Food Chemistry

20.2 Wine 921

by the slight oxidation of ethanol that occurs on
storage of red wine.

(20.12)

20.2.6.7 Nitrogen Compounds

The nitrogen compounds in must precipitate to
a smaller extent by binding to tannins during
grape crushing and mashing, while most (70–
80%) of them are metabolized by the growing
yeast during fermentation. Free amino acids,
especially proline (about 200–800 mg/l) are
the major nitogen compounds which remain in
wine. Tryptophan, which is present in must in
concentrations of 1–30 mg/l, and acetaldehyde,
which is provided by yeast, are precursors of
1-methyl-1,2,3,4-tetrahydro-β-carbolin-3-carbo-
xylic acid (MTCA). In fact, 0–18 mg/lof
MTCA have been detected in wine. Its formation
(cf. Formula 20.13) is inhibited by SO 2 ,which
traps the precursors. On distillation, MTCA
apparently remains in the residue because only
traces, if at all, are present in brandy and whiskey.
However, MTCA is not restricted to fermented
products like wine, beer and soy sauce as the
precursors are widely found, e. g., in milk, cheese
and smoked foods.

20.2.6.8 Minerals

The mineral content of wine is lower than that of
the must since a part of the minerals is primarily
removed by precipitation as salts of tartaric acid.
The ash content of wines is about 1.8–2.5g/l,
while that of must is 3–5 g/l. The average compo-
sition of ash in %, is: K 2 O, 40; MgO, 6; CaO, 4;
Na 2 O, 2; Al 2 O 3 ,1;CO 2 ; 18; P 2 O, 16; SO 3 , 10;
Cl, 2; SiO 2 ,1.

(20.13)

The iron (as Fe 2 O 3 ) content of wine is 5.7–

13 .4mg/l, but it can rise to much higher levels

(20–30 mg/l) through improper processing of

grapes.

20.2.6.9 Aroma Substances

Most of the volatiles in wine, more than 800

compounds, with a total concentration of

0 .8–1.2g/l, has been identified. For the wines

Gewürztraminer and Scheurebe, it has been

found that the compounds listed in Tables 20.16

are so odor active that they can produce the

aroma in each case. This could be confirmed

for Gewürztraminer in a model experiment.

A synthetic mixture of odor and taste compounds

in the concentrations given in Table 20.16

and 20.17 reproduced the aroma and the taste of

Gewürztraminer.

The two cis-rose oxides and 4-mercapto-4-

methylpentan-2-one, which has an exceptionally

low odor threshold (cf. 5.3.2.5), have been iden-

tified as the cultivar-specific odorants in Gewürz-

traminer and Scheurebe. In addition, ethyloc-

tanoate, ethylhexanoate, 3-methylbutylacetate,

ethylisobutanoate, linalool, (E)-β-damascenone

and wine lactone (cf. structure in 5.2.5) exhibit

high but different aroma values in the two types

of wine. Other typical odorants are listed in Table

20.18. Some red wines, e. g. from Shiraz grapes

contain the bicyclic terpene (−)-rotundone, the

key aroma compound of pepper (cf. 22.1.1.2.1).

Its concentration is high (up to 145 ng/l) in

samples showing an intense “peppery” aroma

note. Ethanol is essential for the aroma of wine.

Since the odor thresholds of many volatiles

increase in the presence of ethanol, e. g., those

of ethyl-2- and -3-methylbutanoate increase by

a factor of 100 (Table 20.19), it influences the

bouquet of wine. Correspondingly, the intensity

of the fruity note increased in an aroma model for

Gewürztraminer when the alcohol content was

lowered. The odorants partly originate from the

grape (primary aroma) and are partly formed on