Flavour precursors: contribution to wine aroma, in-mouth sensory properties and flavour release
A critical aspect of wine quality from a consumer perspective is the overall impression of the wine in the mouth. Wine flavour is driven by volatile compounds (including ethanol), and also by non-volatiles such as sugars, salts, acids and phenolic compounds including tannins, polysaccharides and proteins. Overall flavour is also potentially derived from non-volatile precursor compounds, such as glycosides (compounds joined to sugars) or amino acid conjugates that can be broken down in-mouth, thereby releasing volatile aromas. The individual wine components alone are not sufficient to achieve a desirable in-mouth perception; this can only be obtained through the interaction of multiple components.
This project has the overall aim of establishing the extent to which wine flavour is due to precursor compounds releasing flavour compounds in-mouth. The persistence and intensity of fruit flavour are very important aspects of wine quality, and grape-derived compounds that are non-volatile, but which can be degraded during the tasting process to produce flavour, are a potentially important source of desirable flavour that has not been previously investigated.
Volatile aroma compounds are released from grape-derived precursors by saliva
Experiments showed that detectable monoterpenes are released in vitro through interaction of saliva with glycosides. Up to approximately 80% release of monoterpenes was achieved during incubation of glycosides isolated from Gewurztraminer wine with saliva. When glycosides were tasted in-mouth, monoterpenes were also detected using an expired air trapping method. The ability of an individual’s saliva to break down the glycosides appeared to be quite consistent but varied across different individuals, in line with previous work on volatile phenol glycosides and saliva.
Monoterpenes can contribute important flavour from in-mouth release
Using a large panel of assessors, the release of flavour from a synthetic geraniol glucoside and from glycosides isolated from Gewurztraminer wine in a water system was confirmed. Both glycoside samples were rated significantly higher than the control in ‘fruity’/’floral’ flavour.
In earlier studies it was observed that not all individuals can perceive a flavour upon tasting these glycosides. In this study, approximately half of the judges could consistently and repeatably discriminate between the samples containing monoterpene glycosides and water. When the same judges were tested with a synthetic guaiacol glucoside, close to 65% could consistently perceive a ‘smoky’ flavour. This highlights the possibility that flavour released in-mouth from glycosides may contribute to differences in perception of wine between individual consumers.
The relative effects of glycoconjugates and free volatiles in Riesling and Gewurztraminer were assessed in a more challenging, but more realistic, model wine system at wine-like concentrations using a timeintensity methodology. The panel consisted of a screened group of assessors who had previously demonstrated that they could perceive flavour from these compounds in the water system described above. When combinations of the glycoconjugates and volatiles were tested at concentrations closely comparable to those found in Riesling and Gewurztraminer wines, monoterpene glycosides were found to enhance the duration and intensity of the perceived flavour. A third of the judges were most responsive to the flavour from the glycosides, and for this subgroup, the glycosides isolated from Riesling contributed significant flavour, with the combination of Riesling volatile aroma compounds and the precursors giving the longest duration of aftertaste compared to the volatiles alone. This result means that at wine-like concentrations, with the influence of ethanol and wine pH, release of monoterpenes from glycoside precursors is relatively subtle, but likely to be an important source of flavour and flavour persistence for varieties such as Riesling. It may be that the effect is larger for wines of lower alcohol or higher pH, and more pronounced for some consumers.