Part 7 – Esters
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v Esters
Alcohols
(ethanol and high alcohols) and acids
(acetic-, lactic-, fatty-, tartaric- and succinic acid) are key components in
wine. These two components, alcohol and acid, can enter into a chemical
reaction with each other to form a new compound. When this happens, the –OH-group of the alcohol links up with the –COOH-group of
the acid, whereby H2O splits off. This is called a ‘condensation
reaction’ and the new compound is called an ester.
l The esters are named as follows: the alcohol (ethanol) loses
the -OH group and; the suffix-anol is replaced by -yl (e.g., ethanol à ethyl); the acid loses an H-ion and gets the suffix-ate
(e.g. acetic acid à acetate).
l An ester, depending on its concentration (detection
threshold), gives off an odor to wine which can be
positive or negative. The volatility of the ester (boiling point) is dependent on the length of the compound: generally
the longer the chain, the less volatile. Common esters in wine can be divided
into 4 groups:
Group 1. Ethanol
+ acetic/lactic acid
These volatile esters do not contribute
positively to the wine aroma. A high content of these esters indicates wine
fault.
Group 2. Ethanol + fatty acid
Fatty acid esters give off fruity, sweets
odors at low concentration, but at high concentration they may gives off a slightly
unpleasant soapy, rancid-like smell.
Group 3. High alcohol + acetic acid
High alcohol esters play an important role in the aromas
of wine, especially in young white wines. In particular isobutyl acetate and
isoamyl acetate, which give off odours of pear and banana. These attractive
aromas dominate in many simple, industrially produced young white wines. The
high content of these esters is obtained by using substantial quantities of at specific
enzymes selected yeasts and cool but fast fermentation. A strong pear and
banana flavor is not exactly a quality indicator. It is an aroma of transient,
which usually completely disappears within 1 or 2 years by hydrolysis, making
these wines quickly uninteresting.
Group 4 Diethanol + Dicarboxylic acid
Esters from group 1 – 3, are formed
during fermentation. They are catalysed by enzyme and go very fast.
The esters in this group, group 4, are formed during wine aging. They are not catalysed by enzyme and they occur very, very
slowly.
Tartaric acid comes from grapes. Succinic
acid is formed in the TCA cycle (see Part 2)
and have 2 carboxylic acid
groups (COOH). They need 2 ethanols to form an ester. These esters, at normal concentrations, have no sensory impact.
v Detection
threshold
The detection threshold is
the lowest concentration of a certain compound
that is perceivable by a human.
This threshold is measured through extensive testing
using human subjects in laboratory settings. Detection thresholds
are not exact values; not everyone is equally sensitive to a certain smell or
taste. The detection threshold of a substance may be different in different
literatures.
v Concentration
Scents like musk,
amber or faeces, smell unpleasant in itself, but at very low concentration,
they can be an essential part of the composition of a perfume. Some compounds
and esters in wine can smell pleasant at low concentration but unpleasant at
high concentration. Also, not everyone is equally sensitive to certain
smells. Even worse, our sensory capability is not constant, it changes
depending on physical conditions like being tired or having a cold. That explains
why there can be so many different odor descriptions for one and the same wine.
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P.S.
Esters are mainly of interest in
aromas in the young white wines than in reds. Why?
In the next post we'll take a brief look
at the yeast cell membrane and a number of aerobic biosyntheses to find out the
reason.
