Grape aroma substance

Monoterpenes, Ci3-norisoprenoids, and some benzenoid compounds are the most important grape aroma substances present in the pulp and skin of berries in both free and glycoside forms. Profiles of these compounds in the grape are mainly dependent on the variety, even if environmental variables and agricultural practices influence their contents (Marais et al., 1992). These compounds are transferred to the wine in winemaking and depend on the process used. 

The next step of the biotechnical sequence, yeast fermentation, is of the utmost importance to the chemistry of winemaking as well as to the formation of flavor substances. We have investigated this previously using 14C-tagged compounds (16). Amino acids, for example, enter the yeast fermentation with a quasi biochemical valence with regard to the formation of metabolic side products like alcohols and esters. In that respect, the composition of the fermentation substrate, the grape must, is highly important to the formation of aroma substances by yeasts. 

Dufour, C., Sauvaitre I. (2000). Interactions between anthocyanins and aroma substances in a model system. Effect on the flavor of grape-derived beverages. J. Agric. Food Chem., 48, 1784-1788. 

The skin is considered to be the principal source of aromatic substances, but the pulp does contain significant concentrations of these compounds. In certain muscat varieties, the must can contain up to two-thirds of the terpenol heterosides. The pulp is characterized in particular by the accumulation of a diverse variety of alcohols, aldehydes and esters which participate in grape aromas. 

Several hundred different chentical substances participate in grape aroma. In this complex mixture, hydrocarbides, alcohols, esters, aldehydes and other carbon-based compounds can be distinguished (Schreier et al 1976). 

Cyclopentenolones with a planar vicinal enol-oxo configuration are known to be powerful aroma active substances with distinct caramel notes. By methylation of the enolic function, this flavour impression is changed drastically to a sweet, mildew, and mouldy odour in the case of 2,5-dimethyl-4-methoxy-3-[2H]-furanone (2). This so-called mesifurane as well as pineapple ketone (1) were stereodifferentiated with modified cyclodextrin [103], Although (1) and (2) can be stereoanalyzed without any racemization, both compounds were detected in strawberries, pineapples, grapes and wines as racemates (Fig. 6.43). 

It is estimated that wines contain 400-600 flavour-active compounds in a total amount of 0.8-1.2 g/1. In wine terminology, aroma is the term used for the smell of young wines. The transformation of aroma during agingby chemical reactions leads to a wine s bouquet. Wine flavour is classified into primary - the bearer of which are substances present in grapes and musts - and secondary, the bearer of which are substances produced during fermentation. 

The ripe grape skin also contains considerable amounts of aromatic substances and aroma precursors. In certain muscat varieties, the skin can contain more than half of the free terpenols of the berry (Bayonove, 1993). Other chemical families of aromatic snbstances may also be contained in the skin. Finally, the skin is covered by epicuticu-lar wax, essentially constituted of oleanolic acid. 

The biochemical processes of maturation have traditionally been summarized by the transformation of a hard, acidic green grape into a soft, colored fruit rich in sugar and aromas. As already indicated, these transformation can only occur when the grape is attached to the rest of the plant. In this case, the increase in the concentration of a substance in the berry can be due to importation of this substance, on-location synthesis or water loss in the vegetal tissue. Conversely, its diminution can result from exportation, degradation or water gain in the tissue. 

Unfortunately, no simple methods currently exist that permit an aromatic substance maturation index. Tasting the grape remains, in this respect, the only available criterion for judgments but this does not estimate the subsequent revelation of other aromas. 

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