Several sulfur-containing substances are found or are formed in grape juice that have a pronounced affect on the wine fermentation and wine quality. Hydrogen sulfide (H2S) and various organic forms of sulfur, especially the mer-captans which are formed from H2S, impart highly offensive odors in the wine. They are produced in trace amounts by grape yeasts during fermentation.
The other major group of sulfur compounds found in wine are sulfur dioxide (SO2) and related aqueous forms that exist as sulfite ions. These substances are produced naturally by yeast, and are invariably present in wine, albeit at concentrations usually less than 50 mg/L. However, sulfur dioxide and bisulfite salts are now commonly added to must due to their strong antimicrobial, antioxidant, and antibrowning properties. It is important to recognize that these activities occur only when the SO2 is in its free, un-bound form.When bound or fixed with other wine compounds, such as acetaldehyde, reducing sugars, and sugar acids, SO2 activity is diminished. How SO2 specifically functions in wine and its important role in wine making will be further discussed later.
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