Residual sugar is one of those terms that is really very simple, once it’s explained to you. If you drink rieslings, this is the best phrase you can know for determining whether or not you’ll like a riesling, because residual sugar describes how sweet a wine is.
So what is it? Residual sugar is the sugar left in the wine after fermentation. During the fermentation process, the yeast converts all the sugar in the grape juice into alcohol; if fermentation is stopped (whether naturally or by the winemaker) before all the sugar has been converted, you are left with a lower-alcohol, higher-sugar wine. This is why sweeter wines tend to be lower in alcohol.
How is it measured? It’s measured in grams per liter, often expressed in a percentage. A wine that has 1 gram of sugar per 1 liter of wine will have a residual sugar of .1%.
Wines are categorized as dry, off-dry, or sweet. Dry wines will have a very, very small amount of sugar (think .01-.03% residual sugar) or none at all (in which case they are often called “bone dry”). Off-dry wines are 1%-5%ish residual sugar (you will learn quickly that the wine industry is rarely precise about terms like this). Off-dry rieslings are incredibly popular these days, and incredible tasty. Dessert wines will be anything more than 5%, although they can get very high.
So, if you are shopping for rieslings and you want to know how sweet it will be, take a look at the back label for one of these categories, or pop online to see what the technical notes from the winery’s website say about the residual sugar!