Was a question posed to us today so here’s what we came up with as an answer. Residual sugar seems like an obvious concept. Residual sugar. Sweet stuff, left over. In wine. But like many concepts in the wine world, it’s not that simple.Yeast eat sugar to make alcohol. This much we all know. So why would the yeast stop before all the sugar is gone? It’s a good question, and it’s one that’s not always easy to answer. But one (or more) of the following is often the matter:

  • Too much sugar ­– and too much alcohol resulting from too much sugar. It’s kind of gross to think of it this way, but yeast will die from wallowing in the alcohol left over after they eat the sugars. Like people, different yeast strains have different alcohol tolerances, but most cop out around 14-18% alcohol by volume (ABV).
  • Fortification –  fortification is both a way to make and a way to deal with accidentally sweet wine. If you want to stop yeast from finishing off the sugar remaining in a wine mid-ferment, adding a bunch of distilled grape spirit (enough to bring the batch up to at least 18% ABV) is the easiest way to do the trick. Fortification is also an efficient way to keep an accidentally sweet wine from spoiling. With all of that sugar sitting around sweet wine is an easy target for spoilage microorganisms, but there are very few yeast or bacteria that can grow in 18% alcohol. This is how port and other sweet fortified wines are made.
  • Malnutrition – Probably the most common reason why yeast lose their mojo. Yeast need more than just sugar to survive, and if they run out of something else first—nitrogen and cell wall components are the most common limiting factors—they won’t finish the fermentation.
  • Something killed them – if a winemaker wants to create a sweet wine without fortifying it, he or she can filter the wine to remove the yeast and then optionally add preservatives to ensure that a few cells don’t multiply and restart the fermentation or contaminate the batch. In addition to sulfur dioxide, potassium sorbate or potassium benzoate is often added to keep yeasts and fungal spoilers at bay.

There are a loads of other causes of “stuck fermentations”—fermentations that stop before all of the sugar is gone when the wine isn’t intended to be sweet—but they’re quite technical and really boring. Why is residual sugar important, well residual sugars add to the sweetness of wines as perceived in the mouth by the tongue, and an old trick to make half decent plonk is to leave a small amount of residual sugars to mask the flaws in the wine. Now that is not to say that all wines with deliberate residual sugars is flawed some of our favourite wines are off dry style whites like Spatlese from Germany