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1 Sección Enología, Cátedra Ciencia y Tecnología de los Alimentos, 2 Cátedra de Farmacognosia y Productos Naturales, Facultad de Química, Universidad de la República, 11800-Montevideo, Uruguay.
* Corresponding author [Email: fcarrau{at}fq.edu.uy]
| Abstract |
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Key words: yeast, anthocyanin, red grape must medium, anthocyanin-derived pigments
Wine color depends on grape pigments and how those pigments are modified during vinification, storage, and aging (Ribereau-Gayon 1982, Boulton 2001, Vivar-Quintana et al. 2002, Schwarz et al. 2003). More recently it has been demonstrated that, during fermentation, yeast releases secondary metabolic products into the medium, such as pyruvic acid and acetaldehyde, some of which react with anthocyanins to produce derivatives such as vitisin A, vitisin B, and ethyl-linked anthocyanin-flavanol pigments (Morata et al. 2003b, Asenstorfer et al. 2003, Lee et al. 2004, Eglinton et al. 2004).
Wine color stability during aging can also be affected by anthocyanin interactions with other polyphenolic compounds, proteins, and polysaccharides (Glories 1984, Escot et al. 2001). Because of the importance of color in red wines and the amount of pigment removed by yeast, in the present work we report the effect of different Saccharomyces cerevisiae strains on anthocyanins extracted from Tannat wines.
The presence of grape skins and other solids during red wine fermentation makes it difficult to study yeast-anthocyanin interactions during winemaking. Previous studies reported complications because of the presence of solids, the natural flora present in grapes, and significant variations in the maceration technology (Cuinier 1997, Morata et al. 2003a, Caridi et al. 2004).
A model red grape juice (RGJ medium) was developed using an anthocyanin extract from grape skins of Vitis vinifera cv. Tannat. This cultivar, widely grown in Uruguay, is one of the richest varieties in polyphenolic compounds (Carrau et al. 2001, Gonzalez Neves et al. 2001). Its name is derived from its unusually high tannin content compared with other red grapes. The RGJ medium allowed us to standardize fermentation conditions and achieve reproducible yeast-anthocyanin interactions during vinification. This strategy also reduced anthocyanin losses from other causes, such as skin adsorption (Ribereau-Gayon et al. 2002). The method represents an improved approach for specifically studying the effect of yeast-anthocyanin interactions.
| Materials and Methods |
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Red grape juice medium. A simulated red grape juice (RGJ medium) was designed for fermentations. It consisted of a Chardonnay white must (pH 3.6; total reducing sugars, 200 g/L; total acidity as H2SO4, 3.25 g/L; free amino nitrogen, 213 mg/L) chemically analyzed according to Zoecklein et al. (1995) and then sterilized by filtration through a 0.45 µm cellulose acetate membrane. This white grape must was stored at 20°C until used. The must was then supplemented with an anthocyanin extract prepared as follows. Tannat grape skins were macerated for 72 hr with 5 g/L potassium metabisulfite under agitation. The liquid was separated by filtration, supplemented with 2 g/ L tartaric acid, and evaporated under vacuum at 40°C to one-third of the initial volume. The extract was then lyophilized and stored under nitrogen at 20°C until used. The anthocyanin extract (2 g) was dissolved in one liter of white grape must and then sterilized by filtration (0.2 µm). Total anthocyanin concentration in the final RGJ medium was ~750 mg/L.
Fermentation conditions. Fermentations were conducted using sterilized 125 mL Erlenmeyer flasks filled to 50 mL with RGJ medium and stopped with Müller valves containing concentrated sulfuric acid. Duplicate flasks were inoculated with 105 cfu/mL (Carrau 2003) and incubated at 20°C without shaking. Carbon dioxide was measured by weight loss and the number of dead cells determined by the methylene blue-staining technique. Total cell population was determined in an improved Neubauer chamber.
Color intensity and hue. Wines were diluted 1:10 (v/v) in a pH 3.2 buffer (0.05 M Na2HPO4, 0.75 M citric acid) in order to avoid effects because of pH differences and co-pigmentation (Boulton 2001). Absorbance at 420 nm, 520 nm, and 620 nm was determined using a Spectronic Genesys 2 spectrophotometer (Spectronic Instruments, Rochester, NY), with a 1 cm path-length quartz cell. Hue (H = A420/A520) and color intensity (IC = A420 + A520 + A620) values were calculated from these measures.
Quantitative analysis of total anthocyanins. Upon removal of yeast cells by centrifugation and filtration, total anthocyanin concentration in each wine was measured spectrophotometrically at 540 nm, after dilution in water-ethanol-hydrochloric acid (70:30:1), according Di Stefano et al. (1989).
Analysis of anthocyanins by HPLC-DAD. Anthocyanins were analyzed by high-performance liquid chromatography (HPLC) using a Hewlett-Packard Series 1050 (Palo Alto, CA) equipped with an HP Chem Station and photodiode-array detector (DAD). Gradients of solvent A (water-formic acid, 95:5, v/v) and solvent B (acetonitrile) were reversed-phase 150 mm x 4.6 mm column applied over a C18 (Phenomenex, Torrance, CA) as follows: 10 to 30% B linear (1.0 mL/min) from 0 to 40 min, then 30 to 35% B (1.0 mL/ min) from 40 to 48 min, and 35 to 10% B (1.0 mL/min) from 48 to 49 min, then 10% from 49 to 55 min to reequilibrate the column to initial conditions (Rivas-Gonzalo et al. 1995). Anthocyanins were detected by scanning from 280 to 600 nm. Quantification was performed against an external standard at 530 nm and expressed as a function of malvidin-3-glucoside concentration. Samples (20 µL) of previously filtered wines (200 µL) were injected into the HPLC. Determinations were made in duplicate. The following anthocyanins were identified in all wines: delphinidin, cyanidin, petunidin, peonidin, and malvidin, as well as their 3-glucosides, 6-acetylglucosides, 6-p-coumarylglucosides, and 6-caffeoylglucosides.
Identification of anthocyanin-derived pigments by HPLC-MS. HPLC-mass spectrometry (MS) analyses were performed using a Finnigan LCQ MS detector (Thermo-quest, San Jose, CA) equipped with an atmospheric pressure ionization source, using electrospray ionization (ESI) interface. Both the sheath gas and the auxiliary gas were a mixture of nitrogen and helium. Sheath gas flow was 1.2 L/ min and auxiliary gas flow was 6 L/min. Capillary voltage was 4V and capillary temperature was 195°C. Spectra were recorded in positive ion mode between m/z 120 and 1500. The mass spectrometer was programmed for a series of three consecutive scans: a full mass, a MS2 scan of the most abundant ion in the full mass, and a MS3 of the most abundant ion in the MS2. The normalized energy of collision was 45%. The anthocyanin-derived pigments were identified by similar M+ and fragmentation patterns (Alcalde-Eon et al. 2004).
Statistical analysis. Significant differences among replicates and treatments (control and each yeast strain) were assessed using a two-way ANOVA (2 replicates x 9 treatments) for the anthocyanin compounds. Statistical analysis was performed using GenStat 5.3.2 software (VSN, Herts, UK).
| Results |
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Yeast effect on total anthocyanin and color parameters of wines.
Total anthocyanin concentration immediately after alcoholic fermentation for the wines obtained with different yeast strains is shown in Figure 2
. An average decrease of 17% in anthocyanin concentration was found for all yeast strains studied when compared with unfermented RGJ medium, with no significant differences among strains (main difference 16 mg/L, 3.3%). However, color intensity was affected by yeast strain, with a significant decrease of color between some fermentations and the control (Figure 3
), but without significant differences for alcohol content and final pH (data not shown). The smallest decrease in color intensity, compared to the unfermented control, was obtained for strain CP882 (1.5% decrease) while the largest loss was found for strain M522 (14% decrease). The values for hue showed no significant differences among treatments in our experimental conditions (Figure 3
).
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Anthocyanin derivatives were identified by HPLC-MS analysis of the wine samples for all the yeast strains studied (Table 1
, Figure 5
). Five derivatives were also quantified by HPLC-DAD: petunidin-3-glucoside-8-ethylcatechin (isomers), malvidin-3-glucoside-8-ethylcatechin, malvidin-3-glucoside-4-vinylcatechol, malvidin-3-(p-coumaroyl)glucoside-8-ethylcatechin, and malvidin-3-glucoside-4-vinyl-guaiacol.
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| Discussion |
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From the data presented in Figures 2
, 3
, and 4
, it can be concluded that total anthocyanin content was similarly affected, according to the polarity of the different compounds, during fermentation by all yeast strains studied. However, color intensity was differently affected according to the yeast strain considered.
The relationship between the polarity of anthocyanins and their adsorption by yeasts was in agreement with a previous report (Vasserot et al. 1997), where delphinidin and petunidin were the most adsorbed. This loss of polar anthocyanins could be explained by the affinity of these compounds to the cell wall (Fleet 1991, Cuinier 1997, Vasserot et al. 1997). This emphasized the importance of this interaction in red wine production from varieties with high concentrations of delphinidin and petunidin, such as Tannat (Gonzalez Neves et al. 2001).
Our finding that acylated compounds were the compounds least removed by yeasts contrasts with the results of Morata et al. (2003a). They reported that the acyl derivatives (acetyl and p-coumaryl) were most strongly adsorbed in fermentations with grape maceration using Saccharomyces strains from a Spanish collection. The absence of solids in our experimental model may allow the more accurate analysis of this effect. Further work would be required to investigate the importance of solids in the context of the present study.
In this work, we demonstrated that the anthocyanin-derived pigments identified were formed during fermentation. The presence of some of these compounds was recently reported in wines (Asenstorfer et al. 2003, Morata et al. 2003b, Alcalde-Eon et al. 2004, Lee et al. 2004). Interestingly, the color intensity of these pigments at the acidic pH of wine is greater than that of the anthocyanins, which exist mainly as colorless hemiacetal structures in wine (Bakker and Timberlake 1997). This pH effect could also explain the direct correlation found between anthocyanin derivative concentration and color intensity.
We also found that the presence of anthocyanin-derived pigments depended on the yeast strain used and resulted in a significant contribution to the preservation of wine color. Finally, these novel results challenge anthocyanin adsorption by yeasts as one of the main factors affecting wine color and demonstrate that yeast contribute to maintaining or improving wine color through anthocyanin-derived pigment formation.
Further studies are required to understand how yeast metabolites interact with the formation of these compounds and how enological practices could be altered to achieve higher concentrations of important anthocyanin-derived pigments.
| Conclusions |
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| Footnotes |
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Manuscript submitted October 2004; revised December 2004
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