Glutathione content evolution during spontaneous alcoholic fermentations of Sangiovese grapes

Three compounds, 3-mercaptohexanol (3MH), 3-mercaptohexyl-acetate (3MHA) and 4-mercapto-4-methylpentan-2-one (4MMP), also known as varietal thiols, have been identified to contribute positively to wine aroma and are responsible for the distinct gooseberry, grapefruit, guava and box tree character found in Sauvignon blanc wines. Certain volatile thiol compounds though, can cause off-aromas of onion, garlic, rubber and rotten egg, this group of molecules is known as reductive sulphur compounds (RSC). This study looks into how the addition of sulphur-compounds to Sauvignon blanc juice contributes to the varietal thiol (3MH and 3MHA) concentration and reductive sulphur compound concentration in South African Sauvignon blanc wine.

Malolactic fermentation (MLF) is a secondary step in the vinification process and it follows alcoholic fermentation (AF) which is predominantly carried out by Saccharomyces cerevisiae. These two processes result in the degradation of metabolites to produce secondary metabolites which also contribute to the final wine flavour and quality. AF results in the production of ethanol and carbon dioxide from sugars and MLF stems from the degradation of L-malic acid (a dicarboxylic acid) to L-lactic acid (a monocarboxylic acid). The latter process results in a smoother texture as the acidity of the wine is reduced by the process, it also adds to the flavour complexity of the wine.

Light-struck taste is a defect prevalent in white wines bottled in clear glass light-exposed for a considerable amount of time leading to a loss of color and appearance of sulfur-like odors. The reaction involves riboflavin (RF), a highly photosensitive compound that undergoes to intermolecular photoreduction by the uptake of two electron equivalents from an external donor, the methionine. The reaction includes different steps forming methional which is extremely unstable and decomposes to methane thiol and acrolein. The reaction of two molecules of methane thiol yields dimethyl disulfide. Methane thiol is highly volatile, has a low perception threshold (2 to 10 µg/L in wine) and confers aroma-like rotten eggs or cabbage.

The genetic material preservation is a priority issue in winemaking research. The recovery of minority grape varieties can control the genetic erosion, contributing also to preserve wine typical characteristics. In D.O.Ca. Rioja (Spain) the number of grown white varieties has been very limited, representing Viura the 91% of the cultivated white grape area in 2005, while the others, Garnacha Blanca and Malvasía riojana, hardly were grown. For this reason, a recovery and characterization study of plant material was carried out in this region. In 2008, the results obtained allowed the authorization of three minority white varieties: Tempranillo Blanco, Maturana Blanca and Turruntés.

Condensed tannins (also called proanthocyanidins) are a widely distributed throughout in plants kingdom and are one of the most important classes of secondary metabolites, in addition, they are part of the human diet. In wine, they are extracted during the winemaking process from grape skins and seeds. These compounds play an important role in red wine organoleptic characteristics such as color, bitterness and astringency. Condensed tannins in red wine are oligomers and polymers of flavan-3-ols unit such as catechin, epicatechin, epigallocatechin and epicatechin-3-O-gallate. The monomeric units can be linked among them with direct interflavanoid linkage or mediated by aldehydes.