Stuck Fermentation

Stuck fermentation occurs when alcoholic fermentation slows dramatically or stops before yeast has converted the intended amount of grape sugar into alcohol. The wine may retain more residual sugar than planned and become vulnerable to unwanted microbial activity. A stuck fermentation is a technical problem that requires diagnosis rather than simply adding more yeast or sugar.

Common causes

• High sugar concentration: very ripe grapes can create high osmotic stress for yeast at the start of fermentation.
• High alcohol: ethanol can inhibit yeast as fermentation progresses, especially when the yeast strain is not suited to the conditions.
• Nutrient deficiency: insufficient yeast-assimilable nitrogen or other nutrients can slow yeast activity.
• Temperature stress: excessively high or low temperatures can inhibit or kill yeast.
• Unfavourable pH or sulphur dioxide conditions: chemistry and additions may affect yeast viability and activity.

Recognising a stuck fermentation

Winemakers monitor sugar depletion or density over time. If the reading stops changing while fermentable sugar remains, the fermentation may be stuck or sluggish. Laboratory analysis and checks of yeast viability, temperature, nutrients and microbial condition help identify the cause.

Managing the problem

Corrective action depends on the diagnosis and may include temperature adjustment, nutrient management or carefully preparing an active yeast starter to restart fermentation. Any intervention must follow sound winery practice and local regulations. Simply adding sugar or an arbitrary dose of yeast can make the problem worse.

Example: a high-sugar red must in a warm vintage may stop fermenting because of alcohol stress or nutrient deficiency, leaving more residual sugar than intended.

Chef’s note: a stuck fermentation is not the same as a deliberately sweet wine. Sweetness may be intentional, but a stuck fermentation is an incomplete or stalled process that requires control and stability checks.