of symmetry passing through C1 and C4. This polyol is widespread in the animal and plant kingdoms. It is a vital growth factor for many microorganisms, especially certain yeasts.
TABLE 2.4 Aliphatic Fatty Acids Among the Volatile Components in Wine(Ribéreau‐Gayon et al., 1982)
| Formula | Name | Boiling point (°C) | Concentration (g/l) | Comments |
|---|---|---|---|---|
| H−COOH | Formic | 101 | 0.05 | |
| CH 3 −COOH | Acetic | 118 | 0.5 | |
| CH3−CH2−COOH | Propionic | 141 | Traces | |
| CH3−CH2−CH2−COOH | Butyric | 163 | Traces | |
|
|
Isobutyric | 154 | Traces | 2‐Methyl‐propionic acid |
| CH3−CH2−CH2−CH2−COOH | Valerianic | 186 | Traces | |
|
|
Isovalerianic | 177 | ? | 3‐Methyl‐butyric acid |
|
|
2‐Methyl‐butyric | ? | ||
| CH3−(CH2)4−COOH | Caproic | 205 | Traces | Hexanoic acid |
| CH3−(CH2)5−COOH | Enanthic | 223 | Traces | Heptanoic acid |
| CH3−(CH2)6−COOH | Caprylic | Traces | Octanoic acid | |
| CH3−(CH2)7−COOH | Pelargonic | 253 | ? | Nonanoic acid |
| CH3−(CH2)8−COOH | Capric | 270 | Traces | Decanoic acid |
It is difficult to attribute any organoleptic role to C6 polyols.
2.4 Aliphatic FattyAcids
These aliphatic fatty acids are shown in Table 2.4. The most important of these compounds is acetic acid, the essential component of volatile acidity. Its concentration, limited by legislation, indicates the extent of bacterial activity (lactic acid bacteria or acetic acid bacteria) and the resulting spoilage of the wine. As yeast forms a small amount of acetic acid, there is some volatile acidity in all wines. Other C3 (propionic acid) and C4 acids (butyric acids) are also associated with bacterial spoilage.
The C6, C8, and C10 fatty acids are formed by yeast. As they are fermentation inhibitors at concentrations of only a few milligrams per liter, they may be responsible for stuck fermentations (Volume 1, Section 3.6.2).
Unsaturated long‐chain fatty acids (C18 and C20) are related to the sterol family. These compounds are fermentation activators, particularly under anaerobic conditions (Volume 1, Section 3.5.2). The most important of these are oleic (C18 with one double bond) and linoleic (C18 with two double bonds) acids. They are active in trace amounts and come from the waxy cuticle of grape skins. Under the name “fatty acids,” there are thus two families of compound with fundamentally different properties.
2.5 Esters
Esters are formed when an alcohol function reacts with an acid function and a water molecule is eliminated (Figure 2.7). It is a reversible reaction, limited by the reverse reaction of hydrolysis of the ester. When the system is in equilibrium, there is a constant ratio between the concentrations of the substances present, governed by the law of mass action.
There are a large number of different alcohols and acids in wine, so the number of possible esters is also very large. Ethyl esters are the most common for kinetic reasons, i.e. owing to the large quantities of ethanol present and the fact that primary alcohols are the most reactive.
Very few esters are present in grapes. Some contribute to aroma, such as methyl anthranilate, which is responsible for the foxy odor in Vitis labrusca grapes and wines made from them. There are also methoxy groups in pectins that release methanol by hydrolysis (Section 2.2.1).
FIGURE 2.7 Esterification equilibrium of an alcohol.
Esters in wine have two distinct origins: enzymatic esterification during the fermentation process and chemical esterification during long‐term aging. The same esters may be synthesized in either way.
2.5.1 Ethyl Acetate
The most prevalent ester in wine is certainly ethyl acetate. A small quantity is formed by yeast during fermentation, but larger amounts result from the activity of aerobic acetic acid bacteria, especially during aging in oak barrels. Apparently, lactic acid bacteria are not able to produce this ester. Ethyl acetate is responsible
