There’s a well known phenomenon in the world of wine that so far has been unexplained. It’s the way that as sweet wines age, they taste less sweet. If you get a 30 year old Sauternes, it isn’t nearly as sweet tasting as a younger wine, even if both started out with the same level of sugar to start with. This dissapearing sweetness is described as the wine ‘eating sugar’.
So what could be happening? I don’t know of any studies that have tracked the actual sugar level in a particular wine over time, to see whether the sugar is in some way disappearing. It could be that there is an as-yet undiscovered reaction taking place consuming the sugar. But I don’t think this is what is happening here.
I have an alternative explanation: the effect of volatile compounds on the perception of sweetness. It’s well known already that when we taste a wine, its sweetness or dryness isn’t just dependent on the level of sugar. Acidity has an important role to play. Champagnes with 10 g/litre of sugar and 10 g/litre of acidity taste bone dry. Put that level of sugar into a wine with less acidity and it will taste quite sweet.
So it should come as no surprise that the smells we detect when we taste a wine will influence how sweet we find that wine. In particular, fruity aromas enhance the perception of sweetness. We can’t smell sweetness, but fruity aromas smell sweet. Why? Because we have learned to associate them with sweetness.
A young Sauternes, for example, usually has very high levels of fruity aromas, typically apricot, passionfruit and peach. These in part come from volatile thiols such as 3MH and 3MHA: botrytised wines usually have sky high levels of these. With time in bottle, these fruity aromas diminish. Smell an old Sauternes: it smells much less fruity than a young one, and usually has complex savoury aromas that have developed with time. It isn’t surprising, therefore, that if the wine smells less sweet, it should also taste less sweet.
The brain is computing the sensation of sweetness not only from the sugar level, but also from the smell of the wine. Even if the old wine still has the same level of sugar, the perception we have of its sweetness has changed.
Support for this idea comes from a couple of recent studies on tomatoes and strawberries, carried out by Linda Bartoshuk and her colleagues at Florida University. They looked at the composition of a range of tomato varieties, testing the levels of sugar and also a group of volatile compounds. They then got a sensory panel to taste these tomatoes, rating them for a range of attributes, including sweetness. They then looked at which compounds contributed to this perception of sweetness: it turned out not to not only be sugar, but also a group of seven volatiles.
For example, one variety had 45 g/l of sugar and was given a score of 13 on the perceived sweetness scale, while another had less sugar (just under 40 g/litre) but got a score of 25. It got this big score because it had about twice the level of a group of six volatiles that were correlated strongly with sweetness.
In the strawberry work, the researchers found 24 volatile compounds that showed significant correlations with perceived sweetness intensity, independent of glucose or fructose levels, and 20 that did this independent of sucrose concentration. Of these, six altered sweetness perception independent of all three sugars.
What is happening here? There are two ways in which we smell something. The first is through the nose, and it’s called orthonasal olfaction. The second is by aromas entering the nose round the back, from the mouth, which is called retronasal olfaction. The brain deals with these two types of smell differently. ‘Taste’ is computed from the taste sensations plus the retronasal olfaction, which are processed together. Sweet smells are likely to result from previous pairings of volatiles and sweet tastes, such as the smell of strawberries, peaches, vanilla and caramel.
So, as a sweet wine ages its smell changes. And even if the sugar level stays the same, the perception of sweetness will change. It’s likely that this is the explanation for wines that ‘eat sugar’ as they age, although I can’t rule out the possibility that sugar levels are also declining in some mysterious way through chemical reactions.