Using glycosides and other flavour precursors for improved wine flavour
It has recently been demonstrated that purified non-volatile glycosides present a valuable, novel opportunity to increase flavour in wine. They can contribute flavour through enzymatic release of volatile aroma compounds during fermentation and winemaking, and by in-mouth breakdown during wine consumption, boosting flavour intensity and aftertaste.
This project will assess the effect of enhancement of grape glycosides in juices and wines. Characterisation of glycoside extracts from various grape varieties will be achieved using LC-MS and GC-MS analysis. Glycosides from different varieties will be extracted and partially purified, their stability investigated during fermentation and wine ageing, and their sensory impact investigated.
The concentration of residual precursors in wines following fermentation will also be measured, to determine whether these may act as quality markers. Additional flavour release systems such as thiol precursors will also be assessed, providing a complementary avenue for building additional flavour. A better understanding of the factors underlying individual variability in sensory response to in-mouth release of aroma from precursors will also be obtained.
Enhancing flavour precursors in winemaking
Isolating glycoside flavour precursors from grapes is relatively simple, although the conventional method also extracts less desirable phenolic compounds, which contribute bitterness. In a novel approach, phenolic-free glycosides were obtained from a substantial quantity of ripe Gewürztraminer grape skins, using straightforward pilot-scale methods, with the assistance of an engineering company. The phenol-free glycoside concentrate contained large amounts of bound monoterpenes, which are major contributors to ‘floral’, ‘citrus’ and ‘fruity’ flavours, especially in white wines. This material was added to a Chardonnay and a Riesling juice prior to fermentation, and to wine prior to bottling, in a winemaking experiment conducted during 2016 and 2017, with in-depth data analysis recently completed. Phenol-free glycosides were added at single strength (equivalent to the amount already in the juice) or double strength.
Chemical analysis showed that the glycoside material greatly boosted free monoterpenes in the finished Chardonnay and Riesling wines. The important fruit flavour-enhancing compound β-damascenone was also substantially increased by the glycoside addition. Whether the glycoside material was added before or after fermentation made only a small difference in final monoterpene concentration. There was only a negligible change in phenolic composition and colour due to the glycoside addition, indicating that the glycosides were indeed phenolic-free. Apart from the free volatile compounds, the intact glycosides in the wines were also enhanced, with the concentration of geraniol glycoside increased more than 60-fold.
Sensory analysis showed that there was a strong increase in ‘fruity’/‘floral’ aroma and flavour attributes due to the glycoside additions, with descriptors such as ‘rose’, ‘citrus’ and ‘confectionary’ used by the panel. Importantly, there was no significant difference in viscosity, acidity, astringency or bitterness caused by the additions, showing that the flavour-active glycosides only affected ‘fruity’ characters, with no detrimental effect on mouth-feel.
A group of white wine consumers in Sydney also tasted wines from the study and their preferences were recorded. A considerable number of the consumers liked the Riesling with a single addition of glycosides, while those wines with a double addition were not well appreciated, suggesting the flavour effect for these wines was too strong. A group of winemakers also assessed wines from the study and there was agreement that the wines made with added glycosides were greatly different from the controls.
One aspect of interest in this study was whether, for the wines with added glycosides, the release of flavour from glycosides during tasting might have been partly responsible for the observed increases in flavour and aftertaste. Some evidence for this was found by examining the sensory scores for those panellists who were tested as having the ability to perceive flavour from model systems containing added glycosides but no free volatiles. The individuals who could taste flavour from glycosides in the model system rated the wines with glycoside additions as more intense in ‘floral’ flavour and aftertaste than those who could not detect flavour from the model glycosides.
A study aimed at assessing factors contributing to individual differences in sensory response to glycosides was conducted. The capacity of people to detect the flavour of free volatiles and their glycoside counterparts was assessed with a group of 41 individuals. In addition, the ability of their saliva to complete glycoside hydrolysis was tested. Results showed that more than 80% of people tested had a significant flavour response to at least one of the glycosides tested, with a complex pattern of response to free volatiles and precursors. Data from saliva tests are currently being analysed.