If you drink aged dessert wines you’ve probably noticed that as a wine ages it seems to taste less sweet. Old Tawny ports are not as sweet as a young ruby, German Auslese or French Sauterne lose some of the over sweetness they have when young. Does the aging process reduce sugar levels? Apparently there is no science to answer the question.
Wine science writer Jamie Goode has an interesting hypothesis about why this might be the case–although the sugar levels remain the same in the aged wine we perceive less sugar because the mind plays tricks on us. We have learned to associate sugar with fruit and so as the primary fruit flavors diminish with age and are replaced with nut and oxidized notes we interpret that loss of fruit as a loss of sweetness. And there is some science to support this hypothesis:
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.
It may be that the aging process modifies the sugar molecules just as it seems to modify acid molecules; perhaps perceived sweetness is a combination of both processes. But there are good evolutionary reasons why we might associate fruit aromas with sugar since doing so helped supply nutritional needs when our forbears were traipsing about the East African plains.
Once again it appears taste is not just a sensation but an idea.