Stuck Fermentation - Possible Causes

Possible Causes

There are several potential instigators of a stuck fermentation. One of the most common found in winemaking is a nitrogen deficient must. Nitrogen is a vital nutrient in the growth and development of yeasts and is usually provided from the wine grapes themselves. Grapes grown in vineyards with soils lacking in nitrogen or grape varieties, such as Chardonnay and Riesling, which are naturally prone to have low nitrogen to sugar ratios will be at greater risk for having a stuck fermentation. Another cause rooted in the vineyard is from overripe grapes. Grapes that are overripe will have high levels of sugars that translates into higher alcohol content. Yeast are unable to reproduce in an environment with 16-18% ABV but in an environment with multiple stressors the fermentation could get stuck even before the alcohol level reaches that point.

A byproduct of the energy created during fermentation is heat which raises the temperature of the fermenting must as the yeast work. When temperatures near 104°F (40°C), activity slows and yeast start to die. If temperatures stay high and the yeast stop reproducing then the fermentation is at grave risk of getting stuck. Even if the must is cooled back down, fermentation will be very difficult to restart due to a chemical compound released by the dying yeast that serves as an inhibitor to the growth of future yeast cells in the batch. Modern winemaking equipment includes temperature control mechanisms such as stainless steel fermenting tanks with a cooling jacket to help regulate temperatures. Another potential cause is a combination of lack of oxygen and yeast nutrients found in the lipids suspended in grape solids. In the absence of oxygen, yeast will seek out the lipids in grape solid as a source of nutrients to sustain its activity. If the wine is lacking grape solids (such as if the wine's been clarified) then the yeast are at risk of starving and causing the fermentation to get stuck. This risk is lower for red wines which are often fermented with their skins in open air containers or oak wine barrels which provide plenty of oxygen and grape solids for the yeast to feed on.