Glycosides were previously thought to be flavourless, needing the action of enzymes during fermentation or slow chemical reactions during wine ageing to release and express their flavour. This project has established that the presence of glycosides in wine and their concomitant breakdown during tasting can boost desirable ‘fruity’ and ‘floral’ lingering flavour attributes. As a persistent aftertaste is a hallmark of quality wines, flavour release from glycosides may be a key factor differentiating between good and excellent wines.
In this project, experiments were conducted with sensory panels tasting glycosides from white wines and parallel measurement of flavour compounds in saliva or in the mouth. The results confirmed that enzymes in the saliva act like a key to a locked door, releasing a wave of additional flavour that can be perceived over the time after swallowing, creating a positive long-lasting fruity flavour sensation.
Adding purified glycosides to a juice or wine resulted in increased flavour with no negative characteristics; it was also established that white grape skins are a readily available source for extraction of glycosides which can be used as a natural flavour boost that has potential for an easily controlled new way of enhancing a wine’s sensory properties. Finally, knowledge about the profile and/or concentrations of glycosides in grapes or a wine could be used by winemakers as a quality or style indicator.
This project demonstrated the potential of non-volatile glycosides as flavour precursors during wine consumption. The studies on floral varieties such as Riesling and Gewurztraminer showed that there is a surprising ability of in-mouth enzymes, most likely from salivary bacteria, to quickly liberate volatile aroma compounds from their bound form during wine drinking, enhancing flavour and contributing to a lingering aftertaste. The quantity of flavour release and/or retronasal perception of flavour released by this mechanism seems to be fairly variable across individuals, suggesting one reason for variation in people’s sensory perception of wine and, potentially, their preferences.
To further corroborate this work, a winemaking experiment was completed to explore various methods winemakers might use to intensify the contribution of these precursors in their wines. Several sets of grape juices were treated to increase their level of glycosides, and following fermentation, sensory and chemical analyses were completed on the finished wines, as well as wines with glycosides added prior to bottling. The addition of glycosides had a major effect on wine aroma and flavour, enhancing ‘fruity’/‘floral’ attributes with no effect on bitterness or astringency. Chemical data showed an increase in key aroma compounds as a result of glycoside addition, as well as higher levels of intact glycosides, acting as flavour precursors and contributing to an enhanced persistence of ‘fruity’/‘floral’ flavour.
The demonstrated benefit of increasing the amount of glycosides present in wines to boost desirable flavour and flavour persistence opens up the option for wine producers to make additions of glycosides isolated from grape skins, or indeed changing vineyard management practices to increase glycoside concentrations. Further studies of the practical application of glycoside preparations from grapes in a production setting should be completed, as well as work on other commercially important grape varieties and investigations of other precursor classes, notably amino acid conjugates of sulfur aroma compounds.