Chapter 1

Chemical Components of Grapes and Wine

Sugars in Grapes and Wine

Sugars are important because they represent a water‐soluble energy source for yeast. Sugars in berries account for nearly all the soluble solids (approx. 90%) in the pulp and juice. The main sugars in grapes are glucose and fructose depending on the variety and maturity of the grapes. Sucrose is rarely found in Vitis vinifera grapes, but it may constitute up to 10 percent of the sugar content in non-V. vinifera cultivars. Fructose, glucose, and sucrose differ significantly in sweetness. The order of sweetness: fructose is sweeter than sucrose, which is sweeter than glucose. In other words, on a sweetness scale, if fructose is 100, then sucrose is 84 and glucose is 66. That is quite a difference. This information is important to a winemaker. For example, if a winemaker wishes to sweeten a wine, less fructose is needed than sucrose to reach the same degree of sweetness.

Residual Sugars

Residual sugar (RS) refers to the natural grape sugars left over in a wine after the alcoholic fermentation is complete. Residual sugar along with alcohol and acidity form the balance of all wines. During fermentation with most yeast strains, the consumption of glucose is faster than that of fructose, and toward the end of fermentation, most of the residual sugar is therefore fructose. Residual sugar may vary from 1 to 3 g/L (0.1–0.3%) in very dry table wine up to 50 to 150 g/L in port style wines or late harvest Botrytis dessert wines.


A polysaccharide is a large molecule made of many smaller monosaccharides. Monosaccharides are simple sugars, like fructose and glucose. Special enzymes bind these small monomers together creating large sugar polymers, or polysaccharides. In wine, polysaccharides may be derived from both grape and yeast cell walls. The major polysaccharides in grapes are cellulose, pectins, glucans, and hemicellulose. Polysaccharides are extracted during the mechanical operations applied to the grapes (destemming-crushing, pressing, and pumping of the crushed destemmed grapes) and during some later stages of winemaking.


Pectin is a structural polysaccharide, a chain of various sugar molecules. The skins and pulp of the grape varieties are rich in pectin compounds. The high viscosity of pectin, which is dissolved after berry crushing impedes juice extraction, clarification, and filtration.


When Botrytis cinerea infects grape berries, it secretes a specific glucan into the grape juice. This polysaccharide is highly viscous.