238 M. Luz Sanz and Isabel Mart ́ınez-Castro
- Derivatization:the active hydrogen of all hydroxyl groups can be substituted by
non-polar groups such as methyl, acetyl or trimethylsilyl. The obtained deriva-
tives are relevant for chromatographic analysis.7.3.3 Biochemical Reactions
Sugars (mainly glucose and fructose) are converted to ethanol, carbon dioxide and
several by-products by the action of yeast during alcoholic fermentation, the most
important process in winemaking. Carbohydrates also participate in different bio-
chemical reactions where the action of different enzymes (pectinases, cellulases,
hemicellulases, glycosidases, etc.) is involved. As a result of enzymatic hydrolysis,
different carbohydrate fractions are produced, whereas the formation of several by-
products takes place. Polysaccharides are also affected by malolactic fermentation
due to the high production of glycosidase activity by lactic acid bacteria which can
hydrolyse their glycosidic bonds (Dols-Lafargue et al. 2007).
The addition of enzymes during winemaking is a common practice to improve
the extraction of colour and aroma compounds, the clarification of the white wines
and the filtration processes of musts and wines (Doco et al. 2007). More informa-
tion about the behaviour of enzymes during winemaking is described in Chapter 4
(Enzymes in winemaking).
7.3.4 Physical Properties
Solubility: whereas monosaccharides are very soluble in water and in polar solvents
such as alcohols, pyridineand dimethylsulfoxide, solubility decreases when molec-
ular weight increases; thus some polysaccharides are totally insoluble. Formation
of some polysaccharides in wines can produce defects (e.g. dextrans appearing in
wines cause ropiness).
Ta s t e: sweetness is the most characteristic property of soluble sugars. Sucrose is
the reference substance for sweet taste; fructose is sweeter and glucose is less sweet
than sucrose. There are some exceptions like gentiobiose which has bitter taste.
Sweetness decreases when molecular weight increases, and the higher oligosaccha-
rides are tasteless (Table 7.1). Polyalcohols also have sweet taste.
Ta b l e 7. 1Sweetness of some
carbohydrates of wine
(relative to that of sucrose)
Compound Relative sweetness
D-Fructose 114
D-Glucose 69
Sucrose 100
, -Trehalose 45
Xylitol 100
Sorbitol 55
Mannitol 50