Stuck Fermentation

Definition - What does Stuck Fermentation mean?

Stuck fermentation is a winemaking issue that occurs during the fermentation process. During the fermentation process, yeast converts sugar to alcohol; stuck fermentation is when the yeast becomes dormant (or is used up) before the sugar is completely converted. Stuck fermentation is different from arrested fermentation - in the latter, the winemaker intentionally stops fermentation to get the desired sweetness and flavor profile in the wine. When a wine is "stuck", the result could be spoiled by bacteria or oxidization. Winemakers can fix the problem, but in order to save the wine they have to catch it immediately.

WineFrog explains Stuck Fermentation

When not treated, stuck fermentation results in wines that are semi-sweet and taste "off". There are many causes of stuck fermentation. The main two causes are high/low temperatures that kill the yeast, and a lack of nitrogen-based food for yeast. Other causes include:

  • Dirty or unsanitary equipment that introduce wild killer yeasts and bacteria into the wine
  • Old yeast used from the onset
  • Wrong yeast used from the onset
  • Dry yeast used from the onset
  • High Sulfite levels
  • Pesticide residue on the grapes
  • Insufficient or an overabundance of sugar in the must
  • High carbon dioxide levels in the wine
There are many things that a winemaker can do to fix stuck fermentation, with adding more yeast usually the last option of the list. Each of these fixes changes the flavor profile of the final wine, from minutely to drastically:
  • Adjust the temperature of the wine (keep it between 70 - 75°F)
  • Swish or stir the lees to get the yeast moving and fermentation going again
  • Add 25 mg of yeast energizer (Thiamin HCL)
  • Remove the old yeast and add new, fresh yeast (removing the old yeast is necessary as the presence of "dying" yeast will affect the movement of the new yeast)
  • For nitrogen deficiency, add DAP (diammonium phosphate dibasic)