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Article

Sweet Red Wine Production: Effects of Fermentation Stages and Ultrasound Technology on Wine Characteristics

by
Zhao Feng
,
Ana Ruiz-Rodríguez
and
Miguel Palma
*
Department of Analytical Chemistry, Center of Agri-Food and Wine Research (IVAGRO), Faculty of Science, University of Cadiz, 11510 Puerto Real, Spain
*
Author to whom correspondence should be addressed.
Appl. Sci. 2024, 14(19), 8864; https://doi.org/10.3390/app14198864
Submission received: 15 August 2024 / Revised: 22 September 2024 / Accepted: 26 September 2024 / Published: 2 October 2024
(This article belongs to the Special Issue Innovative Technology in Food Analysis and Processing)
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Abstract

:

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It is possible to produce sweet red wines with distinct characteristics by applying different conditions in the production stage. This study presents the effects of stopping alcoholic fermentation at different stages on the resulting wines. Additionally, it examines the effects of using ultrasound technology to compensate for the reduced time of alcoholic fermentation in contact with grape skins and seeds. Winemakers will find valuable information to help them to select the most convenient winemaking conditions to produce sweet red wines according to the characteristics they find most desirable.

Abstract

Three different sweet red wines were produced using Tempranillo grapes with three different sugar concentrations: 25, 50 and 75 g/L, using sulfur dioxide and low temperature to stop the alcoholic fermentation. They were compared to the wine produced without stopping the alcoholic fermentation. Cold pre-fermentation macerations and ultrasound during the post-fermentation stage were applied to try to improve the organoleptic properties specifically for sweet wines. The treatment with ultrasound after stopping the fermentation enhanced the extraction of anthocyanins compared to the wines produced without ultrasound, increasing the red color of the final wines, resulting in increments in the range of 16–30%. In any case, significant differences were found between the regular dry red wine and the sweet wines in terms of polyphenolic content, anthocyanin, tannin concentration and absorbance at 520 nm, with lower contents for the sweet wines. The wines were evaluated by different tasting panels showing different results depending on the tasting panel composition. First, an inverse relationship was observed between the sweetness of the wines and their acidity, bitterness, and astringency descriptors. Additionally, wines with higher sugar levels were described as having the most fruity aromas. Finally, no differences in vegetal aromas were found in the different wines.

1. Introduction

From 2000 to 2019, wine consumption in China grew by around 90% [1]. China imports mainly from France, Australia, Chile, Spain and Italy, with 3.4 million hectoliters imported in 2022. Although the consumption of wine per capita (1.3 L per person) remains low, there is great potential for the wine market [2,3,4], so that countries with a long wine-making tradition are increasingly interested in exporting wine to China in order to counteract the declining consumption they are experiencing in their own countries [5,6,7].
The Chinese population mainly consumes dry and sweet red wines of medium and low alcohol content, which are highly aromatic and with a refreshing taste [8,9,10]. For example, the study by Li et al. [8] investigated taste among 414 university students in China and found that 92% of the young people preferred red wine to white wine. Another study by Somogyi et al. [9] revealed that Chinese respondents preferred sweet-tasting wines or that they mixed red wine with soda to increase the wines’ sweetness and decrease its alcohol content. Another survey by Chu et al. [10] with 3421 participants, concluded that the Chinese have two clear preferences for wines, either dry red wine with a mild, refreshing taste and a moderate aroma, or sweet wine.
Sweet wines are classified as sweet (total sugars ≥ 45 g/L) or semi-sweet (12 g/L < total sugars < 45 g/L) [11]. A wine with a high sugar content can be produced by starting with a grape with a very high sugar concentration, which produces a must of such a concentration that it causes fermentation to stop, usually by dehydrating the grapes. In hot climates, sun drying is the most widespread method for dehydrating grapes and increasing sugar concentration in wines [12]. An alternative method consists of artificially drying the grapes by means of climatic chambers that allow factors such as temperature, relative humidity or air flow during the grape drying process to be controlled [13]. Another approach involves a late harvest, with the grapes dehydrated because of over ripening on the vine. A completely different alternative involves the use of frozen grapes, for which the grapes are harvested from the vine while heavy frosts take place (Ta ≤ −8 °C). When pressed, a large part of the water is retained in the skin of the berries in the form of ice, while a juice with a high concentration of sugars, acids and aromatic compounds is extracted [14]. The same technique can be artificially applied by freezing the grapes in a cold chamber. Any of the above described processes will produce wines with such a high sugar concentration that the medium itself prevents the yeasts from carrying out their metabolism in a normal way and consuming all the sugars in the must. However, these kind of processes are difficult to implement in the industrial production of red wines, since the pressing is carried out after the alcoholic fermentation.
Another method of producing sweet wines consists of harvesting the grapes in the traditional way and then, during the winemaking process, stopping the fermentation process by cooling down the grapes and/or by adding SO2, which preserves a significant amount of the original grapes’ sugar content in the wine [15]. This winemaking process is similar to the one used to produce traditional red wines, but with the inconvenience that, because of stopping the fermentation process, the contact time between the must and the solid parts is shortened. This results in a poorer color extraction compared to the one obtained from traditionally produced dry red wines, where maceration is carried out until fermentation is completed.
In order to improve the extraction of the components of interest during the alcoholic fermentation—and particularly, the aroma-related compounds—a common practice is to extend the contact time between the must and the berry skins by means of a controlled maceration that favors the extraction of the compounds in the grapes’ solid parts [16,17]. Prefermentative maceration is maceration prior to alcoholic fermentation at low temperature (5 to 10 °C), which facilitates the extraction without loss of the aromatic fraction [18]. A study on the red variety Pinot noir from Burgundy revealed that the effect of cold pre-fermentation resulted in wines with a richer polyphenolic content [19]. The effects of two prefermentative treatments (cold maceration at 6–8 °C and cold maceration at 0–2 °C using dry ice) were studied on the Monastrell grape variety, and an increase in the phenolic and aromatic fractions was reported in both cases. For phenolics, including anthocyanins, increases in the range of 10–20% were found, while increased levels for acetates and esters reached up to 23%, in some cases. However, both the starting level of sugars and the final content of the ethanol affect the extraction efficiency of compounds from the grape skins. In the case of anthocyanins, the effects were less intense in the musts with higher levels of sugars and higher levels of ethanol [20]. In warm climates, cold maceration for 10 days at 4 °C was tested on three red grape varieties: Tempranillo, Merlot and Syrah. The final wines showed an increment in total phenolic content (6–8%), anthocyanins (15–24%), tannins (6–8%) and aromatic compound content (up to 63%) in comparison with the control wines, thus improving the quality of the wines [21]. Maceration after freezing the berries at different speeds was also evaluated for the Muscat variety, and although the final wines were considered to be more aromatic, no significant (p = 0.05) color differences were observed [22]. The effects of prefermentative maceration and fermentation at low temperature were studied for Cabernet Sauvignon. The wines that were produced at low temperature proved to be more aromatic, although with a lower color intensity, while those that had been fermented at room temperature showed a greater color intensity and less aroma. However, different maceration times did not show any effects [23]. It could, therefore, be concluded that prefermentative maceration, in most cases, resulted in a significant increment in the extraction of compounds from the skins. This is particularly interesting with regard to volatile compounds, even if the extraction of phenolic compounds was also enhanced. In the case of the vinification of red wines, the effects of this winemaking technique are masked by the extractions that take place during the fermentation of the must while in contact with the solid parts of the grapes.
Another approach used to enhance the diffusion of the compounds from the skins into the must is the application of an external source of power during the maceration or fermentation to facilitate the transferring of the compounds. One of the techniques that has gained popularity recently is the application of ultrasound in combination with maceration [24]. This technique is based on a phenomenon called “cavitation” where small bubbles collide with the surfaces of the solids that are in contact with the liquid that carries the ultrasound waves, thereby facilitating the transference of the solid compounds into the liquid [25,26,27]. This technique has been employed to extract a wide variety of organic compounds from different matrices, including the compounds with antioxidant activity that are found in grapevine leaves [28], the phenolic compounds from grape pomace, skin and seeds [29] or the phenolic compounds present in grapes as part of the method to determine ripeness indices [30,31]. These abilities revealed by ultrasound are quite attractive for application to winemaking. A prefermentative maceration with ultrasound was applied to the white Muscat variety and resulted in wines that were more intense and with a different aromatic profile from the reference wine [32]; the Syrah variety was used to produce red wine by applying ultrasound to the must-skin macerate during fermentation, which resulted in wines with an enhanced color [33]. Bautista-Ortín et al. [34] applied the ultrasonic technique to the vinification of a red wine and the extraction of the phenolic compounds from the grape skin into the must during its maceration was accelerated in comparison to traditional maceration. This technique was approved by OIV, through the OENO 616-2019 resolution [35]. Ferraretto et al. [36] applied ultrasound to skin maceration and the aging on the lees, improving the extraction of phenolic substances (up to 120% for anthocyanins and 45% for total phenolics in the Petit Verdot variety) and accelerating the aging process on the lees. Thus, the application of ultrasound allows the recovery of components from the skins to be improved, which makes an attractive option of this technique when the relative levels of the compounds in wines are low or when regular maceration times do not achieve the desired standards with regard to the transferring of components from the skin to the must.
The main objective of this work is to produce sweet red wine through a fermentation stop applied at three different levels of sugar content: 25, 50, 75 g/L. Given that the wines obtained were expected to have low levels of anthocyanins and, therefore, a lower color intensity, pre-fermentative maceration would be applied and ultrasound during the postfermentative maceration. These assays were conducted on Tempranillo grapes, whose berries are small in size, have good anthocyanin content, relatively low tannin content, and are highly aromatic [37]. Once the wines had been produced, they were characterized and different tasting sessions were conducted by a Chinese and a Spanish panel of judges to discuss any of the differences and preferences between the wines from the different assays.

2. Materials and Methods

2.1. The Winemaking Processes

Tempranillo red grapes grown in the Chiclana de la Frontera area (lat. 36°25′48.5″ N, long. 6°06′30.1″ W) were used. Their initial characteristics were: 252.3 g of total sugars L−1, 3.76 g of titratable acidity (tartaric acid/L) and pH of 4.03.
Four different types of wines were produced (Figure 1), distinguished by their final sugar contents. Table 1 summarizes the characteristics of the four assays, identified by their abbreviations. The control wine, with less than 4 g/L of total sugars, was labeled RW (reference wine). The wines with 25 g/L, 50 g/L, and 75 g/L of sugar were labeled SL25, SL50, and SL75, respectively. All the assays included a 24 h prefermentative maceration stage to extract more phenolic and aromatic compounds, following previous research [23].
Sulfur dioxide (40 mg/L) was added to the musts in the form of pure potassium metabisulfite (Agrovin, Alcazar de San Juan, Spain) and pure tartaric acid (Agrovin) was used to adjust their pH to 3.7. Fermentation was carried out in 50 L tanks by inoculating the active dry yeast Saccharomyces cerevisiae var. bayanus (VINIFERM Revelacion, Agrovin) (30 g/HL). Before adding the yeast into the must, it was rehydrated for 15 min using 150 mL of warm (25 °C) water. No pasteurization was applied to the grape must. The alcoholic fermentation was conducted in an air-conditioned chamber at 22 °C, with the musts punched down twice daily during pre-fermentation maceration until fermentation was complete. Fermentation progress was monitored every 6–12 h by checking temperature and sugar content. Malolactic fermentation was not encouraged in this study. When the musts reached the expected sugar concentration (25 g/L, 50 g/L and 75 g/L), fermentation was stopped by adding additional potassium metabisulfite to the tanks until reaching 60 mg of SO2 /L and the temperature was lowered to 2–4 °C for 5 days. No ethanol was added.
To enhance the red color of the SL wines, the pulp was subjected to two daily 20 min cycles of ultrasound treatment (ACM-200E model, Ultratecno, Massalfassar, Spain) over 5 consecutive days, at a power of 28 kHz–2 kW. The wines were checked after each cycle to ensure fermentation did not restart. These parameters were based on previous research [32,33]. Then, the sweet wines, SL25, SL50, and SL75, were pressed, stabilized, and filtered. Potassium metabisulfite up to 100 mg/L SO2 and potassium sorbate (Agrovin) at a concentration of 200 mg/L were added according to regulations (OIV 2015) [38] to preserve the wine and to prevent any refermentation. The wines were stored in bags in boxes.
The reference wine, RW, was produced using traditional methods; i.e., under the same conditions as the rest of the wines but without a fermentation stop, nor any ultrasound, while the same amounts of potassium metabisulfite were added.

2.2. Must and Wine Analytical Control

Both the musts and the wines were characterized by determining their density, pH, titratable acidity and alcohol content [39]. The density measurements were carried out by means of a density meter DMA 4500M (AntonPaar, Graz, Austria). The acidity and pH were measured using an automatic analyzer pHmatic 23 (Crison Instruments, Düsseldorf, Germany) based on acid-base titration. The ethanol in the final wines was determined by distilling the samples in a DE 2000 distiller (TDI, Barcelona, Spain) and then their density was measured using a DMA 4500 M digital density meter in order to determine the alcoholic strength of the hydroalcoholic mixtures that had been obtained.

2.3. Determining Individual and Total Anthocyanins

The individual anthocyanins were determined by ultra-high performance liquid chromatography (UHPLC) using an Ultra LaChrom system (Hitachi, Tokyo, Japan) fitted with a UV–Vis L-2420U detector and a Phenomenex Kinetex C18 column (Torrance, CA, USA) of 2.1 × 50 mm and a particle size of 2.6 µm. Throughout the process, the column was kept at a constant temperature of 35 °C. Two mobile phases, A and B, were used. Acidified water (5% formic acid, Merck, Darmstadt, Germany) was used as solvent A, and methanol (Merck) was used as solvent B. The mobile phase had a flow rate of 1.0 mL·min−1. The gradient used for the separation was as follows: 0 min 10% B; 1.50 min 15% B; 3.30 min 25% B; 4.80 min 35% B; 5.40 min 50% B; 5.90 min 55% B; 6.60 min 100% B [40].
The method proposed by The Australian Wine Research Institute with certain modifications previously published in other papers was used to determine total anthocyanins [23]. Absorbances were measured at 520 nm using an Agilent Technologies Cary 60 UV–Vis spectrophotometer (Santa Clara, CA, USA).

2.4. Determining Individual and Total Phenolic Compounds

The individual phenolic compounds were determined using the ACQUITY UPLC H-Class System (Waters Corporation, Milford, MA, USA) coupled to a photodiode array detector, a fluorescence detector and a quaternary eluent management system. For the identification of phenolic compounds, the wavelength range 200–400 nm was used, the quantification of most compounds was carried out at 320 nm, and for the quantification of catechin, a fluorescence detector was used by comparing their retention times against those obtained from commercial standards (Sigma-Aldrich, St. Louis, MO, USA). The column was kept at a constant temperature of 47 °C for the entire analysis.
Acidified water (2% acetic acid, Merck) (phase A) and 2% acetic acid in acetonitrile (Merck) (phase B) at a flow rate of 0.6 mL/min were used as the mobile phases for the procedure. The gradient used for the separation was as follows: 0 min, 0% B; 1.00 min, 0% B; 3.00 min, 5% B; 4.00 min, 10% B; 4.50 min, 10% B; 5.00 min, 20% B; 7.00 min, 20% B; 8.00 min, 30% B; 10.00 min, 30% B; 11.00 min, 100% B. [23,31].

2.5. Determining the Absorbance at 520 nm

The samples’ colors were determined by means of a Cary 60 UV–Vis spectrophotometer (Agilent Technologies) after filtering the samples through a 0.45 µm nylon filter (Phenomenex, Torrance, CA, USA).

2.6. Determining Organic Acids and Volatile Acidity

An ion chromatography system (Metrohm AG, 930 Compact IC Flex, Gallen, Switzerland), combined with a conductivity detector and a Metrosep-250/7.8 organic acid column (Metrohm AG), was used to determine organic acids, including acetic acid, formic acid, butyric acid, propionic acid, malic acid and lactic acid following the conditions suggested by Metrohm. Specifically, a mixture of 0.4 mmol/L sulfuric acid (Merck) and 12% acetone (Merck) was used as eluent at an isocratic flow rate of 0.4 mL·min−1 for 20 min. To obtain the volatile acidity, the concentration of acetic acid was determined, while any other volatile acids were below their limits of quantification.

2.7. Sensory Analysis of the Wines

The wines that were obtained were evaluated by two panels of tasters. The tasting was carried out in a tasting room at the Instituto de Investigación Vitivinícola y Agroalimentaria (IVAGRO, Puerto Real, Cadiz), using individual booths and under temperature, light, ventilation and environment conditions in accordance with UNE-EN ISO 8589:2010 standard [41].
The members of the judging panels were previously trained in the detection and recognition of aromas according to UNE-EN ISO 5496:2007/Amd 1:2019 [42] and were separated into different sessions according to their origin, Spanish or Chinese. For the Spanish session, 21 judges were present, all of them were of Spanish nationality (13 men and 8 women, ranging from 25 to 50 years of age). For the Chinese tasting sessions, 12 judges were present, all of them were Chinese nationals (7 men and 5 women, ranging from 24 to 30 years of age).
Three tasting sessions were conducted on different days, one test per day. Two of the exercises conducted were of a discriminative nature and the third exercise focused on wine description and preferences.
The first discriminative sessions were conducted according to the UNE-EN ISO 4120:2008 standard [43]. During the sessions, three glasses were presented, two of them containing the same wine and a third one containing a different wine. Each judge was asked to identify the different one according to their aromas. These tests were carried out using the 4 types of wines RW, SL25, SL50 and SL75.
In the first session, a series of exercises were performed to compare the sweet wines, SLs, produced in the assays, against the reference wine, RW, and in a second session, the sweet wines, SLs, were compared against each other.
Subsequently, a comparative-descriptive exercise was conducted where all the wines were compared against each other and scores were given according to the organoleptic characteristics of each type of wine (ISO 8587:2006) [44] and each judge’s preferences. The selected attributes were color intensity, red fruit aroma, vegetal aroma, sweetness, acidity, bitterness and astringency. All of these attributes were evaluated using a scale from 0 (not present) to 4 (maximum intensity). The judges were also asked to rank them according to their preferences.

2.8. Statistical Analysis of the Results

All the data were subjected to analysis of variance (one-way ANOVA) using SPSS 22.0 (IBM, Armonk, NY, USA). The Student’s t test was applied to discriminate amongst the means of chemical data. The sensory data registered in the triangular tasting sessions were analyzed according to the methodology UNE-EN ISO 4120:2008—Sensory analysis [43].

3. Results and Discussion

3.1. Basic Oenological Parameters of the Final Wines

The basic oenological parameters of the wines that had been produced are presented in Table 2. Regarding the alcohol content, the sweet wines with different sugar levels had less alcohol than the reference wine (RW). This was a consequence of stopping the fermentation to obtain the sweet wines. The values ranged from 8.94 (% vol) for SL75 to 14.68 (% vol) for the RW. On the other hand, the sweet wines, which presented different sugar levels, also had a lower titratable acidity than the RW. Thus, the reference wine had the highest acidity at 6.66 g tartaric acid/L, while SL25’s titratable acidity was 15% lower (5.64 g tartaric/L), SL50’s was 21% lower (5.24 g tartaric/L) and SL75’s was 23% lower (5.11 g tartaric/L). The reason for this lower titratable acidity in the sweet wines was the cooling procedure that had been applied to stop the fermentation, which caused a greater precipitation of potassium hydrogen tartrate and calcium tartrate [15].
With respect to their acetic acid contents, the SL wines presented a significantly lower acetic acid concentration than the RW. The reference wine had a concentration of 0.40 g acetic acid/L, while SL25 had 30% less (0.28 g acetic acid/L), SL50 had 45% less (0.22 g acetic acid/L) and SL75 had 65% less (0.14 g acetic acid/L). Acetic acid, which is the main volatile acid found in wines, is formed during its fermentation by yeasts without the intervention of bacteria [18]. As can be seen, the interruption of the fermentation affects the final acetic acid content level; hence, those wines that undergo a longer alcoholic fermentation reach higher levels of acetic acid.
With respect to malic acid, the wines produced were found to be in the range between 2.65 and 2.83 g/L. In general, malic acid content levels correspond to the vigor of the vines, the climatic conditions of the specific vintage and the ripening stage, and range from 1 to 5 g/L in musts [15]. This data did not present any significant differences, which is consistent with the fact that none of the four wines produced had undergone malolactic fermentation. Lactic acid levels are closely related to those of malic acid and normally reach concentrations between 0.16 and 0.19 g/L. In the wines, this value was below 0.30 g/L, which is within the normal ranges for red wines [15]. Consequently, stopping their fermentation did not affect the levels of malic acid or lactic acid in the final wines. It has been proven that the levels of malic acid and lactic acid can be influenced by fermentation conditions. However, when the same yeast strains and temperature conditions are used, the variations are small throughout alcoholic fermentation [45], consistent with the results obtained in the present trial.

3.2. Impact of Ultrasound Applications

The wines whose fermentation had been stopped (SL25, SL50 and SL75) exhibited a significantly lighter red color than traditional wines. This effect is a consequence of the temperature decrease applied to stop the alcoholic fermentation. Wines produced as sweet wines are kept at a temperature of 2 °C after the fermentation halt, during the 5-day period in which the ultrasound treatment is applied before pressing them. The reference wine, RW, continued alcoholic fermentation at 22 °C under the same conditions of daily punch-downs, so the extraction of phenolic compounds, particularly anthocyanins, continues for a period that is nearly twice as long as in the case of the SL75 wine, for example. The application of ultrasound in the case of the SL25, SL50, and SL75 wines aimed to compensate for this shorter extraction time. Therefore, ultrasonic treatments were applied to the wines still containing the grape skins in an attempt to enhance the extraction of anthocyanins from the skins into the wine. As can be seen in Figure 2, the initial intensities of red color, before the application of the ultrasounds, ranged between 48% absorbance for the SL75 wine and 61% of the SL25 wine vs. the RW.
The progress of the extraction was monitored, Figure 2 shows the 3 types of sweet wines, regarding their relative absorbance at 520 nm during the 10 ultrasound cycles. The RW wine, which is the assay processed as red dry wine, is the experiment that presents the highest absorbance and is given a value of 100. The SL25 assay showed an increase in the intensity of the red color after the application of the ultrasonic treatment, reaching its maximum absorbance at cycle 10, with 71% in respect to the RW. The use of ultrasound resulted in an absorbance increment of 16% for the SL25 wine. On the other hand, SL50 and SL75 experienced greater increments than SL25, with 23% for SL50 and 27% for SL75 after 7 ultrasound cycles. However, after the first 7 cycles, their absorbance at 520 nm began to decrease.
In general, the wines that initially presented a low color intensity increased their absorbance by at least 16%, with the greatest increases exhibited by the wines that had started with the lowest levels of absorbance, particularly SL75. It was also observed, regarding the evolution of the absorbance, that its increment was not unlimited and that it decreased, most likely due to the oxidation or degradation of the anthocyanins that actually took place in the SL50 and SL75 wines. This effect has already been described in the production of sweet red wines with extended macerations, where malvidin derivatives experienced decreases of up to 20% in 2–4 day macerations after initial increased levels [46]. Therefore, the effect of ultrasound significantly increases the extraction of anthocyanins and is especially beneficial in wines produced with a higher sugar content; that is, in wines with shorter contact time between the must and the grape skins.

3.3. Total Phenolic Compounds and Total Anthocyanins

Table 3 shows the concentration of total phenolic compounds and anthocyanins of the four different types of wine produced. It has been described that the effects of maceration duration during alcoholic fermentation, although primarily modulated by the grape variety, generally result in reductions in total phenolic compound levels with shorter maceration times at constant temperatures. However, in the case of anthocyanins, these reductions are more significant [47]. Some similar results were obtained in this study (Table 3).
The RW wine was the one with the highest concentration of total phenolic compounds, 2263 mg/L, which was significantly higher than the levels in the sweet wines. The RW showed levels in the same ranges previously reported for Tempranillo wines produced using regular winemaking procedures [48,49]. The SL25 wine reached 92% (2086 mg/L) with respect to the RW, SL50 reached 88% (2005 mg/L) and they were not significantly different from each other. The SL75 wine reached 78% (1786 mg/L), which represented a difference of 22% with respect to the RW, and differed significantly from the rest of the wine types. These data explain their absorbances at 520 nm, since the phenolic compounds that have very low absorbance at 520 nm, play a significant role in the stabilization of the red color instead.
Regarding total anthocyanins, the difference between dry wine and wines with higher sugar contents is greater. The RW, with the highest concentration, was significantly different from the rest. The SL25 wine reached 66% (369 mg/L), SL50 58% (326 mg/L) and SL75 54% (305 mg/L) with respect to the RW. The SL75 wine again differed significantly from the rest of the wines except from SL50, similar to the analysis of total phenolic compounds.

3.4. Individual Anthocyanins

Table 4 shows the concentrations of the individual anthocyanins in mg/L. The three mayor anthocyanins were malvidin 3-glucoside (M3G), malvidin 3-transcumaroylglucoside (M3tCG) and malvidin 3-acetylglucoside (M3AG), as previously described for Tempranillo wines [50]. The RW wine presented the highest concentrations of individual anthocyanins, with significant differences when compared to the rest of wines.
There were few differences between the sweet wines, probably due to the effect of the ultrasound. SL25 reached 75%, SL50 reached 73% and SL75 69%, with respect to the RW wine. The anthocyanin malvidin 3-glucoside in the SL25 wine reached 81%, 74% in SL50, but in SL75 it did not exceed 69% with respect to the RW wine. As for malvidin 3-transcumaroylglucoside, it did not reach beyond 65% of the RW level despite the application of ultrasound, with 64% in SL25, 53% in SL50 and 48% in SL75.
Therefore, SL25 differed significantly from SL75 with respect to the anthocyanins malvidin 3-glucoside and malvidin 3-transcumaroylglucoside.
On the other hand, SL50 had intermediate values between SL25 and SL75 presenting only a remarkable difference with SL25 with regard to M3tCG. The SL50 and SL75 wines proved to be very similar with respect to their anthocyanin contents, which clearly agreed with their absorbance trends at 520 nm. The wine with the highest absorbance at 520 nm was RW, with a more intense color and higher anthocyanin concentration while the sweet wines registered a lower absorbance and lower anthocyanin concentrations accompanied by a less intense color.

3.5. Individual Phenolic Compounds

Table 5 displays the individual phenolic compounds identified in the four wines. First of all, it should be noted that the reference wine (RW) always presented the highest content of each phenolic compound with significant differences compared to the concentrations reached by any of the three sweet wines with different sugar levels.
All of the sweet red wines, with different sugar levels, presented concentrations of some of the phenolic compounds analyzed that were in the order of 50% or even lower than the reference wine. In other words, in general, stopping the fermentation resulted in a greater difference in the level of simple phenolic compounds between the reference wine and the sweet wines than in the case of the anthocyanin compounds, where the differences were lower. For example, the concentrations of rutin in SL25, SL50 and SL75, reached 54%, 48%, 46%, respectively, and quercetin-3-rhamnoside was detected at between 55% and 46%. The contents of gallic acid in SL25, SL50 and SL75 reached 41%, 38%, 31%, respectively. The wine SL25 showed a 54% content for these compounds in relation to the RW wine, ranging from 39% for caffeic acid to 82% for epicatechin. Contents in SL50 ranged from 38% to 79% (52% as average content), and SL75 ranged from 31% to 71% (46% as average content). No significant differences were found between SL50 and SL25 and SL75, but SL75 was significantly different from SL25. It is well known that anthocyanins are mainly extracted at the beginning of the alcoholic fermentation; therefore, their levels were more similar across the wines than the other phenolics, that have additional extraction at the end of the alcoholic fermentation.
Catechin and epicatechin are compounds that provide astringency and color stability to wine. With respect to the RW, the sweet red wines reached epicatechin levels of 82% in SL25, 79% in SL50 and 71% in SL75; the only significant difference was between SL25 and SL75. In the case of catechin, similar results were registered, but with values around 20% lower than those for epicatechin, i.e., 57% in SL25, 56% in SL50 and 47% in SL75. Therefore, SL25 and SL50 wines were statistically similar with respect to these parameters, while the SL75 wine was considered statistically different from the rest of the wines, either sweet or the RW.
For the rest of the individual phenolic compounds, there were no marked differences between the three types of sweet wines, with rather similar values.

3.6. Sensorial Analysis

Two types of tastings, i.e., triangular and descriptive tasting, were carried out in order to determine whether the wines were sensorially different, and which were the descriptors that characterized them.

3.6.1. Comparison between the Wines

In the triangular tasting session, the judges discriminated the sweet wines from the control (RW) wine by olfactory perception, i.e., without using the gustatory channel. Significant differences were found for all sweet wines vs. the reference wine.
Regarding the comparison among the sweet wines (Table 6), only six tasters were able to distinguish between SL25 and SL50, and the same result was registered for SL50 and SL75. In contrast, nine tasters (p < 0.01) distinguished between SL25 and SL75. In other words, according to the judges, only SL25 and SL75 were significantly different.
Thus, it was concluded that the sweet wines were significantly different from the control one. However, the differences between the sweet wines were not so clear, with the exception of SL25 and SL75.

3.6.2. Comparative–Descriptive Tasting of the Four Wines

In the comparative–descriptive tasting (Figure 3), all the judges considered the RW to have more color and awarded it with a maximum score of 4, the sweet reds scored around 1.3 for color. This agrees with the concentrations of individual phenolic compounds and anthocyanins that had been measured in our study, where the RW had significantly higher concentrations than SL75, which stabilized its color and resulted in a more intense color. The RW and SL25 wines had less red fruit aromas (2.2 score) than SL50 and SL75 (2.8 score). Regarding vegetal aromas, no differences were found between the four wines, while with respect to flavors, RW was awarded a score of 0 for sweetness and SL75 was awarded a score of 4. Both SL50 and SL25 presented intermediate values (3.15 and 2.05, respectively). The RW was given the highest score for astringency, bitterness and acidity, while SL75 was rated as the lowest with respect to these parameters. Astringency and bitterness are associated with certain chemical compounds such as catechin or epicatechin, and generally to non-anthocyanic phenolic compounds. The RW had a significantly higher concentration than SL75, which resulted in a notable effect on its flavor. On the other hand, an inverse relationship was observed between the level of sweetness of the wines and their acidity, bitterness and astringency descriptors. For example, sweet wines were perceived as less bitter. Thus, SL75 reached a value of 1.2, SL50 1.75, and SL25 2.75. The same results were observed for acidity and astringency. These aspects are obviously important for Chinese consumers.

3.6.3. Judges’ Wine Preferences

Figure 4A shows that in general, the dry reference wine was the favorite one for the Spanish judges, with 55% of votes. The judges valued the intense color of this wine against the lighter color of the sweet wines. On the other hand, of the judges that preferred the sweet wines, 20% opted for the SL25 wine, 15% preferred SL50 and 10% chose the SL75 wine as their favorite.
In contrast, 37.5% of the Chinese judges (Figure 4B), preferred the SL75 sweet wine and 37.5% of the panelists preferred the SL50 wine for its bright pink color and sweetness, while 25% of them preferred the SL25 wine and none chose the dry reference wine because of its bitterness and astringency.
Therefore, two different results were obtained in terms of the taster’s preferences according to their origins. The Chinese tasters had a preference for the wines with a bright pink color and a sweet flavor compared to the Spanish judges, who valued the intense color and the palate sensation provided by the notes of acidity, bitterness and astringency.

4. Conclusions

It has been proven that treatment with ultrasound after stopping the fermentation enhances the extraction of color, resulting in increments of 16–30%. This increasing effect does not continue forever and at a certain point it remains stable or even decreases and presents a lower absorbance value at 520 nm. Therefore, using ultrasound in the way applied in this study (5 days, 2 applications of 20 min per day), it is only possible to partially improve the lack of color found in sweet wines produced by stopping the fermentation.
On the other hand, significant differences were found between the control and the sweet wines in terms of polyphenolic content, anthocyanin, tannin concentration and absorbance at 520 nm, particularly between the sweetest wine (75 g/L) and the reference one. However, the differences in these parameters between the three sweet wines were not so noticeable in spite of their differences in sugar concentration, with the wines from the 25 g/L assay differing the most from the wines from the 75 g/L assay, while the wines from the 50 g/L assay remained at an intermediate position between the other two sweet wines.
It has been demonstrated that the production of sweet wines by stopping the fermentation process is particularly interesting. From the organoleptic point of view, it was found than the higher the level of sugars, the larger the red fruit aromas, the lower the astringency and the lower the bitterness. On the other hand, the higher the levels of sugars, the lower the red color and the lower the acidity. This means that stopping the fermentation not only affects the levels of sugars but also significantly contributes to several organoleptic properties that will condition the preference of the consumers. Additional studies would be needed to increase the extraction of red compounds while maintaining the fruity aromas and the low astringency of the sweet wines.

Author Contributions

Conceptualization, A.R.-R. and M.P.; methodology, A.R.-R. and Z.F.; investigation, A.R.-R. and Z.F.; original draft writing, Z.F.; review and editing, A.R.-R. and M.P.; supervision, A.R.-R. and M.P.; funding acquisition, M.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was co-financed by the 2014–2020 ERDF Operational Programme and by the Department of Economy, Knowledge, Business and University of the Regional Government of Andalusia. Project reference: FEDER-UCA18-108366.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data are included in the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

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Applsci 14 08864 g001
Figure 1. Flowchart showing the experimental work during the winemaking.
Figure 1. Flowchart showing the experimental work during the winemaking.
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Figure 2. Evolution of the percentage of absorbance with respect to the reference wine as a result of applying ultrasound.
Figure 2. Evolution of the percentage of absorbance with respect to the reference wine as a result of applying ultrasound.
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Figure 3. Sensorial analysis of the wines.
Figure 3. Sensorial analysis of the wines.
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Figure 4. Preferences of Spanish (A) and Chinese (B) wine tasters.
Figure 4. Preferences of Spanish (A) and Chinese (B) wine tasters.
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Table 1. Acronyms and processing conditions of the different wines produced.
Table 1. Acronyms and processing conditions of the different wines produced.
Wine AcronymsFinal Sugar ContentPrefermentative
Maceration		Postfermentative Ultrasound
(2 Daily Cycles for 5 Days)
RW<4 g/L24 h0 min
SL2525 g/L24 h20 min/cycle
SL5050 g/L24 h20 min/cycle
SL7575 g/L24 h20 min/cycle
Table 2. Measured values of the main oenological parameters in the final wines.
Table 2. Measured values of the main oenological parameters in the final wines.
Final WineRWSL25SL50SL75
Sugar concentration (g/L)2.11 ± 0.12 a27.13 ± 0.69 b52.84 ± 0.74 c77.23 ± 0.91 d
Alcoholic strength (% vol)14.68 ± 0.03 a11.63 ± 0.01 b10.32 ± 0.04 bc8.94 ± 0.02 c
pH3.76 ± 0.03 a3.71 ± 0.01 a3.72 ± 0.02 a3.73 ± 0.03 a
Titratable acidity
(g of tartaric acid/L)		6.66 ± 0.04 a5.64 ± 0.07 ab5.24 ± 0.03 b5.11 ± 0.06 b
Acetic acid (g/L)0.40 ± 0.01 a0.28 ± 0.01 b0.22 ± 0.01 b0.14 ± 0.01 c
Malic acid (g/L)2.79 ± 0.022.83 ± 0.052.71 ± 0.082.65 ± 0.07
Lactic acid (g/L)0.17 ± 0.010.19 ± 0.010.19 ± 0.010.17 ± 0.01
Mean value and standard deviation (x ± SD) (n = 3). Different superscripts (a, b, c, d) in the same row indicate statistical differences (p < 0.05) according to the Student–Newman–Keuls test.
Table 3. Concentration of total phenolic compounds and anthocyanins in the Tempranillo wines produced.
Table 3. Concentration of total phenolic compounds and anthocyanins in the Tempranillo wines produced.
WineTotal Phenolic Compounds (mg/L)Total Anthocyanins (mg/L)
RW2263 ± 35.5 a561 ± 1.4 a
SL252086 ± 26.8 b369 ± 11.0 b
SL502005 ± 21.5 b326 ± 6.1 bc
SL751786 ± 11.4 c305 ± 12.1 c
Mean value and standard deviation (x ± SD) (n = 3). Different superscripts (a, b, c) in the same column indicate statistical differences (p < 0.05) according to the Student–Newman–Keuls test.
Table 4. Concentration of individual anthocyanins in Tempranillo wines expressed in mg/L: M3G: malvidin 3-glucoside, M3AG: malvidin 3-acetylglucoside and M3tCG: malvidin 3-transcumaroylglucoside.
Table 4. Concentration of individual anthocyanins in Tempranillo wines expressed in mg/L: M3G: malvidin 3-glucoside, M3AG: malvidin 3-acetylglucoside and M3tCG: malvidin 3-transcumaroylglucoside.
WineM3GM3AGM3tCG
RW197.28 ± 13.6 a11.96 ± 1.61 a38.48 ± 2.92 a
SL25159.24 ± 10.13 b9.01 ± 1.30 b24.58 ± 1.76 b
SL50146.7 ± 11.21 bc8.68 ± 1.22 b20.37 ± 2.7 c
SL75135.06 ± 9.02 c8.3 ± 0.87 b18.33 ± 2.03 c
Mean value and standard deviation (x ± SD) (n = 3). Different superscripts (a, b, c) in the same column indicate statistical differences (p < 0.05) according to the Student–Newman–Keuls test.
Table 5. Concentration of individual phenolic compounds in the wines in mg L−1.
Table 5. Concentration of individual phenolic compounds in the wines in mg L−1.
WineGallic AcidCatechinCaffeic AcidEpicatechinRutinQuercetin-
3-Glucoside		Quercetin-3-Rhamnoside
RW21.56 ± 1.66 a34.4 ± 2.13 a4.79 ± 0.89 a14.54 ± 2.03 a4.73 ± 0.52 a9.52 ± 1.15 a1.56 ± 0.07 a
SL258.92 ± 0.96 b19.73 ± 1.12 b1.84 ± 0.25 b11.87 ± 0.91 b2.57 ± 0.30 b5.27 ± 0.72 b0.78 ± 0.07 b
SL508.10 ± 0.66 bc19.18 ± 0.96 b1.87 ± 0.16 b11.42 ± 1.06 bc2.29 ± 0.39 b4.59 ± 0.32 b0.86 ± 0.03 b
SL756.76 ± 0.82 c16.34 ± 1.31 c1.58 ± 0.41 b10.37 ± 1.52 c2.17 ± 0.12 b4.46 ± 0.51 b0.72 ± 0.02 b
Mean value and standard deviation (x ± SD) (n = 3). Different superscripts (a, b, c) in the same column indicate statistical differences (p < 0.05) according to the Student–Newman–Keuls test.
Table 6. Successful discrimination between pairs of sweet wines by the 12 judges.
Table 6. Successful discrimination between pairs of sweet wines by the 12 judges.
Triangular TastingNumber of Judges Successfully Distinguishing the Winesp
SL25 vs. SL506NS
SL25 vs. SL759<0.01
SL50 vs. SL756NS
NS: non-significant differences.
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Feng, Z.; Ruiz-Rodríguez, A.; Palma, M. Sweet Red Wine Production: Effects of Fermentation Stages and Ultrasound Technology on Wine Characteristics. Appl. Sci. 2024, 14, 8864. https://doi.org/10.3390/app14198864

AMA Style

Feng Z, Ruiz-Rodríguez A, Palma M. Sweet Red Wine Production: Effects of Fermentation Stages and Ultrasound Technology on Wine Characteristics. Applied Sciences. 2024; 14(19):8864. https://doi.org/10.3390/app14198864

Chicago/Turabian Style

Feng, Zhao, Ana Ruiz-Rodríguez, and Miguel Palma. 2024. "Sweet Red Wine Production: Effects of Fermentation Stages and Ultrasound Technology on Wine Characteristics" Applied Sciences 14, no. 19: 8864. https://doi.org/10.3390/app14198864

APA Style

Feng, Z., Ruiz-Rodríguez, A., & Palma, M. (2024). Sweet Red Wine Production: Effects of Fermentation Stages and Ultrasound Technology on Wine Characteristics. Applied Sciences, 14(19), 8864. https://doi.org/10.3390/app14198864

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