2009_Book_FoodChemistry

924 20 Alcoholic Beverages
Table 20.23. Aging of Gewürztraminer in a steel stank
(I) and in an oak barrel (II) – changes in the concentrations
of important aroma substances a
Compounds
Concentration (mg/l)
Acetaldehyde 1.86 4.32
3-Methylbutanal <0.001 0.051
3-Methylbutylacetate 2.9 0.450
Methional <0.0005 0.0099
β-Damascenone 0.00084 0.0028
Guaiacol 0.0036 0.056
Vanillin 0.045 0.335
Quercus lactone n.a. 0.134
a Storage 14 months; n.a. not analyzed.
rels, as shown in Table 20.23 for Gewürztraminer.
β-Damascenone and vanillin also increase.
The aroma substances formed on storage of bottles
include 1,1,6-trimethyl-1,2-dihydro-naphthalene
(TDN). After longer storage, it exceeds the
aroma threshold (ca. 20 µg/l water) and contributes
a kerosine-like aroma note to the aroma
profile in particular of old Riesling wines.
In Riesling wines from southern European
countries, this aroma substance can increase to
such an extent on aging that they acquire a very
unpleasant taste after even a short storage time
(kerosine/petrol note, Table 20.24). As a result of
the intensive sunshine and high temperatures, the
precursor carotinoids are formed in relative high
concentrations and are then degraded to TDN in
this cultivar.
The monoterpene pattern can be used to differentiate
cultivars. For example, a clear distinction
can be made between wines from the grape
cultivar “White Riesling” and wines from
other grape cultivars which are also sold as
“Riesling”. As shown in Fig. 20.7, the monoterpene
concentrations (especially of linalool,
hotrienol, α-terpineol, and 3,7-dimethylocta-
1,5-trans-dien-3,7-diol) in “White Riesling” are
considerably higher than in the other “Riesling”
wines.
Methoxypyrazines (Table 20.18) in concentrations
of 10–20 ng/l are characteristic of
Sauvignon wines. They are exceptionally odor
active (cf. 5.3.1.7) and produce a paprika note in
the odor profile.
I
II
20.2.7 Spoilage
As with beer, defects in wine are reflected in appearance,
odor and taste and, if not controlled, result
in complete spoilage. A full explanation of all
defects is beyond the scope of this book; hence
only a general outline will be provided.
Of importance is browning due to oxidative
reactions of phenolic compounds which, in red
wine, may result in complete flocculation of the
color pigments. This oxidative darkening process
is as much chemical as enzymatic (polyphenoloxidases).
Sulfurous acid is the preferred agent to
prevent browning. Once the wine is affected by
browning, it may be lightened by treatment with
activated charcoal. The charcoal treatment can
also remove other defects, such as the taste of
mash or rotten grapes. Iron-induced turbidity
(white or greyish casse) appears as a white,
greyish-white or greyish haze or cloudiness and
consists mostly of ferric phosphate (FePO 4 ). It is
formed by the oxidation of ferrous compounds in
wine. Proteins, tannins or pectins can participate
in the build-up of such cloudiness (black casse).
The so-called copper casse or turbidity is based
on the formation of Cu 2 S and other compounds
with monovalent copper. It originates from the
Cu 2+ ions present in wine and their reduction in
the presence of excess SO 2 . Other taste defects
compiled in Table 20.24 can be divided into:
• those produced by the cultivar (e. g., strawberry
note, fox note)
• those produced in fermentation by other
microbial processes (e. g., “boeckser”, mousy
note, medicine note)
• those formed during wine storage and aging in
wood barrels or by contamination (e. g., cork
note, musty note, kerosine note, untypical aging
note).
A medicine note is detected when the phenols
listed in Table 20.24 are formed in excessively
high concentrations on the degradation of ferulic
and p-coumaric acid. This aroma defect has been
observed especially in the cultivar “Kerner” when
the grapes were exposed to intensive sunlight.
The “untypical aging note” (Table 20.24) is produced
by stress during ripening of the berries.
Dryness, low nitrogen uptake with a high yield
can result in the formation of the unwanted odorant
2-aminoacetophenone during fermentation.