Chapter 1

Chemical Components of Grapes and Wine

Alcohol in Wine

In wine, alcohol is formed by yeast via fermentation of sugars, of which there are many different types. The predominant alcohol, by a considerable margin, is ethanol (or ethyl alcohol) and is often abbreviated as alcohol. Trace amounts of other alcohols are also produced including methanol and higher (fusel) alcohols, and these contribute to the flavor of the wine. Wine producers routinely measure ethanol concentrations to track fermentations, for quality control, and for legal obligations. Alcohol content is indicated in terms of percent by volume (vol.%), percent by weight (wt.%), grams per 100 mL, specific gravity, or proof. Under standard fermentation conditions, ethanol can accumulate at up to about 11 to 14 percent. The prime factors controlling alcohol production are sugar content, fermentation temperature, and yeast strain. Because the amount of alcohol produced during fermentation is dependent on sugar concentration, wines from warmer regions with longer growing seasons tend to have higher alcohol concentrations than cooler regions. Red winegrapes are usually harvested later than white winegrapes, and as a result red wines typically have a higher alcohol concentration than whites. Only fortified wines have alcohol percentages of 17 to 20 percent or higher.

Ethanol

The single most significant by-product of fermentation is ethanol. Ethanol is crucial to the stability, aging, and sensory properties of wine. During fermentation, the increasing ethanol content progressively limits the growth of microorganisms and consequently off-odors. The inhibitory action of ethanol, combined with the acidity of the wine, permits wine to remain stable for years in the absence of air. By affecting the metabolic activity of Saccharomyces cerevisiae, ethanol also influences the types and amounts of aromatic compounds produced.

Glycerol

Glycerol is a non-volatile compound which has no aromatic properties, but which significantly contributes to wine quality by providing sweetness, body, and mouthfeel. It is the most import by-product of alcoholic fermentation in quantity after ethanol and carbon dioxide. Wines lacking in body can benefit from an increased glycerol production to improve the sensory characteristics. Glycerol is typically found at concentrations of 4 to 10 g/L in dry wine and in the case of the botrytized late harvest wines, levels in excess of 20 g/L are not uncommon. Glycerol production can be controlled by number of factors, including:

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