7 Carbohydrates 239
Ta b l e 7. 2Rotary power of
some carbohydrates of wine Compound [^ ]D
D-Fructose − 92
D-Glucose + 52. 7
- D-glucose + 112
β-D-glucose + 18. 7
Sucrose + 66. 5
β-Cyclodextrin + 162
Rotary power: the specific rotation [ ]Dis defined using the sodium D-line at
20–25◦C. It is not related to molecular weight, and is very characteristic of every
sugar in solution (see Table 7.2). It depends on concentration, temperature and sol-
vent. When a crystalline sugar is dissolved, the rotary power of the solution changes
until the equilibrium is reached (mutarotation). Each anomeric form has their own
rotary power, and the equilibrium value reflects the individual values of all present
forms.
Crystallization: sugars may easily crystallize. The crystals are usually stable at
room temperature, although in general are hygroscopic and they have to be held
in a closed vessel. Sugars are present in wines at very low concentration; thus they
remain soluble. Nevertheless some problems can arise when some insoluble crystals
appear, such as mucic acid (galactaric).
7.4 Mono- and Disaccharides
Wine contains several sugars. Their structure and the reported concentration ranges
are summarized in Table 7.3.
D-Glucose is the most abundant monosaccharide in nature, and it plays a central
role in biochemistry, since it is the primary fuel for living cells. D-Fructose occurs in
free form in many fruits and honey, as wellas forming polysaccharides (inulin and
fructans) in several plants. D-Glucose and D-fructose are the main sugars in must.
Although both decrease during fermentation, the ratio fructose/glucose increases
since glucose is the preferred substrate of many types of yeast.
D-Mannose is present in polysaccharidesof plants (mannans) and also in com-
bination with other sugars. Mannoproteins are considered one of the main polysac-
charides of wines as it will be described later.
D-Galactose appears free in nature in low amounts, but it forms part of lactose
in milk and appears in complex biomolecules as glycolipids and glycoproteins. Its
content seems to be higher in wines aged in contact with lees (Doco et al. 2003).
L-Arabinose is found in free form inConiferae; it appears in combined form in
bacterial polysaccharides, pectic materials, hemicelluloses and plant glycosides.
Both galactose and arabinose contents were found to be higher in wines made
from selectedBotrytis cinerea-infected grapes than in those made from healthy
grapes (Dittrich and Barth 1992).
D-Ribose is a constituent of nucleic acids, hence it is present in all plant cells; it
is also found in several coenzymes.