7 Carbohydrates 241
D-Xylose is an important component of wood. It is only fermentable by certain
microorganisms such asLactobacilli, TorulaandMonilia.
L-Rhamnose is a constituent of many glycosides and polysaccharides, whereas
L-fucose appears in polysaccharides and glycoproteins.
Sugars in red wines are usually higher than in white wines (Ribereau-Gayonet al. 2006). This can be due to the contact with grape skins, to the longer aging
which liberate sugars from polyphenol conjugates and also to the hydrolysis of
hemicellulose when wine is aged in barrels. Monosaccharides, especially galac-
tose, fructose and xylose vary during aging depending on the type of oak wood
(del Alamo et al. 2000). This variation has been widely studied in brandies (Viriot
et al. 1993).
The infection withBotrytis cynereais responsible for certain changes in the car-
bohydrate composition of musts and wines, mainly by oxidation. Botrytized wines
contain xylosone and 5-oxo-fructose (Ribereau-Gayon 1973), a hexodiulose which
is formed by oxidation of fructose; it has been found in botrytized musts vary-
ing within 80–150 mg/L (Barbe et al. 2000, 2002). Acid sugars also increase (see
below).
Aging in oak barrels changes the proportion of sugars in wine. During aging
average levels of galactose, fructose andxylose were higher than other monosaccha-
rides; type of oak and barrel manufacturer affected the monosaccharide composition
of the aging wine, indicating structural differences between French and American
oak hemicelluloses (del Alamo et al. 2000).
, -Trehalose is a non-reducing disaccharide formed by two units of glucose (1-
O- - D-glucopyranosyl-1-O- - D-glucopyranoside). It is present at low concentration
in fungi and plants. It is the main disaccharide in wines (Bertrand et al. 1975) and it
is formed as a result of the metabolic activity of yeasts. Its level varies within 0–611
mg/L in wines (Bertrand et al. 1975) and within 0–53 mg/L in sherries (Santa-Mar ́ıa
et al. 1983)
Other disaccharides identified in wine (sucrose, isomaltose, lactose and turanose)
are present in small amounts, seldom>50 mg/L and sometimes<5 mg/L (Bertrand
et al. 1975). Sucrose appears in wines at very low levels. It is added occasionally to
musts (chaptalization).
The presence of pentosylhexosides, trisaccharides and tetrasaccharides has been
reported in different wines using positive- and negative-ion electrospray ionization
Fourier transform ion cyclotron resonance mass spectrometry (Cooper and Mar-
shall 2001). Nevertheless, at present there are still a number of minor carbohydrates
in wines without a conclusive identification.
7.5 Polyalcohols
Wine contains different polyalcohols as shown in Table 7.4.
Tetritols and pentitols are not very frequent in nature, but some of them have been
isolated from certain plants, algae and fungi. Hexitols appear more frequently in