal Foch (by 15%), Regent (by 12%), and Zweigelt (by 32%) (

Table 7. Effect of pre-treatment on the glucose content (g/100 g) in the juice of the grape varieties studied.

Color	Variety	Control (C)	Enzymatic Pre-Treatment (E)	Thermal Pre-Treatment (F)
White grapes	Jutrzenka	10.81 ± 0.32 a	9.54 ± 0.38 b	10.41 ± 0.42 a
	Hibernal	8.54 ± 0.25 a	8.75 ± 0.26 a	8.74 ± 0.26 a
	Muscaris	10.21 ± 0.31 a	9.93 ± 0.20 a	11.52 ± 0.23 b
	Seyval Blanc	7.32 ± 0.22 a	7.44 ± 0.22 a	7.22 ± 0.22 a
Red grapes	Marechal Foch	8.66 ± 0.26 a	9.99 ± 0.30 b	9.65 ± 0.19 b
	Merlot	9.18 ± 0.28 a	9.28 ± 0.28 a	9.77 ± 0.29 a
	Regent	7.94 ± 0.24 a	8.88 ± 0.27 b	8.81 ± 0.26 b
	Zweigelt	6.56 ± 0.20 a	8.67 ± 0.26 b	8.48 ± 0.26 b

a, b—average values in a row marked with the same letter are not statistically significantly different (p < 0.05).

). In contrast, a 12% decrease was noted for the variety Jutrzenka. However, the thermal (F) treatment increased the glucose content of the juice from the variety Muscaris by 13%, from the varieties Marechal Foch and Regent by 11%, and from the variety Zweigelt by 32%.

All of the juices obtained from the grape varieties studied were characterized by a low sucrose content, as compared to the monosaccharide (fructose and glucose) content (

Table 8. Effect of pre-treatment on the sucrose content (g/100 g) in the juice of the grape varieties studied.

Color	Variety	Control (C)	Enzymatic Pre-Treatment (E)	Thermal Pre-Treatment (F)
White grapes	Jutrzenka	0.82 ± 0.02 a	1.29 ± 0.03 b	3.11 ± 0.06 c
	Hibernal	0.73 ± 0.02 a	0.77 ± 0.02 a	3.66 ± 0.07 b
	Muscaris	1.26 ± 0.04 a	0.81 ± 0.02	0.70 ± 0,01 c
	Seyval Blanc	3.61 ± 0.07 a	1.05 ± 0.04 b	2.35 ± 0.05 c
Red grapes	Marechal Foch	0.56 ± 0.01 a	1.75 ± 0.04 b	2.14 ± 0.06 c
	Merlot	0.12 ± 0.00 a	0.15 ± 0.01 a	1.23 ± 0.04 b
	Regent	2.83 ± 0.06 a	3.53 ± 0.07 b	0.57 ± 0.01 c
	Zweigelt	2.22 ± 0.07 a	3.77 ± 0.11 b	4.13 ± 0.12 c

a, b, and c—average values in a row marked with the same letter are not statistically significantly different (p < 0.05).

). The application of the enzymatic (E) treatment resulted in an increase in the sucrose content in the juices obtained from the following varieties: Jutrzenka by 57%, Marechal Foch by 212%, Regent by 25% and Zweigelt by 70%. At the same time, as a result of the treatment, the sucrose content decreased in fruit juices from the varieties of Muscaris by 64% and Several Blanc by 71%. Thermal treatment (F) increased the glucose content of the juice obtained from the following varieties: Jutrzenka by 161%, Hibernal by 401%, Marechal Foch by 282%, Merlot by 1025%, and Zweigelt by 86%, while as the result of the treatment the glucose content decreased in fruit juices from the varieties Muscaris by 45% and Reagent by 80%. Table 8 shows that, in most cases, the thermal treatment (F) had a greater effect on the sucrose content of the juice than the enzymatic treatment (E).

In most cases, the application of enzymatic (E) or thermal (F) treatment had a significant effect on the polyphenol content of the juice of the individual grape varieties (

Table 9. Effect of pre-treatment on the polyphenol content (mg/100 mL) in the juice of the grape varieties studied.

Color	Variety	Control (C)	Enzymatic Pre-Treatment (E)	Thermal Pre-Treatment (F)
White grapes	Jutrzenka	13.15 ± 0.26 a	17.09 ± 0.51 b	21.88 ± 0.66 c
	Hibernal	16.76 ± 0.34 a	16.92 ± 0.34 a	24.91 ± 0.49 b
	Muscaris	29.41 ± 0.59 a	30.11 ± 0.91 b	49.39 ± 1.48 c
	Seyval Blanc	20.57 ± 0.62 a	20.45 ± 0.41 a	20.81 ± 0.42 a
Red grapes	Marechal Foch	27.55 ± 0.55 a	21.56 ± 0.43 b	22.74 ± 0.68 b
	Merlot	40.64 ± 1.22 a	42.64 ± 1.28 a	40.36 ± 1.21 a
	Regent	25.75 ± 0.52 a	35.19 ± 0.71 b	27.42 ± 0.55 c
	Zweigelt	21.41 ± 0.64 a	32.48 ± 0.97 b	31.21 ± 0.94 b

a, b, and c—average values in a row marked with the same letter are not statistically significantly different (p < 0.05).

). No effect of pre-treatment on the polyphenol content was observed for the varieties Seyval Blanc and Merlot. After the thermal (F) treatment, the polyphenol content of the juice obtained from the white varieties Hibernal, Jutrzenka, and Muscaris, and from the red varieties Regent and Zweigelt increased markedly. For the variety Marechal Foch, the enzymatic (E) treatment, as well as the thermal (F) treatment, resulted in a decrease in the polyphenol content. Different grape varieties are characterized by different polyphenol content in the skin, pulp, and seeds [36]. According to Jovanović-Cvetković [37], in the case of Morava, Muscaris, and Pinot blanc varieties, the total polyphenol content is higher in the seeds than in the skin. Therefore, the type of pre-treatment may affect the degree of polyphenol extraction from individual morphological parts of grapes and ultimately their content in the juice.

Both enzymatic (E) and thermal (F) treatment had an effect on the ash content of the juice of the grape varieties studied (

Table 10. Effect of pre-treatment on the ash content (%) in the juice of the grape varieties studied.

Color	Variety	Control (C)	Enzymatic Pre-Treatment (E)	Thermal Pre-Treatment (F)
White grapes	Jutrzenka	0.291 ± 0.006 a	0.351 ± 0.007 b	0.362 ± 0.006 b
	Hibernal	0.314 ± 0.006 a	0.391 ± 0.008 b	0.441 ± 0.009 c
	Muscaris	0.442 ± 0.013 a	0.412 ± 0.012 b	0.392 ± 0.012 c
	Seyval Blanc	0.562 ± 0.017 a	0.361 ± 0.011 b	0.343 ± 0.010 b
Red grapes	Marechal Foch	0.473 ± 0.014 a	0.562 ± 0.017 b	0.371 ± 0.011 c
	Merlot	0.432 ± 0.009 a	0.372 ± 0.007 b	0.333 ± 0.007 c
	Regent	0.353 ± 0.011 a	0.362 ± 0.011 a	0.473 ± 0.014 b
	Zweigelt	0.273 ± 0.005 a	0.423 ± 0.008 b	0.472 ± 0.009 c

a, b, and c—average values in a row marked with the same letter are not statistically significantly different (p < 0.05).

). For the varieties Hibernal, Jutrzenka, Marechal Foch, and Zweigelt, after the enzymatic (E) treatment, an increase in the ash content of the juice, ranging from 19% to 55%, was noted. In contrast, for the varieties Muscaris, Seyval Blanc, and Merlot, a decrease was noted, ranging from 7% to 36%. For the thermal treatment (F), an increase ranging from 24% to 74% was noted for the varieties Jutrzenka, Hibernal, Regent, and Zweigelt, whereas a decrease ranging from 11% to 39% was noted for the varieties Muscaris, Seyval Blanc, Marechal Foch, and Merlot.

The present study indicated that the (enzymatic or thermal) pre-treatment of the pulp of the individual grape varieties had different effects on the pressing yield, physical properties, and chemical composition of the juices obtained. According to Shanshiashvili et al. [8], pressing optimization should be carried out for specific grapes harvested in a specific year.

4. Conclusions

The study found that, despite the same climatic and soil conditions, there was considerable variation among the grape varieties studied in terms of the characteristics under analysis. The varietal characteristics had a significant effect on the pressing yield and the quality of the juice obtained. The varieties studied exhibited varying pressability. The variety clearly determined the extract, pH contents, and juice density. Moreover, the juices from the varieties studied varied in terms of glucose, fructose, and sucrose contents. The mineral content (ash content) also varied depending on the variety. Juices from the studied varieties varied significantly in terms of the polyphenol content. The red grape varieties and, for the white grapes, the Muscaris variety, were characterized by a high polyphenol content.

Pre-treatment had different effects on the pressing yield of the individual grape varieties and the quality of the obtained juices. Enzymatic treatment (E) significantly increased the pressing yield of the following grape varieties: Jutrzenka, Seyval Blanc, Regent, and Zweigelt. In contrast, thermal treatment (F) increased the pressing yield of varieties such as Hibernal, Regent, and Zweigelt. In the other cases, no effect of pre-treatment on the increase in the pressing yield was noted, and in certain cases, the process efficiency decreased. In most cases, no effect of pre-treatment on the extract content of the obtained juice was noted. Moreover, pre-treatment had a slight effect on the pH of the juice. The study showed, however, that pre-treatment had an effect on the juice density. The control sample juices (C), except for the variety Regent, were characterized by a higher density than that of the juices obtained after the enzymatic (E) treatment of the pulp. After thermal treatment (F), a decrease in the juice density was noted in all cases. The pre-treatment applied had either no effect or a slight effect on the fructose content of the juice of the studied grape varieties. In contrast, the enzymatic (E) or thermal (F) treatment for certain grape varieties could increase the glucose content of the juice. In most cases, an increased sucrose content of the juice was noted after (enzymatic or thermal) pre-treatment, with a considerably higher increase after the thermal (F) treatment. In the case of several varieties, the pre-treatment applied increased the polyphenol content of the juice. Pre-treatment could either increase or decrease the ash content of the juice.

Among the studied grape varieties, the Muscaris variety deserves special attention due to its high polyphenol content.

The use of local grape varieties promotes biodiversity, the development of small orchards, and the support of rural communities.

Since the study was conducted on a laboratory scale using a single grape harvest, further research should include testing grapes from subsequent harvests and be carried out on a semi-industrial scale. Moreover, it would be useful to conduct sensory tests on juices obtained from individual grape varieties after applying different pre-treatment methods.