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 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.
2.5.2 Fatty Acid Ethyl Esters and Higher Alcohol Acetates
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.
Fatty acid ethyl esters have aromas of wax and honey. They are present at total concentrations of a few milligrams per liter.
Acetate esters of higher alcohols (isoamyl acetate and phenylethyl acetate) should also be included among the fermentation esters. These compounds are present in moderate quantities, but have intense, rather unusual odors (banana, rose, and honey). They contribute to the aroma complexity of naturally neutral wines, but may mask some varietal aromas.
Among these acetate esters, isobutyl acetate (or 2‐methylpropyl acetate) plays a role in enhancing fruity aromas (Cameleyre et al., 2015). In contrast with most acetates of higher alcohols produced during alcoholic fermentation, this substituted acetate increases in concentration during tank/barrel aging and at the start of bottle aging. It is present in red wines in the form of its S enantiomer only, and its aroma is reminiscent of bananas. It is found at average concentrations of 72 μg/l, whereas its aroma perception threshold is about 1,100 μg/l. Nevertheless, despite its infra‐threshold concentration, it is known to contribute to black fruit, fresh fruit, and jammy notes in red wines.
The formation of all these esters is promoted when fermentation is slow (Bertrand, 1983; Dubois, 1993) and difficult, due to the absence of oxygen, low temperatures, and clarified must.
2.5.3 Substituted Acid Ethyl Esters
Short‐chain substituted acid ethyl esters exhibit special behaviors and have a particularly significant sensory impact. Recent studies (Pineau et al., 2009; Lytra et al., 2012, 2013, 2014, 2015), supported by older literature (Ribéreau‐Gayon et al., 1982; Guth, 1997), have established that some of these esters are involved in the fruity aroma of red wines. In contrast with most linear fatty acid ethyl esters produced during alcoholic fermentation, these substituted esters increase in concentration during tank or barrel aging and at the beginning of bottle aging. It seems that these compounds