1. Introduction
Sweet dessert wines are very popular in the world. They are produced using various technologies primarily based on the grape dehydration process. This treatment is aimed at increasing sugar concentration in the must, and can be carried out using several methods, such as cryoconcentration, for German and Canadian Eiswein,
Botrytis cinerea infection for Tokaj wines, stopping fermentation by distillate addition for Porto or Madeira wines, or by partial grape drying. The latter method is the most common and has been used for centuries. Several countries have a long tradition in producing these types of wines. For instance, Italy’s production of Vino Passito and Vin Santo, Austria’s production of Strohwein, France’s production of Vin de Paille, and Spain’s production of Pedro Ximenez. The old methods were based on spreading the grapes on mats and drying them in the sunshine. At present, modern dryers with a forced warm, dry air convection are mostly used, thus avoiding problems associated with fungal growth and ochratoxin, contamination, or insect problems [
1,
2]. The winemaking process itself also provides many problems. A high sugar concentration and increased alcohol content create osmotic stress for yeasts, and only some of them are able to carry out the fermentation process. The end products are wines with a very distinctive taste and aroma, which are often more complex than standard red or white wines made from ordinarily ripe grapes [
3,
4]. Mostly, they are sweet to very sweet, capable of a long life, usually deep golden in colour, with a viscous appearance. In general, the nose and palate is met with a complex, alluring blend of peaches, dried apricots, and marmalade, with flavours of almonds and honey. The intense mouthfeel is balanced by a clean, fresh, and very long finish of dried apricots [
3,
4,
5].
In Poland, in recent years, there has been an increase in the number of vineyards where mostly hybrid varieties are grown.
Vitis vinifera strains are cultivated only in the west of the country. The whole country is located in the cool climate zone, and it is rarely possible to produce wines with higher residual sugar contents under natural conditions. Generally, it is only possible to harvest grapes with a relatively low extract (usually 17–23° Brix), although these can give good, fragrant dry, or semi-dry wines with a balancing acidity. [
5]. The only way to obtain dessert wines is to use special production methods. The climate that prevails in Poland should favor the production of ice wines, however, there is no guarantee that a frosty winter will happen every year. At the turn of 2020 and 2021, only 500 L of ice wine were produced, which accounted for 0.03% of the total wine production in Poland. Straw wines (i.e., wines made of semi-dried grapes being dehydrated indoors) are a safer solution for producers, as they are independent of weather conditions [
1].
A number of studies provide evidence for the beneficial effects of grapes on human health, both in their basic and modified forms. A special role is attributed to polyphenolic compounds contained in grapes, which greatly contribute to the prevention of cardiovascular diseases and cancer [
6,
7,
8]. Most of them are found in red wines, due to their production technology. Maceration, together with skins and seeds, results in the extraction of their ingredients into the must. For white wines, the process is slightly different, as the must is fermented directly after pressing, reducing valuable compound contents such as polyphenols by up to 10-fold. For winemakers, polyphenols are important, primarily for the colour and taste of wine [
9,
10]. Phenolic compounds contained in unfermented grapefruits are not fully assimilated by the human body because they occur as large polymeric complexes. Ethanol formed during the fermentation process is a good solvent for phenolic compounds, hence facilitating their extraction into wine [
11].
One grape variety (i.e., hybrid variety ‘Hibernal’) was used in the experiment, but three different production methods were applied. In commercially available wines, one can also find similar examples of different technologies being used during production in order to produce different wine characteristics (e.g., Amarone della Valpolicella and Recioto della Valpolicella, where the maximum sugar content of the former is 12 g/L, and over 100 g/L of the latter) [
12]. In this study, three wines with different styles and compositional parameters were obtained. Their variability was mainly due to different production technologies (i.e., classic for straw wines: grape drying (wine
A), grape drying and maceration (wine
B) and wine refermentation using dehydrated grapes (wine
C)).
During the long preliminary procedure (i.e., drying, fermentation, and maturation to the final product), straw wine is enriched with several compounds, similarly to classic wines. In wines made from dried grapes, substances dissolved in water, mainly sugars, but also acids, are concentrated [
13,
14,
15].
By applying the red wine production method to straw wine making, a product with a higher concentration of polyphenols, that is comparable to red wines, can be obtained. The objective of this study was to evaluate the production methods of straw wines, considering their oenological characteristics, as well as the content and proportion of essential health-promoting compounds. The results will help improve the quality of straw wines produced under cool climate conditions.
3. Results and Discussion
The physical and chemical parameters of the obtained straw wines were determined in the experiment. The first analyzed parameter was the pH of the juice, which is a factor that influences must and wine stability [
22]. The initial juice pH in the grapes after drying increased from 3.14 to 3.31 (
Table 1). This process is observed when the juice is concentrated, but also during extended maturation [
23]. During fermentation, the pH value increased to 3.91 for sample
A, and the statistically lowest and most desired pH was achieved for sample
C (
Table 3). Aponte and Blaiotta [
24] analyzed the passito wine ‘Moscato di Saracena’, and recorded a pH range from 3.1 to 3.93, which was similar to the values obtained in the present study. Bondada et al. [
23] confirmed that pH was not strictly correlated with acidity (TA), and pH increase did not always show the same trend, which was also reflected in the results of this study.
The main organic acids in wines, such as tartaric acid, citric acid, and malic acid come from fruits; others are fermentation products (i.e., lactic acid, succinic acid or acetic acid), and affect wine sensory characteristics such as colour, and its microbiological stability [
25,
26]. The titratable acidity of wines in the experiment was in the range of 8.18–10.34 g/L; it was statistically the highest in sample B, which was probably due to the high initial acid content in the macerate which was then fermented (
Table 3). Climatic conditions are key factors in determining grape maturity and maturity-related parameters, such as sugar content and total grape acidity. In classic wines, it is assumed that total acidity should be between 5 and 12 g/L [
27]. Croce et al. [
12] studied 302 samples of Italian wines made from dried grapes. The acidity recorded by these authors ranged between 3.73 and 11.31 g/L. The values obtained in this experiment were at the upper end of this range.
Wines made from dried grapes can vary in typology, with sugar concentrations reaching even 600 g/L. Wines in the present study could be divided into two groups in terms of sugar content: wines A and B belonged to very sweet wines with a sugar content higher than 150 g/L, and wine C belonged to the group of sweet wines, with sugar in the range of 60 to 150 g/L. Domizio & Lencioni [
28] demonstrated a high diversity among
passito styles. They summarized that Tuscan wines (Vin Santo) had residual sugar content between 10 and 250 g/L. Their observations indicated that recent trends in consumption have prevailed toward slightly sweet and sweet wines. Laureati et al. [
29] also described the production of traditional Vin Santo, made from three varieties: Trebiano, Malvasia and Grechetto. The wines were divided into three types: semi-sweet wines (10–50 g/L residual sugar), slightly sweet wines, and sweet wines (up to 100 g/L residual sugar content). The described production technology for Vin Santo was very similar to that used by the authors for sample A; however, a residual sugar in the experiment was determined at 182 g/L, which classified it as a very sweet wine (
Table 3). By reducing the degree of dehydration, and thus sugar level in the must, it is possible to match the style of Polish wines to traditional Italian wines. Giordano et al. [
30] described obtaining Passito di Pantelleria DOC wines using a base wine and refermentation to sweeten the product, yielding wines containing 200 to 340 g/L residual sugar. In the present study, the authors used the same method for wine
C but obtained a lower sugar concentration (i.e., 69 g/L). Such a result was due to the low residual sugar content in the base wine and in the dehydrated grapes. No reports were found in the literature on using dried grapes and the maceration method to produce wines, which was applied in sample B. Such a method is sometimes used to produce traditional white wines (very short maceration), and almost always to produce red wines (longer maceration).
The yeast used in the study (i.e.,
S. bayanus), is applied to resume fermentation, refermentation, fermentation of meads, and production of sparkling beverages [
31,
32]. In this case, the fermentation in two samples (wine
A and
B) stopped at the maximum alcohol concentration of 18% vol, whereas the alcohol concentration in sample
C was 15% vol (
Table 3). Croce et al. [
12] recorded alcohol content in
passito wines ranging from 10.18 to even 20.52% vol. Similar results were obtained by Domizio and Lencioni [
28], who examined 63 Tuscan Vin Santo wines and determined that the alcohol concentration was in the range of 10–19% vol. They divided Vin Santo into three styles depending on residual sugar and alcohol content: dry style (16–19% vol alcohol and 10–50 g/L sugar), slightly sweet and sweet style (14–16% vol alcohol and up to 100 g/L sugar), and very sweet (14–16% vol alcohol and up to 250 g/L sugar). Relating the results of the present study to the above division, wine
C can be classified as a slightly sweet style, whereas wines
A and
B would be classified as very sweet in terms of sugar content, and as having a dry style in relation to alcohol concentration.
The current study analyzed the content of major organic acids by isotachophoresis (ITP) (
Table 3). This method is used for electromigration separation and allows the analysis of mixtures of ionic substances [
33]. In the experiment, sample
B had the highest content of tartaric, malic and succinic acids, and together with wine
A, also citric acid. The lowest contents of tartaric, citric, malic, succinic, and acetic acids were measured in sample
C. Aponte and Blaiotta [
24] obtained similar values as the authors for wine
A: tartaric acid content—2.56–2.65 g/L, citric acid—0.54–0.60 g/L, malic acid—2.16–2.11 g/L, succinic acid—1.72–1.89 g/L, acetic acid—0.79–1.51 g/L, and in only one case, lactic acid—0.84 g/L. Giordano et al. [
30] studied three Italian
passito wines and acquired similar contents of the aforementioned acids to those obtained in the present study. Wine
C proved to be the most similar to Passito di Panteleria DOC in terms of organic acid content; the production method of both wines was also very similar. In the experiment, as in a study by Aponte and Blaiotta [
24], wine
A statistically had the highest acetic acid content at 1.5 g/L, and fermentation dynamics and acetic acid production were also evaluated. The maximum concentration of acetic acid formed in wines from dehydrated grapes has been defined in several countries; for example, the maximum value of acetic acid in Canadian Eiswein is 2.1 g/L [
4,
34]. In Europe, the standard for white and rosé wines is 1.08 g/L, and 1.8 g/L for red wines. An elevated acetic acid content in table wines usually indicates a spoilage process; however, in the case of wines made from dried grapes, it is responsible for retaining a redox balance when responding to osmotic stress induced by high sugar levels [
34].
The total polyphenol content of wine is influenced by winemaking techniques, namely maceration, fermentation, and aging [
35]. In white wines, it is assumed to range from 100 to 400 mg/L, in rose wines from 400 to 800 mg/L, and in red wines from 1000 to 2000 mg/L [
36]. The straw wines used in the present study contained 907.5 to 3748.5 mg of gallic acid equivalents per liter (mg GAE/L); sample
B (i.e., the one in which the dried grapes were macerated) was the richest in phenolic compounds (
Table 4). Many authors have shown that the maceration process leads to an increase in total polyphenols due to their high concentration in the skins and seeds. Their greatest increments occur at the end of maceration, when the produced alcohol destroys the skin lipid layer that protects the seeds [
37,
38]. For wines
A and
C, the concentration of polyphenols was over 70% lower than for sample
B in the current study. Fhurman et al. [
39] studied the effect of skin contact with grapes on the polyphenol content and antioxidant capacity of white wine. During maceration, which lasted from 2 to 18 h, the latter authors noted a gradual increase in the concentration of polyphenols (up to 41%). They also analyzed the effect of fresh grape maceration in alcohols of different concentrations (2–18% vol). The 18% vol alcohol resulted in the highest polyphenol extraction, 60% higher than in the wine without any added alcohol. In the present study, wine
A had no contact with skins and seeds, whereas polyphenols present in the dried grapes in wine
C were dissolved in the base wine. A lower alcohol concentration, compared to other samples, may not have been sufficient to fully extract polyphenols from dried grapes. Panceri and Bordignon-Luiz [
26] analyzed the effect of grape dehydration, among others, on the chemical composition of young wines from dark Merlot and Cabernet Sauvignon varieties using a five-day maceration after 22 months of maturation. They determined total phenolic content (TPC, in GAE) in the range of 1221.78–1588.50 mg GAE L
−1 (i.e., they obtained values similar to samples
A and
C after 5 years of maturation). Much lower TPC values (an average of 249 mg/L) were obtained by Loizzo [
35], who analyzed
passito from Saracena that was aged between one to four years; however, he expressed the results as (+)-catechin equivalents. The production of this wine was slightly different from the one used in this study. Fresh must was obtained from overripe grapes and dehydrated grapes were added to it. During the wine aging process, the phenolic composition changes quantitatively and depends on the type of wine and its storage conditions [
40]. The conducted analysis confirmed the differentiation of the total polyphenol content depending on the production method.
The antioxidant properties of the wines were evaluated by FRAP and CuPRAC assays. FRAP is commonly used in many studies of antioxidative compounds, CuPRAC is less known, but is considered to be more sensitive [
41]. The analysis of the obtained values showed that sample
B revealed the strongest antioxidant properties. The results from FRAP and CuPRAC analyses are well correlated with TPC [
42], and as shown above, sample
B was richest in phenolic compounds. This also confirmed that the phenolic compounds in straw wine
B were characterised by a higher antioxidant potential.
Studies of of radical scavenging capacity (RSC), defined as the ability to quench the DPPH free radical, showed that sample
B was 2.7 times more effective as an antiradical agent than sample
A, and 2.5 times more effective than sample
C. In red wines, RSC is usually more than 10 times higher than in white wines [
43]. Panceri et al. [
44] showed the RSC in the range of 2.67–4.67 mM/L (TE). Li et al. (2009) [
45] tested 24 Chinese classic wines and reported activity for red wines ranging from 4.19 to 21.30 mM/L (TE), for rosé wines from 1.40 to 3.41 mM/L (TE), and for white wines 0.08 to 1.12 mM/L (TE). Noting the results of the antiradical capacity with the literature data, it could be observed that the values obtained for samples
A and
C corresponded to the characteristics of typical white wine, compared with the red wines of sample
B.
The polyphenol composition of wine samples was analyzed using high-performance liquid chromatography (HPLC) with the use of standards (
Table 5). Polyphenols were identified as flavonols ((+)catechin, quercetin), hydroxybenzoic acids (vanillic acid and syringic acid), hydroxycinnamic acids (
p-coumaric acid, caffeic acid, chlorogenic acid, ferulic acid,
trans-cinnamic acid), and stilbene (
trans-resveratrol). Two of the phenolic compounds (e.g., syringic acid; (+)-catechin), belonged to the chemical markers of ‘Hibernal’ white wine [
46].
(+)-Catechin content of the wines in the present experiment was in the range of 37.82–64.29 mg/L, with only wine
C having statistically lower contents than wines
A and
B (
Table 5). Loizzo [
35] recorded 71.7 mg/L of catechins in fresh Passito of Saracena wine. Avizcuri-Inac et al. [
47] analyzed the chemical and sensory characteristics of mature sweet wines obtained using different techniques, and determined that catechins in white wines range from nd.–0.12 mg/L, and in red wines from nd.–0.17 mg/L. In twelve white commercial wines, Han et al. recorded catechins in the range of 3,57–16,87 [
48]. Rozdrigez-Delgrado et al. [
49] analyzed 55 fresh red wines from the Canary Islands and obtained values ranging from 17.70 to 30.77 mg/L. Panceri et al. [
44] also analyzed catechin contents in red wines and acquired values ranging from 5.82 to 18.19 g/L. Low catechin content is usually found in older wines and it is related to their polymerization Tannins that contribute to dark colour and astringency of wine mainly belong to condensed tannins (or procyanidins) derived from seeds and skins [
35,
50]. The main forms are polymers of flavan-3-ols ((+)-catechin, (−)-epicatechin, (−)-epigallocatechin, and (−)-epicatechin-3-O-gallate) with C4–C6 or C4–C8 linkages and monomeric units [
51]. Quercetin in the wines was determined in the range of 0.22–0.32 g/L, with sample
A having a statistically lower content. Similar results were obtained by Budic-Leto et al. [
52] in a study on Croatian red dessert wines, where the determined quercetin levels varied from 0.00 to 12.402. In contrast, Panceri et al. [
44] obtained slightly higher values ranging from 2.33 to 15.58 g/L. The concentration of flavonols in wines largely depends on grape variety, but also on environmental factors and winemaking practices [
52].
The examined wines were characterized by a rather high content of vanillic and syringic acids: 1.38–2.83 g/L and 1.27–2.25 g/L, respectively (
Table 5). Avizcuri-Inac et al. [
47] reported much lower values—nd.–0.15 mg/L and nd.–0.25 mg/l, and in one case, only 1.41 mg/L. The scatter in the values obtained for these acids in red wines from dried grapes was high. According to Rozdrigez-Delgrado et al. [
49] these contents were higher—1.71–2.99 g/L and 1.64–2.77 g/L, and similar to the results obtained in this study. On the other hand, the content of these acids in the experiment of Panceri et al. [
44] was significantly higher (i.e., 3.53–13.42 g/L and 1.40–8.85 g/L).
The concentrations of hydroxycinnamic acids determined in wine samples
A,
B, and
C were within typical ranges characteristic of white wines [
6,
48,
52] (
Table 5). Wine
B was distinguished by having the highest content, which, taking into account the maceration process, is the expected result.
A very low content of
trans-resveratrol (0.03–0.18 mg/L) was determined in the examined wines
A and
C, and this stilbene was completely absent in sample
B (
Table 5). Vitalini et al. [
53] tested, among others, three Italian dessert wines from dried grapes, and only in Santelmo—Vin Santo DOC did they determine a
trans-resveratrol content of 0.02 mg/L. A much higher concentration was recorded by Loizzo et al. [
35] in Passito of Saracena wine—4.6 mg/L. Classic red wines, according to a large study by Stervbo et al. [
54] contained between 0 and 14.3 g/L
trans-resveratrol. Grapes, and products made from them, are considered a very good source of
trans-resveratrol, which was attributed as being a potent anti-cancerous compound, although recent studies have not confirmed its unique properties. Numerous analyses of the chemical composition of wines have revealed that these products differ greatly in their
trans-resveratrol content. It is most often found in red wines, less often in rosé wines, and least often in white wines [
53]. The method of beverage production is also of great importance, as also demonstrated in this study. Although the skins of ‘Hibernal’ grapes, despite their golden-pink colour, contain a small amount of
trans-resveratrol, possibly due to their Seibel 7053 genetic lineage, we would expect the highest content of this compound in sample
B but due to the use of maceration, it probably degraded during this process for unknown reasons.
A sensory evaluation of the straw wines was carried out, where the respondents assessed their appearance, aroma, flavor, and quality (
Table 6). Wines
A and
B were rated as wines with an intense amber colour (
Figure 2), full flavor, and a long finish. Both in terms of aroma and taste, flavors defined as typical for
passito wines prevailed (i.e., mainly dried fruit, honey, caramel and nuts [
3,
5,
6]). The high alcohol content of these wines (18%) was fully palpable and also visible through “tears” on the walls of the glass [
55]. Of the two wines, wine
B was rated higher. Wine
C, in terms of aromas, corresponded to the varietal characteristics, which was reminiscent of Riesling, with a slight addition of vanilla, honey, and linden flowers. The flavor was mainly green fruits, grapefruit, and lemon. This wine was well received and assessed to be very good in terms of quality.