Food Chemistry

20.2 Wine 919

sulfured wines which still have an excessively
high acid content can be subjected to biologi-
cal acid degradation (malolactic fermentation). In
this process, lactic acid bacteria (e. g.,Leuconos-
toc oenos)convertL-malic acid (10 g) to lactic
acid (6.7g):

HOOC CHOH CH 2 COOH

→HOOC CHOH CH 3 +CO 2 (20.10)

In addition, the residual sugar, aldehydes and
pyruvate are degraded so that less SO 2 is re-
quired in a subsequent sulfur treatment step.
The multiplication of the lactic acid bacteria is
promoted by increasing the temperature to 20◦C
and stirring up the yeast settlings.
Wine blending is a suitable way of rectifying de-
fects, refreshing old wines, deepening the color of
red wines (table wines) and enhancing the bou-
quet or readjusting the low acid content, thus
producing a uniform quality wine for the mar-
ket.

Tartaric or citric acid can be added to low-acid
wines from southern European countries. The
addition of gypsum or phosphate treatment to
enhance the color of red wines, which is used in
the case of certain southern wines (e. g., Malaga,
Marsala) is based on the increase in the color
yield caused by lowering the pH with CaSO 4 or
CaHPO 4.

20.2.6 Composition

The chemical composition of wine varies over
a wide range. It is influenced by environmental
factors, such as climate, weather and soil, as well
as by cultivar and by storage and handling of the
grapes, must and wine.
Within the scope of wine analysis, wine extract,
alcohol, sugar, acids, ash, tannins, color pig-
ments, nitrogen compounds and bouquet-forming
substances are important. Hence, the value and
quality of a wine is assessed through the content
of ethanol, extract, sugar, glycerol, acids and
bouquet substances. With the large number of

quality-determining constituents, the evaluation
and classification of wine are possible only by
a combination of chemical analysis and sensory
testing.

20.2.6.1 Extract

The extract includes all the components of wine

mentioned above, except the volatile, distillable

ones. Many of the extract components are present

in must and are described in that section; oth-

ers are typical fermentation and degradation prod-

ucts. The extract content of 85% of all German

white wines is about 20–30 g/l (average about

22 g/l), while the extract content of red wines is

somewhat higher – German “Auslese” wines con-

tain about 60 g/l; other sweet wines, 30–40 g/l.

Since the sugar content can be manipulated, the

“sugar-free extract” (extract in g/l minus reduc-

ingsugaring/lplus1g/l for arabinose, which is

also detected in the reductometric determination,

but is not fermentable) is of greater importance

for an evaluation of quality.

20.2.6.2 Carbohydrates

Carbohydrates (0.03–0.5%) present in fully fer-

mented wines are small amounts of the hexoses

glucose and fructose and of nonfermentable

pentoses. Incompletely fermented wines contain

higher concentrations of both hexoses, but

substantially more of the slower fermenting

fructose. The average ratio of glucose to fructose

in the residual sugar of wine is 0.58:1, but it

varies to a great extent. The pentose sugars

which are present in fermented wines consist of

0 .05–0.13% arabinose, 0.02–0.04% rhamnose,

and xylose in trace amounts.

20.2.6.3 Ethanol

The ethanol content of wine varies over a wide

range. It serves as a quality feature (cf. 20.2.3.3).

An alcohol level above 144 g/l indicates addition

of ethanol.

The extent to which ethanol is derived from added

sugar in fermentation can be determined by the

NMR spectroscopic measurement of the ratio of

the hydrogen isotopes^1 Hto^2 H. The method is

based on the fact that the plant-specific^2 H/^1 Hra-

tio (R value, cf. 18.4.3) of the sugar also appears

in ethanol: about 2.24 (corn sugar), about 2. 70

(beet sugar), about 2.45 (wine). The detection