2009_Book_FoodChemistry

20.2 Wine 921
by the slight oxidation of ethanol that occurs on
storage of red wine.
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.
20.2.6.7 Nitrogen Compounds
(20.12)
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-carboxylic
acid (MTCA). In fact, 0–18 mg/l of
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 composition
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.
20.2.6.9 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 identified
as the cultivar-specific odorants in Gewürztraminer
and Scheurebe. In addition, ethyloctanoate,
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
(20.13)