· The appearance of ,, the aroma of ,, the taste and body of , wine come mainly from four sources:
- · Primary compounds originating from the , grape formed in the plant cell.
- · Secondary compounds derived from the , grapes modified by the treatments applied.
- , compounds products of alcoholic fermentation formed by yeasts during fermentation.
- Properties related to the time formed during the maturation of the wine (Rapp and Versini 1991).
Alcoholic fermentation represents the main process in which the active aromatic compounds develop as compared to wine the aroma of grape juice/must is relatively low (Rapp 1988, Jackson 2000, Lambrechts and Pretorius 2000). The development of the aroma during alcoholic fermentation has not yet been fully clarified.
Three main pathways for the development of the aroma during fermentation are recognized. * Some grape-origin compounds remain essentially chemically intact ,; others are metabolised to give aromatically active , metabolites; and others are subjected to intracellular or extracellular , hydrolysis or biotransformation reactions that modify their aromatic properties.
· The production of ,, the release and modification of volatile aromatic compounds derived from , grapes, the processing during the production of ,, the breakdown and modification of non-volatile compounds from yeasts during fermentation have an important role in shaping the aromatic and taste profile of wine (Schreier 1979, Rapp and Mandery 1986, Etievant 1991, Guth 1997, Lambrechts and Pretorius 2000, Cole and Noble 2003, Smyth et al. 2005).
· During the growth phase of , yeasts the metabolism of sugars leads to the formation of a variety of volatile , compounds including the superior , alcohols of , fatty acids of , esters of , carbonyl compounds of sulphur compounds and several organic , acids many of which are involved in the formation of the aroma and taste of wine (Rapp and Versini 1991, Guth and Sies 2002, Francis and Newton 2005, Howell et al. 2005, Smyth et al. 2005, Swiegers et al. 2005).
· Nitrogen compounds , also contribute to the formation of some of these compounds , especially upper alcohols and , esters and additionally regulate the formation of other volatile compounds , such as hydrogen sulfide , thiols/mercaptans and monoterpenes (Henschke and Jiranek 1993, Rapp and Versini 1996, Albers et al. 1998).
· In addition to the production of primary , metabolites, yeasts interact with a variety of non-volatile , tasteless precursors derived from the , grape which are of considerable organoleptic significance. · Glycosides and conjugated cysteine , when hydrolyzed by the enzyme activity of the yeasts can significantly contribute to the varietal character of the wine (Strauss et al. 1986, Francis et al. 1992, 1996a,b, Sefton et al. 1993, Francis and Newton 2005, Swiegers et al. 2005).
* Examples of aromatic compounds derived from grapes are hexyl derivatives that release fruit and flower aromas and some monoterpenes , norisoprenoids (P-Damasquinone) and thiols (Etievant 1991, Moio and Etievant 1995, Cole and Noble 2003, Swiegers and Pretorius 2005).
· Some phenolic compounds in the grapes represent another group of compounds which, by interacting with the yeasts, modify the organoleptic influence they exert on the characteristics of , wine, mainly on colour and body (Mazza 1995, Bartowsky et al. 2004, Eglinton et al. 2004 ,Dillon et al. 2005, Herderich and Smith 2005).
· The production of ,, the release and modification of volatile aromatic compounds derived from , grapes, the processing during the production of ,, the breakdown and modification of non-volatile compounds from yeasts during fermentation have an important role in shaping the aromatic and taste profile of wine (Schreier 1979, Rapp and Mandery 1986, Etievant 1991, Guth 1997, Lambrechts and Pretorius 2000, Cole and Noble 2003, Smyth et al. 2005).
· So , any factor affecting the growth and metabolism of , yeasts such as , nitrogen can have an effect on the composition of wine and its organoleptic characteristics.
Source: Bell S-J and P.A. Henschke , 2005. Implications of nitrogen nutrition for grapes , fermentation and wine. Australian Journal of Grape and Wine Research 11 , 242-295
