Compuestos azufrados volátiles en vinos - Fundación para la ...

y lowering higher alcohols production (Rapp andVersini 1991). However, based on the trend shown inFigure 3, this advice has to be taken cautiously as itmight apply only to fermentations with initial YAN inthe range included in the descending part of higheralcohols production pattern, i.e. YAN >200mg N/L.The production of fatty acids ethyl esters, as well as ofacetate esters, including ethyl acetate, is generally increasedwhen DAP is added to the juice prior to alcoholicfermentation (Figure 3). This can have interestingimplications for wine fl avour as fatty acids ethyl estersand acetates are generally responsible for the fruitycharacter of wine (Guth and Sies 2002). However,ethyl acetate, one of the dominant yeast-derived volatilemetabolites, when present at very high concentrations,can give unwanted sensory characteristics,often described with terms like nail lacquer/solventand volatile acidity. Branched chain esters are, from aquantitative point of view, the less abundant volatilesproduced during fermentation. Although their contributionto wine fl avour has still to be clarifi ed, tentativeevidence is available in the literature for thesecompounds to be important contributors to the redberry fruit character of some red wines (Diaz- Marotoet al. 2005). Their concentration appears to decreasewith increased DAP additions, however.The existence of a variety of diff erent responses forthe various groups of yeastderived volatile compoundsto DAP supplementation arises from the factthat each group of volatiles is derived from a diff erentmetabolic pathway, each of which respond differentlyto DAP supplementation. However, from a practicalpoint of view, understanding the potential of DAPsupplementation as a tool to modulate wine sensorycharacteristics cannot be based simply on compositionaldata. The various volatiles or groups of volatilesillustrated in Figure 3 occur in wine over an extremelybroad range of concentrations. However, this fi guredoes not represent the actual quantitative relationshipbetween diff erent chemical species. For example,higher alcohols, characterised by herbaceous, fusellikeodours, typically occur in concentrations that canbe up to 400 times higher than ethyl fatty acid esters,Odour Activity Value120100806040200Isoamyl alcoholEthyl octanoateIsoamyl acetateEthyl 3-methylbutanoateEthyl acetate100 200 300 400 500YANFigure 4. Theoretical relationship between initialYAN concentration and Odour Activity Values ofselected yeast-derived aroma compounds.Seminario TécnicoCompuestos azufrados volátiles en vinoscharacterised by fruit-like odours. Nevertheless, relativelysmall variations in the concentration of ethyl fattyacid esters, such as those introduced by variations inYAN content, are more likely to aff ect the aroma ofwine than if proportionally similar variations wouldoccur for higher alcohols. This is due to the fact thatsome of the possible sensory modifi cations associatedwith changes in the concentration of specifi caroma compounds depend, among other factors, onthe ability of that aroma compound to generate anolfactory stimulus at a given concentration.This complex relationship is often simplifi ed bymeans of the concept of odour threshold, defi ned asthe minimum concentration at which a given compoundcan be detected by the sense of smell (Guth1997). This is referred to as the odour activity value(OAV). Some of the fermentation-derived volatilecompounds, such as esters, that are generally associatedwith the fruity character of wine, are extremelypowerful odourants (i.e. have a very low odour threshold)and can, therefore, impart specifi c sensory attributeseven when present in low concentrations. Onthe contrary, compounds like higher alcohols possessa much higher odour threshold and, therefore, arelikely to generate variations in the aroma profi le ofa wine only when their concentration varies to a verylarge extent. In Figure 4, a theoretical relationship betweenthe OAV of selected volatile compounds, belongingto the chemical classes of Figure 3, and DAP supplementationis illustrated. It appears clear then thatthe range of variations potentially introduced by DAPin the concentration of acetates and fatty acid ethylesters (isoamyl acetate and ethyl octanoate are usedas reference compounds for these two classes) canhave a dramatic impact on the volatile character ofwine, whereas variations in compounds such as higheralcohols (isoamyl alcohol), although quantitativelyextremely large, are likely to have a limited impact.Although it has to be stressed that OAVs only give aprojection of the potential of a given compound tocontribute to the overall aroma of a wine, the trendsshown in Figure 4 provide a good indication of whichone of the compositional changes associated withDAP supplementation is likely to have a greater impacton wine aroma.IMPLICATIONS OF NITROGEN FOR WINEFERMENTATIONS3.1 Implications of nitrogen for white winefermentationsInterestingly, the results obtained in various winemakingtrials conducted at the AWRI with sub-optimalYAN juices have indicated that, under typical winemakingconditions, DAP supplementation is an extremelypowerful tool for modulating the production of esterswhich, based on the previous discussion, are probablyTextos asociados67