Winemakers Research Exchange
Written by Michael Attanasi
The relative youth of winemaking in Virginia, compared to other more established regions, has led to great stylistic experimentation in the state. This has resulted in the re-evaluation of many winemaking techniques which have either already been well-established in other regions, or written off. One such technique which is being investigated by Virginia winemakers involves the fermentation of juices under increased turbidity conditions. The goal, as always, is to produce wines of richer complexity and depth of character.
Grape particulates can impart benefits to winemaking. Insoluble solids play an important role in providing yeast nutrients during fermentation, such as lipids (Ribéreau-Gayon 1985). Sterols and unsaturated fatty acids are present in grape insoluble solids, which contribute to yeast viability (Casalta et al. 2016). Although some have noted that fermentation rate can be reduced in clarified juices (Liu et al. 1987), this may be due to settling phenomena at the laboratory volume scale. Others have noted that overclarification of musts can be detrimental to wine and fermentation quality, unless supplemented by nutrients such as lipids and sterols (Casalta et al. 2016).
These benefits can be overshadowed by impacts of turbidity on wine aroma. Often, white wines made from high turbidity juice are perceived as lower quality, due in part to the characteristic production of hydrogen sulfide and other reductive aromas associated with high turbidity juices (Singleton et al. 1975; Liu et al. 1987). Higher turbidity juice tends to lower the overall aromatic intensity of wines, and can often result in a stale-tasting wine (Singleton et al. 1975; Liu et al. 1987). Higher alcohols are often increased in wines from juices with higher turbidity, and these can have detrimental impacts on wine aroma (Crowell and Guymon 1963; Liu et al. 1987; Klingshirn et al. 1987). Polyphenol oxidase, laccase, as well as vineyard residues are associated with grape particulates, and these particulates seem to inhibit ester formation (Boulton et al. 1996; Casalta et al. 2016). Despite these effects, turbidity usually does not have an effect on general wine chemistry, all other things being equal (Singleton et al. 1975; Liu et al 1987).
Particle size can also have a strong effect on final wine aroma. Larger particles (gross lees) settle out first, and wine fermented in the presence of these particles will often have higher concentrations of higher alcohols (such as isobutanol, active amyl alcohol, and isoamyl alcohol) (Klingshirn et al. 1987). Lighter weight particulates have less of an impact in this regard, and in general turbid fermentations should not include heavy lees.
Because of the apparent impact of turbidity on the redox characteristics of must and wine, turbidity could have an effect on the evolution of varietal thiols in Sauvignon blanc wine. The major volatile thiols found in Sauvignon blanc wines are 3-mercaptohexanol (3-MH), 3-mercaptohexyl acetate (3-MHA), and 4–mercapto-4-methylpentan-2-one (4-MMP). They are largely responsible for the citrus, grapefruit, boxtree, and other tropical aromas typical in Sauvignon blanc (Arn and Acree 1998; Tominaga et al. 1998; Tominaga et al. 2006; Coetzee and du Toit 2012). They are odor active at very low concentrations (in the ng/L magnitude) (Tominaga et al. 1998), and impart qualities which many consumers look for in Sauvignon blanc wines (King et al. 2011). 3-MH is often found in grape must in cysteinylated and glutathioylated forms (des Gachons et al. 2002). The cysteinylated and glutathioylated precursors of 3-MH and 4-MMP are linked by disulfide bridges, which are an oxidized form of sulfur. Thus, either chemical or biological reduction is needed to liberate varietal thiols. Indeed, Herrmann (2016) found significant and substantial increases of 3-MH with increasing levels of turbidity. In this study, however, no differences were found between turbidity increases of 50 NTU and 250 NTU, but 500NTU was significantly higher, and 900NTU was the highest. 3-MHA and 4-MMP were not affected by turbidity. As opposed to liberation of varietal thiols through reducing cysteine and glutathione precursors, the increase in 3-MH in this study was attributed to the biological combination of hydrogen sulfide with 2-hexenal (as discovered by Schneider et al. 2006). Recent studies, however, suggest that neither 2-hexenal nor the cysteinylated and glutathioylated precursors are correlated to final wine concentrations of 3-MH (Pinu et al. 2012). Other, unknown juice factors seem to have an effect on the final concentration (Pinu et al. 2012).
Rachel Vrooman of Stinson Vineyards investigated the impact of juice turbidity on varietal characteristics in Sauvignon blanc (Vrooman 2016). In her study, Sauvignon blanc juice from a settling tank was separated into two stainless steel barrels, one of which had its NTU adjusted to 50 NTU while the other had its turbidity adjusted to 250 NTU by adding back fine lees from the same lot of Sauvignon blanc. All other treatments between wines were equal. Consistent with previous research (Singleton et al. 1975; Liu et al 1987), the turbidity treatments did not greatly affect general wine chemistry. However, the concentration of 3-MH (grapefruit and passionfruit aromas) (Tominaga et al. 2006) and 4-MMP (boxtree aromas) (Arn and Acree 1998) was greatly increased in the high turbidity treatment. The increase in 3-MH is consistent with Herrmann’s findings (2016), but the 4-MMP increase is unique. 3-Mercaptohexyl acetate (3-MHA) was lowered in conditions of high turbidity in Vrooman’s study (2016). 3-MHA has no direct precursor, but is instead formed by yeast enzymatic activity (Swiegers et al. 2005). Although some have not found a correlation between 3-MH and 3-MHA (Herrmann 2016), others have noted a moderate correlation (Pinu et al. 2012).
| Wine Chemistry | ||||||||||
| Ethanol (%vol/vol) | Residual Sugar (g/L) | pH | TA (g/L) | Volatile Acidity (g/L) | Malic Acid (g/L) | Lactic Acid (g/L) | Total SO2 (ppm) | Free SO2 (ppm) | Molecular SO2 (ppm) | |
| Low NTU | 12.11 | <1.0 | 3.49 | 6.55 | 0.13 | 3.79 | <0.15 | 116 | 15 | 0.43 |
| High NTU | 11.91 | <1.0 | 3.47 | 6.81 | <0.12 | 3.74 | 0.16 | 93 | 7 | 0.21 |
| Lab Results from ICV from Late February, 2017 |
| Variety Characteristics | |||
| 3-mercaptohexan-1-ol (ng/L) | 3-mercaptohexylacetate (ng/L) | 4-methyl-4-mercaptopentan-2-one (ng/L) | |
| Low NTU | 499 | 16 | <0.3 |
| High NTU | 796 | 9 | 0.8 |
| % Change | 60% | -44% | |
| Lab Results from ETS from Late February, 2017 |
Sensory analysis found the wines to be significantly different (p<0.001). Judges were split with regard to which wine they preferred. Some judges remarked that the high NTU treatment wine was slightly reduced, a common result of turbid fermentations. There was a slight tendency for the high NTU treatment to have higher overall aromatic intensity and varietal character than the low NTU treatment, as well as less body. This tendency was not significant, however.
well as less body. This tendency was not significant, however.
Pectinases added at pressing may have had an impact on the grape particulates in the high NTU wine in this study, potentially resulting in greater production of 3-MH. However, due to the lack of correlation of 3-MH with precursors (Pinu et al. 2012), it seems more likely that a reductive phenomenon is occurring. The reductive conditions of high turbidity must could play a role in reducing these compounds, either chemically or through biological mediation. Additionally, the study by Pinu et al. (2012) investigated 55 different juices to determine that no correlation could be found; however, the tendency of wines to equilibrate to their unique redox potential over time (Geloso 1931; Deibner 1956; Deibner 1957) suggests that these juices may have had different redox environments which could have affected 3-MH evolution.
The impact of redox potential, as well as overall reductive strength, on the formation of volatile thiols in Sauvignon blanc warrants investigation. However, until then, fermenting Sauvignon blanc at higher (yet moderate) turbidity could be a simple and viable means to increase varietal character. The risk of hydrogen sulfide production is still present, but might be mitigated by judicious selection of yeast strains, appropriate yeast nutrient management, using fine lees to adjust NTU as opposed to gross lees, and modifying NTU to a higher (but still moderate) level. However, more work in Virginia is needed to ensure that this is a reproducible, consistent result.
For more information on Rachel’s study, visit www.winemakersresearchexchange.com.