What is the definition of “sweet wine”?

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Dear Dr. Vinny,
What is the definition of “sweet wine”?
—Barb B., Edmond, Okla.
Dear Barb,
There is a technical definition to “sweet wine”: If a wine has more than 30 grams per liter of residual sugar in it, it’s considered “sweet.” If there are less than 10 grams per liter, it’s considered “dry,” or the opposite of sweet in the world of wine. Anything in between is considered “off-dry.”
Where does that sugar come from? The grapes. The riper the grapes, the more sugar they have. Remember that fermentation converts that sugar into alcohol with the help of yeast. It’s not unusual for trace amounts of sugar to be left after the fermentation, and some wines are made in styles that purposefully leave some extra sugar behind.
That’s the technical definition, and not very useful to most regular folks. When wine folks talk about “sweet” wines, I think they are usually referring to dessert wines, or wines that are made in a deliberately candied style, like some very popular wines often found in supermarkets. That extra sugar can add a richer texture and smooth out the rough edges of an inexpensive wine.
Keep in mind that everyone’s tolerance and perception of sweetness is different, and while I may like cotton candy, you might find it overly sweet. Sometimes wines might taste sweet but are technically dry. The wine’s ripe fruit flavors and the relationship of the flavors to the alcohol, glycerin, tannins and acidity will influence how you perceive it.
—Dr. Vinny