Part 7 - Esters

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.