Reaching Your Nose

Glycosides are small molecular weight compounds that have sugars attached to them. Grape berries, like many, if not all plants contain so-called secondary plant metabolites that often, but not always, accumulate as glycosides in the vacuoles of plant cells. Many famous flavour compounds, such as the monoterpenoids geraniol and linalool (muscat flavour of muscat grapes), certain ketones of raspberry (raspberry flavour) and benzaldehyde of almonds (nutty flavour), accumulate both as aglycones (ie. without sugars) and as glycosides. In many cases, but most likely not always, the glycosides have reduced volatility, due to the higher water solubility imparted by the sugar component, than the corresponding aglycones. With a lowered volatility, the chances that the glycosides will reach your olfactory system in your nose, either through your mouth or through smelling, are less than for the corresponding aglycones. Therefore, most often, the glycosides are regarded as “flavourless”, whilst the aglycones, if they indeed are capable of provoking a stimluation of your olfactory system, have a much better chance of inducing flavour perception.

Grapes contain hundreds of potential flavour-active compounds.

In grapes, several hundred different potential flavour compounds (ie. secondary metabolites) accumulate as both glycosides and aglycones. It is also possible (although I don’t know of any study clarifying this) that some major flavour active compounds present in grape berries are present as aglycones only.