In the fourth part of our article series on alcoholic fermentation, Westgarth Wines wine specialist Maurizio Broggi discusses the dreaded ‘stuck’ fermentation, as well as actively stopping the fermentation process and the reasons behind doing so.

As mentioned in the previous article, if temperatures are too high or too low during fermentation this may slow the activity of the yeasts or even see them struggle to survive, causing fermentation to become sluggish or stuck.

The winemaker’s nightmare is a stuck fermentation. Fermentation may become sluggish and stop suddenly (stuck) while the sugars have not yet been completely converted into alcohol. Besides undesirable residual sugar in the wine, a stuck fermentation is at risk of spoilage from oxidation and bacteria and may lead to the production of foul-smelling hydrogen sulfides and high levels of volatile acidity.

A sluggish or stuck fermentation is typically caused by yeasts not being in the most ideal conditions to grow and ferment. This may occur due to a lack of sufficient nutrients (nitrogen) to sustain yeast reproduction and metabolism or extreme temperatures (too cold or too hot) for yeasts to grow and ferment efficiently. Interestingly, grapes that are harvested at very high levels of sugar can also be challenging to ferment as yeasts struggle when sugar levels are too high.

Restarting a stuck fermentation can be quite difficult, particularly towards the end, when there is little sugar left to ferment, however, there are some potential solutions. For red wines, aeration may help to reactivate yeasts. In white wines, warming the ferment may help to wake up yeasts.

Sometimes, the addition of more and/or different yeast strains may restart inactive yeasts again. The addition of nutrients and vitamins may promote yeast metabolism and activity. Often, the best solution is to progressively blend the stuck ferment into a vat where a healthy, active fermentation is taking place.

A healthy and successful fermentation naturally comes to an end when the wine is fundamentally dry and almost all the fermentable sugars have been consumed and converted into alcohol. This is the case for many dry table wines. In some cases, however, the winemaker may want to stop fermentation earlier to retain a certain amount of sugar in the wine. This can be achieved through the following actions:

• Fermentation can easily be stopped by lowering the temperature to or below 5°C (41°F) to deactivate yeasts.

• The carbon dioxide produced during fermentation can be retained to increase the vessel’s internal pressure to about seven atmospheres, at which point the activity of the yeast stops.

• Yeasts can also be physically removed by filtration or centrifugation.

• The addition of sulfur dioxide inhibits yeast activity.

• Finally, the fortification process adopted in fortified wine such as Port, stops the fermentation by adding alcohol to the fermenting must, killing the yeasts.

Want to read more? Take a look at some of our other blogs:

• • Alcoholic fermentation: yeasts
  • Alcoholic fermentation: vessels
  • Alcoholic fermentation: monitoring