In preparation for our Four Decades of Bordeaux tasting, I reread an excellent book on wine by Phillip Hills called ‘Appreciating Wine – The Flavour of Wines Explained’. Although slightly technical, it gives an excellent explanation of what factors contribute to the aromas and flavours found in wine and in particular the aging process.
Hills is the first to admit that there are plenty of gaps in the scientific (biochemical) understanding of exactly what is going on. For instance, we don’t know what gives cabernet sauvignon its characteristic black currant flavour. Apparently it’s not the same stuff that gives black currants themselves their taste.
It appears that the tannins derived from the grape skins and pips and contact with toasted oak barrels account for most of the wine’s flavours and aromas and for the way that these change with age. This is particularly true of cabernet sauvignon.
Tannins are part of the group of chemicals called phenols which are smelly and often referred to as aromatic compounds. Small changes in these complex polymers can result in very different aromatic characteristics. As a wine ages, this is exactly what happens. The phenolic compounds change and combine with each other, many eventually becoming so large and heavy that they come out of solution and form the sediment that you find in older bottles.
The effect of this is to remove some of the harsher bitter tastes that ‘young’ tannins often have and also to remove some of the colour of the wine. By implication some of the primary black currant and fruit flavours are also eventually lost, and secondary more subtle flavours are either formed or allowed to come out from under the shadow of the young wine’s strong cassis, graphite and wood notes. Taking this to its logical conclusion, at some point the aromatic compounds may just retreat entirely.
For a wine to have good aging potential it needs good acidity because the acids play a part in the polymerisation of the tannins. They also change and reduce. From experience, I’ve noticed that wines which are too old offer fleeting interest and then seem to quickly oxidise. Apparently this is because there is not enough acid left to resist oxidisation.