Wine Chemistry and Biochemistry

252 R. Baumes

Glycoconjugates

Postfermentary stage

Carotenoids

Prefermentary stage

Varietal aroma

Compounds

Impact on wine aroma

Cysteine conjugates

Fermentary stage

S-methylmethionine

Fermentary and

postfermentary stages

Phenolic

Acids

Fermentary stage

Unsaturated

Lipids

Prefermentary

GenerallyGenerally stage

positivepositive

GenerallyGenerally

negativenegative

Fig. 8A.1Wine aroma precursors, their main stages of degradation during the wine biotechnolog-
ical sequence and general impact on wine aroma of the odorants generated

fermentation, constituents of the so-called fermentative aroma, nor those starting

compounds, involved in more or less complex biological or chemical processes

occurring under different wine aging conditions, such as Maillard reactions or bio-

logical and chemical oxidative aging. All these compounds are described elsewhere

in this book.

8A.2 Unsaturated Fatty Acids

The majority of grape acyl lipids are esters of unsaturated fatty acids, and the major

ones, linoleic and linolenic acid esters, are wine aroma precursors (Fig. 8A.2).

However, their total contents in unsaturated fatty acids, about 350 mg/kg in grape

berries, hardly depend on the cultivar and decrease with maturity (Bayonove 1998).

These compounds, primarily located in the solid parts of the berry, are degraded

by grape enzymes to the so-called C6-compounds, when grapes are crushed in

air in the prefermentary stage; The powerful odorants, hexanal and 2-hexenal, are

the major C6-compounds produced in grape must, and hexanol and hex-2- and

3-en-1-ols are minor products (Drawert et al. 1966; Roufet et al. 1986, 1987;

Ferreira et al. 1995). The main enzymes involved in this degradation, are the grape
lipoxygenase, which regiospecifically oxidizes only unsaturated lipids containing

a1-cis,4-cis-pentadiene system, to hydroperoxides with acis,trans-diene system

(linoleic and linolenic lipids), and grape hydroperoxide-lyase, which cleaves the

hydroperoxides to the C6-aldehydes (Drawert et al. 1966). Thus, the potential in