Pascal Ribéreau-Gayon - Handbook of Enology, Volume 2

meso ‐Inositol is a normal component of grapes and wine. It is a cyclic polyol with six carbon atoms, each carrying a hydroxyl radical. Among the nine inositol stereoisomers, which are all diastereoisomers of each other, meso ‐inositol has a plane 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 2.4Aliphatic 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
CH 3−CH 2−COOH	Propionic	141	Traces
CH 3−CH 2−CH 2−COOH	Butyric	163	Traces
	Isobutyric	154	Traces	2‐Methyl‐propionic acid
CH 3−CH 2−CH 2−CH 2−COOH	Valerianic	186	Traces
	Isovalerianic	177	?	3‐Methyl‐butyric acid
	2‐Methyl‐butyric		?
CH 3−(CH 2) 4−COOH	Caproic	205	Traces	Hexanoic acid
CH 3−(CH 2) 5−COOH	Enanthic	223	Traces	Heptanoic acid
CH 3−(CH 2) 6−COOH	Caprylic		Traces	Octanoic acid
CH 3−(CH 2) 7−COOH	Pelargonic	253	?	Nonanoic acid
CH 3−(CH 2) 8−COOH	Capric	270	Traces	Decanoic acid

It is difficult to attribute any organoleptic role to C6 polyols.

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.

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.

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 for the olfactory characteristics in wines affected by acescence: a solvent‐ or glue‐like odor. These wines also have high volatile acidity, but acetic acid is not only responsible for acescence.

The aroma perception threshold of ethyl acetate is approximately 160 mg/l. Even below this value, while it may not be identifiable, it may spoil wine bouquet with an unpleasant, pungent tang.

Furthermore, ethyl acetate affects the somatosensorial sensations provoked during tasting. At relatively high concentrations (above 120 mg/l) that are still below the aroma perception threshold, it gives red wines a burning sensation, which reinforces the impression of harshness on the aftertaste. Ethyl acetate contributes to hardness and firmness in red wines. From field observations, an acetic acid concentration of at least 0.90 g/l is required to produce a noticeable acrid, sour aftertaste, which is, in fact, due to the mixture of acetic acid and ethyl acetate.

FIGURE 2.8 Biosynthesis mechanism of fatty acids.

Ethyl esters of fatty acids, mainly ethyl hexanoate and octanoate, are produced by yeast during alcoholic fermentation. They are synthesized from forms of the acids activated by coenzyme A (HS‐CoA): acyl‐ S ‐CoA. Acetyl‐ S ‐CoA, from pyruvic acid, may be involved in a Claisen condensation with malonyl‐ S ‐CoA, producing a new acyl‐ S ‐CoA with two additional carbon atoms ( Figure 2.8). Acetyl‐ S ‐CoA thus produces butanoyl‐ S ‐CoA, then hexanoyl‐ S ‐CoA, etc. Specific enzymes then catalyze the alcoholysis of acyl‐ S ‐CoA into ethyl esters of fatty acids. At the same time, coenzyme A is regenerated.

Fatty acid ethyl esters are formed by yeast, under anaerobic conditions, in quantities greater than those predicted by the mass action law. Consequently, they are hydrolyzed during aging and concentrations tend to decrease. Garofolo and Piracci (1994) determined the kinetic equations for the hydrolysis of esters of fatty acids in model media and in wines, at various pH values, over a period of 29 months according to their results shown in Table 2.5.