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.
Factors contributing to individual differences in sensory response to glycosides
Forty-one people were assessed for their ability to perceive flavour from two types of glycosides, tasted at a moderate concentration. The proportion of glycosides broken down by each person’s saliva was also assessed chemically using GC-MS and LC-MS methods. While there was a large difference among individuals in the ability of their saliva to release volatiles from the glycoside precursors, surprisingly the percentage released did not correlate with the individual’s flavour perception. AWRI bioscientists completed a detailed metagenomics study and found that there were two distinct groups of people with different bacterial communities. The group with low ability to release glycosides had a different bacterial community from the group who could release a high proportion of glycosides. However, the ability to perceive flavour from the precursors did not relate to the groups identified from the metagenomic study, nor the saliva hydrolysis data.
The last pieces of the puzzle – the sensitivity of each individual to the flavour of the glycosides and their sensitivity to the free volatile compound – were determined using a large series of sensory detection tests at low concentrations with 24 of the test subjects. The sensory detection values for each person were found to relate very well to the classification of people as glycoside ‘tasters’ or ‘non-tasters’.
Thus it seems that although individuals can differ in the innate ability of their saliva to break down glycosides and release flavour, it is the sensitivity of the olfactory system that is the true predictor of whether someone can easily taste flavour from these compounds. Some individuals’ saliva has a low ability to release the flavour from the precursors, but they can nevertheless taste the glycosides, as enough flavour can be released. The saliva hydrolysis was not the limiting factor for the people tested. These results show that most people can perceive flavour from glycosides if they have the olfactory ability to perceive the released flavour compound, providing evidence that in-mouth release of flavour during wine consumption from this source is a common experience, rather than being limited to only a fraction of people.
Understanding flavour precursors in winemaking
To better understand the types of flavour precursors present in grape skin, numerous marc samples from different varieties were extracted and analysed. Muscat varieties showed the highest concentration of bound monoterpenes, with Muscat Gordo the highest by two-fold. Viognier extract showed the highest release of 2-phenylethanol (‘floral’, ‘rose’) after acid hydrolysis, showing it may be useful for increasing specific attributes in wine.
Studies of glycoside stability over time were conducted, with different sugar structures under different conditions. This is relevant to consideration of the timing of possible glycoside additions in wine production at different pH values. No differences in degradation were seen under the different conditions studied. As such, optimisation of the glycoside profile (type of sugar) in the source material is not required, when considering producing marc extracts for addition to wine. With respect to volatile evolution, lower pH wines resulted in higher concentrations of rearranged monoterpenes, with the likelihood of a shorter shelf-life of wines made at lower pH with enhanced glycosides. More extensive studies will be conducted.