Gg

G.G1—G.G4

'Adapted from Amerine et al., 1980 and Boulton et al., 1996 2Mainly sucrose and various pentoses 3Mainly tartaric, malic, citric, lactic, and acetic 4Mainly potassium, magnesium, and calcium

'Adapted from Amerine et al., 1980 and Boulton et al., 1996 2Mainly sucrose and various pentoses 3Mainly tartaric, malic, citric, lactic, and acetic 4Mainly potassium, magnesium, and calcium sugar may vary, depending mostly on maturity. Other sugars also may be present, but at very low concentrations, including the sugar alcohol—sorbitol—and the pentoses—arabinose, rhamnose, and xylose. It is common practice among wine makers to refer to the total sugar concentration in units of Brix or °Brix. The °Brix value is actually a measure of density or specific gravity and is easily and quickly determined using a hydrometer. Juice from mature grapes at 20% sugar is ordinarily about 21°Brix to 24°Brix.

As will be discussed later in more detail, glucose and fructose (and to a lesser extent, sucrose) serve as the major growth substrates for the fermenting yeasts. In fact, the main wine yeast, Saccharomyces cerevisiae, ferments only these simple sugars, and all of the ethanol that is produced is derived from sugar fermentation. In addition, a small amount of the sugar is converted to esters, aldehydes, higher alcohols, and other volatile organic compounds (formed also from metabolism of fatty acids and amino acids) that contribute important flavor and aroma characteristics. When nearly all of the sugar is fermented, and only residual (0.1% to 0.2%) amounts remain, the wine is considered "dry." In contrast, when the residual sugar concentration at the end of fermentation is 10 g/L

or higher, a "sweet" wine is produced. Very sweet wines, such as those consumed as dessert wines, can contain as much as 100 g/L to 200 g/L (10% to 20%). Finally, it should be noted that when the sugar concentration in the must is low, as might occur in grapes grown in cool climates, it is permissible (in some countries, but not in California) to add sugar to the must, a process known as chaptalization.

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