Sugar in wine
First of all we need to define the question itself: what do we actually mean when talking about sugar in wine?
It can be the sweetness a producer adds: either early in the process to be able to ferment the grapes into wine at all (that is, if the grapes are so unripe that there is not enough sugar in them for the yeast to have something to eat, and thus to be able to form alcohol), or as an addition after the wine is finished to mitigate an excessively high acidity. An example of the latter can be found in, for example, Champagne, where a dry wine can have up to 12 g of sugar per liter. But it is not simply white, refined sugar that is poured into the bottle, but usually unfermented grape juice from the same grapes as the fermented wine itself. "Sugar in wine" can also refer to the types of wine that have a natural so-called residual sweetness: due to, for example, grape variety, climate, late harvest, noble rot or that the fermentation process has somehow stopped before all the sugar has been eaten by the yeast.
Through history, some parts of Europe have been so cold during the months when wine is typically fermented that the yeast simply died from the cold, leaving behind a residual sweetness in the wine. Riesling from the Mosel is a classic example of this, where today instead this historical flavor profile is recreated by harvesting the wines later, when the grapes have a higher sugar content, alternatively by engaging cooling elements in the fermenter, in order to stop the fermentation in a controlled manner.
Historically speaking, Barolo wines are also a type of wine with residual sweetness, here too it was the cooler climate that stopped the fermentation process. Amarone is a modern concoction, the result of a cask of Recioto being forgotten and left to ferment completely. The final product was thus drier, but since the dried grapes contained so little liquid, the sugar content was still high enough for the yeast to have time to eat itself to death before the sugar ran out completely. And this is the simple principle behind Amarone to this day.
Dessert wines such as Tokaijer, Sauternes, Barsac, Coteaux du Layon, Beerenauslese, Icewine and so on are also wines made from grapes that have such a high sugar content when they are plucked and pressed that the final product can reach residual sugar levels of up to 200 g/liter. The sweetness is balanced by the acidity of the grapes, which is why many of the examples above are made from grapes with naturally high acidity levels; such as Riesling, Furmint, Sauvignon Blanc and Sémillon.
Examples such as Port, Madeira and Pedro Ximenez are wines where the producer has stopped the fermentation with grape spirits (usually with an alcohol level of 70 percent, and the amount controlled in the region's regulations). Residual sweetness is often very high, up to 350 g/liter, but is balanced out by the higher alcohol content.
In general, it can be said that dry wines (and this applies to both red, white and rosé wines) have a residual sweetness of approximately 1-3 grams per liter, with the exception of wines from Valpolicella which differ somewhat. A very well-produced, high-quality Amarone contains about 5-8 g/liter, while an inferior variety can contain over 20 g/liter; the higher sugar content to some extent masks the lower quality and makes the wine "easy to drink". Unfortunately, the cheapest Amarone-like wines rarely contain anything other than the aforementioned refined, plain white sugar!
If, when you are at a restaurant, you ask your waiter which wine is the driest, because you are afraid of too much sugar, you can reassure yourself with the knowledge that the sugar that is naturally found in wine grapes is significantly lower than that found in in the grapes you buy to eat, table grapes. And probably lower than in most things you find on the menu! A dry wine can also be perceived as "sweeter" than it is, as many aromas and aromatic compounds are associated with sweetness and thus trick us into thinking that the sugar content is higher. The tropical fruitiness of a warm climate Sauvignon Blanc, for example, tells the brain that it is ingesting sugar, with aromas of melon and yellow fruit as sweet messengers.
Sugar in wine is a hot potato and only factual information can change the common man's erroneous view of wine with residual sweetness; because what one tends to forget is that the alcohol itself in wine is more calorie-rich than the paltry grams of sugar per liter!