Wine is a complex composition of, inter alia, more than 800 volatile constituents.
Some are derived from the grapes, while others are formed during fermentation or maturation. The aroma or flavour of wine is probably the most important aspect of the enjoyment and quality thereof. Aroma constituents represent a wide variety of chemical classes, of which nitrogen-containing constituents (Fig. 1) make up a small but significant section. Nitrogen is an essential constituent of amino-acids, nucleotides, proteins and nucleic acids, it plays an extremely important role as yeast nutrient and eventually ends up as a number of flavourful constituents in wine. It may also be formed in the grapes and subsequently transferred to wine. Amines are some of the most common nitrogen-containing constituents and examples of volatile amines are ethyl-, phenylethyl-, methyl- and isoamylamine. At present there is still uncertainty about the contribution to flavour or taste made by these constituents.
An important group of nitrogen-containing constituents are the pyridines, for example 2-acetyl-1,4,5,6-tetrahydropyridine and 2-acetyl-3,4,5,6-tetrahydropyridine which are formed from lysine by certain Brettanomyces yeasts as well as certain Lactobacillus bacteria. They cause the so-called mousiness, unpleasant malodours linked to the urine of mice (Heresztyn, 1986). Other nitrogen-containing constituents also responsible for mousy flavours and also formed microbiologically, are 2-ethyltetrahydropyridine and 2-acetyl-1-pyrrolene (Grbin et al., 1996). With a threshold value of 0,1 g/l in water (Teranishi, Buttery & Guadagni, 1975), the latter is an extremely important, negative aroma impact constituent. The mousy flavour is usually observed on the palate after the wine has been swallowed. Another effective means of identification is by rubbing a few drops of wine on the hand and then smelling it.
Some of the most important cultivar-typical aroma constituents in Sauvignon blanc and Cabernet Sauvignon grapes and wine are the nitrogen-containing methoxypyrazines. These components are secondary products of amino-acid catabolism. However, the exact chemical way for its formation is unknown. The most important pyrazine, usually occurring in the highest concentrations, is 2-methoxy-3-isobutylpyrazine (ibMP), while 2-methoxy-3-isopropylpyrazine (ipMP) and 2-methoxy-3-secbutylpyrazine (sbMP) also occur, but usually in much lower concentrations.
The above three methoxypyrazines have extremely low threshold values; ibMP, for example, can theoretically be detected sensorially at a concentration of 2 ng/l in water (Buttery et al., 1969). This may be compared to one berry in 500 000 tons of grapes. Other literature sources provide slightly higher threshold values and these could vary with the composition of the wine. Methoxypyrazines differ with regard to their aromas, which range from bell peppery(typical of ibMP), asparagus-like (typical of ipMP) to vegetative, grassy and gooseberry-like (Allen et al., 1988).
Some of the most prominent malodours in wine are corky or mouldy flavours. Apart from the well-known 3,4,6-trichloro-anisole, various other chemical constituents are involved in these flavours (Tanner & Zanier, 1978). They include nitrogen-containing constituents such as 2-methyl-3-ethylthiopyrazine and 2,5-dimethylpyrazine.
Fairly recently 2-amino-acetophenone was identified in German Vitis vinifera wines as the constituent causing an atypical maturation character related to a soapy, acacia flower or naphthalene-type flavour (Rapp, Versini & Ullemeyer, 1993). It has a threshold value of 0,7 to 1,0 g/l in wine. This component is said to occur in higher concentrations in wines made from grapes subjected to drought/stress conditions. This component is also responsible for the well-known, so-called foxy taint of American non-Vitis vinifera hybrids (Acree et al., 1990).
Furthermore, 2-amino-acetophenone is closely related to methyl anthranilate which is a common occurrence in the above hybrid wines (Nelson et al., 1977). Recently, it was also proven that methyl and ethyl anthranilate contribute to the fruity, berry-like, sweet flavours of Pinot noir wines and are therefore considered to be important aroma impact constituents of this cultivar (Moio & Etivant, 1995).
Thiazoles are ring compounds containing both sulphur and nitrogen atoms. The component, 5-(2-hydroxyethyl)-4-methylthiazole was observed in grape and wine distillates and causes a medicinal, peanut-like aroma (Jackson, 1994).
It is obvious that although nitrogen-containing flavour constituents make up a very small percentage of the total number of volatile constituents, they can play a particularly prominent role, both as positive cultivar-typical flavour constituents, but also as negative malodours. From a flavour/quality point of view, nitrogen-containing constituents are therefore of great importance in grapes and wine.
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