2009_Book_FoodChemistry

20.2 Wine 919
sulfured wines which still have an excessively
high acid content can be subjected to biological
acid degradation (malolactic fermentation). In
this process, lactic acid bacteria (e. g., Leuconostoc
oenos) convert L-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 required
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 defects,
refreshing old wines, deepening the color of
red wines (table wines) and enhancing the bouquet
or readjusting the low acid content, thus
producing a uniform quality wine for the market.
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 pigments,
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; others
are typical fermentation and degradation products.
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 contain
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 reducingsugaring/l
plus1g/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 fermented
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 Hratio
(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