Residual sugar is a phrase that crops up time and time again when discussing the flavour profile and style of wine. But how many people really know what it means? Or what purpose it serves? And haven’t we been told not to consume sugar anyway?

First, don’t panic. No one is spooning bagfuls of refined white sugar into your wine. The residual sugar refers to the natural grape sugars left over after fermentation has stopped. When the juice is first pressed, it is intensely sweet – and this sweetness is essential. The fructose provides fuel for the yeast to do its job and slowly that sugar is converted into alcohol.

Fermentation is key

The point at which fermentation stops can largely determine the sweetness of the wine, but that may not be the reason why it ceases at that point. The yeast used in winemaking has an alcohol toxicity level – different strains can tolerate different strengths of booze. A weaker strain will die off before all the sugar is used, whereas more robust strains will happily carry on until almost all the sugar is used. If a winemaker is looking for a flavour profile with a certain level of sweetness, or a lower alcohol wine, they can chill the fermenting wine which artificially stops the process early, leaving more residual sugar behind.

Sticky ends

Of course, some wines are naturally very sweet, like dessert wines. These grapes, such as muscat, are naturally very sweet and are often partially dried before the process begins to increase the sugar to water ratio. The noble rot Botrytis cinerea will suck the water out of grapes, making them perfect for turning into a sticky wine.

In the balance

Residual sugar is important because it balances out acidity. A highly acidic wine, like riesling, needs residual sugar to lift the flavour profile and lighten the mouthfeel. It’s a delicate balancing act managed by the winemaker, but essential to bringing you a full and rounded flavour. The right level of residual sugar will also help a wine age gracefully in the cellar.