2.25). These two reactions can be summarized as follows:
During transamination, PLP is temporarily transformed into pyridoxamine phosphate (PMP). The PLP aldehyde group is bound to a lysine residue ε‐amino group on the active site of the aminotransferase to form an intermediate (E‐PLP) (Figure 2.26). The α‐amino group of the amino acid that is the transamination substrate displaces the lysine ε‐amino group bound to PLP. The cleavage of this intermediate liberates PMP and the keto acid corresponding to the amino acid substrate. PMP can in turn react with another keto acid to furnish a second amino acid and regenerate PLP.
The partial reactions can be written in the following manner:
the balance for which is:
FIGURE 2.26 Mode of action of pyridoxal phosphate (PLP) in transamination reactions. Formation of intermediates between PLP and aminotransferase or the amino acid substrate.
Some amino acids, such as serine and threonine, possess a hydroxyl group on their β carbon. They can be directly deaminated by dehydration. A dehydratase catalyzes this reaction, producing the corresponding keto acid and ammonium (Figure 2.27).
2.4.4 Formation of Higher Alcohols and Esters
Yeasts can excrete keto acids originating from the deamination of amino acids only after their decarboxylation into aldehyde and reduction into alcohol (Figure 2.28). This mechanism, known as the Ehrlich reaction, explains in part the formation of higher alcohols in wine. Table 2.4 lists the principal higher alcohols and their corresponding amino acids, possible precursors of these alcohols.
Several experiments clearly indicate, however, that the degradation of amino acids is not the only pathway for forming higher alcohols in wine. In fact, certain ones, such as propan‐1‐ol and butan‐1‐ol, do not have amino acid precursors. Moreover, certain mutants deficient in the synthesis of specific amino acids do not produce the corresponding higher alcohol, even if the amino acid is present in the culture medium. There is no relationship between the amount of amino acids in must and the amount of corresponding higher alcohols in wine.
The production of higher alcohols by yeasts appears to be linked not only to the catabolism of amino acids but also to their synthesis via the corresponding keto acids. These acids are derived from the metabolism of sugars. For example, propan‐1‐ol has no corresponding amino acid. It is derived from α‐ketobutyrate that can be formed from pyruvate and acetyl‐CoA. α‐Ketoisocaproate is a precursor of isoamyl alcohol and an intermediate in the synthesis of leucine. It too can be produced from α‐acetolactate, which is derived from pyruvate. Most higher alcohols in wine can also be formed by the metabolism of glucose, without the involvement of amino acids.
FIGURE 2.27 Deamination of serine by a dehydratase.
FIGURE 2.28 Formation of higher alcohols from amino acids (Ehrlich reactions).
The physiological function of higher alcohol production by yeasts is not clear. It may be a simple waste of sugars, a detoxification process of the intracellular medium, or a means of regulating the metabolism of amino acids.
With the exception of phenylethanol, which has a rose‐like fragrance, higher alcohols smell bad. Most, such as isoamyl alcohol, have heavy solvent‐like odors. Methionol is a peculiar alcohol because it contains a sulfur atom. Its cooked‐cabbage odor has the lowest perception threshold (1.2 mg/l). It can be responsible for the most persistent and unpleasant off‐odors of reduction, especially in white wines (Volume 2, Section 8.6.2). In general, the winemaker should avoid excessive higher alcohol odors. Fortunately, their sensory impact is limited at their usual concentrations in wine, but it depends on the overall aroma intensity of the wine. Excessive yields and rain at the end of ripening can dilute the must, in which case the wine will have a low aroma intensity and the heavy, common character of higher alcohols can be pronounced.
The winemaking parameters that increase higher alcohol production by yeasts are well known: high pH, high fermentation temperature, and aeration. In red winemaking, the extraction of pomace constituents and the concern for rapid and complete fermentations impose both aeration and high temperatures. In this case, the production of higher alcohols by yeast cannot be limited. In white winemaking, a fermentation temperature between 20 and 22°C limits the formation of higher alcohols.
Ammonium and amino acid deficiencies in must lead to an increased formation of higher alcohols. Under these conditions, the yeast appears to recuperate all of the available amino nitrogen by transamination. It releases the unused carbon skeleton in the form of higher alcohols. Settling of white must for clarification purposes also limits the production of higher alcohols (Section 13.5.2).
The nature of the yeast (species and strain) responsible for fermentation also affects the production of higher alcohols. Certain species, such as Hansenula anomala, have long been known to produce a lot of these compounds, especially under aerobic conditions (Guymon et al., 1961). However, production by wine yeasts is limited, even in spontaneous fermentation. More recently, various researchers (Kishimoto, 1994; Masneuf, 1996; Masneuf‐Pomarède et al., 2010) have shown that most Saccharomyces bayanus strains (formerly called uvarum) produce considerably more phenylethanol than does S. cerevisiae. Lastly, the production of higher alcohols in S. cerevisiae depends on the strain. Limited production of higher alcohols (with the exception of phenylethanol) should be among the selection criteria for wine yeasts.
TABLE 2.4 The Principal Higher Alcohols Found in Wine and Their Amino Acid Precursors
| Higher alcohol | Concentration in wine (mg/1) | Amino acid precursor |
|---|---|---|
|
|
80–300 |
|
