Graceful Age

Following the end of all kinds of fermentation, the wine will mature in wood, tank and bottle. During this process, hydrolysis will still continue in the acid environment of the wine. A lot of famous and not so famous wine lovers have time and time again stated that a wine improves with ageing.

(Personally, I disagree., Never have I tasted a wine that has improved with ageing beyond a couple of years, but then again, I have only on few occasions tasted really “great” wines when young and again after ageing. Storage conditions may have something to do with it, but personal taste also.)

If the characters of old wine, or at least of one to three year old wines, owes at least part of it’s flavor generating profile to the hydrolysis of glycosides, then one can truly see glycosylation as a kind of “storage reserve”. Although I think grapevines are great as they are, the potential for manipulation in the form of increased glycosylation may merit attention. The different chemistry of different forms of glycosides also merit attention in this respect. If any readers are still reading, I hope we haven’t turned you off wine. Instead, next time you sip a great wine, at least ponder about the biochemical background that at least in part may be responsible for what you are enjoying.

(Patrik Jones, 10th of September, 2001. Many of these ideas and all facts originate in the many papers produced by Pat Williams and his co-workers at the AWRI. For a more detailed scientific investigation into this topic start by having a look at some of their papers.)

Fast or Slow?

In the making of wine, the must and resultant wine is essentially sparged with carbon dioxide for a considerable amount of time due to alcoholic and malolactic fermentation. This sparging, I believe, results in a great loss of potential flavour (Another source of flavour loss may also be chemical transformations as mentioned above, but here in a negative direction!). Glycosides may very well be spared from this evaporative and/or chemical loss and can therefore be regarded as a protection from flavour-loss (and by now we are well and truely into not much more than mere speculation…). One possibility, hence, given that glycoside hydrolysis will proceed as soon as the berries are crushed, would be that a rapid alcoholic fermentation, followed by a very mild and slow malolactic fermetation therefore may yield a maximum pool of potential flavour. Many winemakers regard slow fermentation as beneficial since the slower the fermentation, the lesser the evaporative loss of volatile compounds may be. However, a fine line between fast (ie. finish the ferment before most of the glycosides have been hydrolysed) and slow (reduce the rate of carbon dioxide production and temperature to minimize the evaporative loss of volatiles) may be worthwhile to consider.

Crush and Squash

When the grapes are crushed the glycosides will enter the water phase of the must (read wine!), and it is tempting to speculate that some of the aglycons may be less soluble than their glycosylated equivalents. Nevertheless, due to the acid environment of the must and the presence of a rich microbiological fauna (bacteria and yeast) that contain glycoside-hydrolyzing enzymes, some, but not all glycosides will be cleaved and yield flavour-active aglycones. This relase of flavour may very well be responsible for the generation of the flavour that we recognize in wine, since the grape berries and freshly crushed berries seldom taste the same as the resultant wine. Another source for the transformation from a rather boring crushed berry flavour (crushed grapes often have little interesting flavour) to some of the marvellous characters associated with wines of differing origin and cultivars, may be attributed to the chemical and/or microbiologically induced chemical transformation of grape-derived potential flavour compounds. Thirdly, the presence of alcohol may also enhance the volatility of certain compounds.

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.

Glycosides & Wine

The flavour of wine is a major part of what makes wine interesting. Now, what are glycosides and what do they have to do with wine flavour? This short story is an attempt to give curious followers of arakoon wines an insight into some of the biochemical aspects relating to the flavour of wine made from Vitis vinifera (The grape species used to produce wine grapes).