Dukasaur wrote:I was thinking that maybe they gain sweetness with time.
That's basically the gist of it. You measure sugar concentration in grapes throughout the season in degrees brix (°Bx), and the acid level in pH. We also measure Titratable acidity (or total acidity) which is actually kind of more relevant than pH but harder to understand/explain so I'm going to leave that out of this post. The other thing people test for alot in red grapes is phenolics but that has nothing to do with sugar, it's really just about color in the skins.
As the growing season moves along, slowly the acid levels go up in pH terms (the grapes become less acidic over time), and the sugar concentration goes up slowly (the grapes become slightly less bitter/tart) and then spike quite a bit in the last month or two of the season (the grapes become really sweet). For white wines, you are usually picking grapes with a pH around 3.5-3.9 and a brix level of around 24-25 °Bx, and red grapes people tend to push sometimes all the way up to 28-29 °Bx if the area has a growing season long enough to allow for that. The reason for the big spike in the last month or so is less about the plant creating a ton more sugar, but also about the grapes losing some of their water weight which leaves the grapes with a greater concentration of sugar.
If you multiply °Bx number by .59 you have a rough idea of the alcohol in the wine made from these grapes (assuming you ferment all the sugar and don't leave some sugar in the wine). So 25°Bx grapes would produce a 14.8% alcohol wine. It's a little more complicated than that but that's the basic idea.
If you Duku ate wild grapes in like may/June/july they are gonna taste horrible. If you went back to that same plant and taste them in late Aug/Sept they are going to taste much better. If no one picked them you could come back in late October and you would basically have raisins still hanging on clusters which are also super tasty but almost too sweet.
Icewine which you canadians are leaders in, are made from incredibly sweet grapes that are left on the vine way too long for normal winemaking. They have little to no water weight and brix numbers that are off the charts. Because of their lack of water you get almost nothing out of a grape when you press it so it requires massive amounts of fruit to create a single bottle and it's both very high in alcohol and very sweet since there is more sugar than the yeast can convert to alcohol before the yeast die off.