What might cause alcoholic fermentation to stop prematurely?

Alcoholic fermentation can stop prematurely due to various factors, but one of the most significant is the combination of low temperatures or high alcohol levels.

When fermentation occurs, yeast converts sugars into alcohol and carbon dioxide. Yeast has an optimal temperature range in which it functions best, usually between 20°C to 30°C (68°F to 86°F) for many wine yeasts. If the temperature drops below this range, the yeast can become inactive or slow down significantly, leading to a halt in fermentation.

On the other hand, as the fermentation process progresses, alcohol levels rise. Most yeast strains have an alcohol tolerance level, typically ranging from about 12% to 15% alcohol by volume, depending on the strain. Once this threshold is exceeded, the yeast can become stressed or even die, thus stopping the fermentation process.

This combination of low temperatures and excessive alcohol concentrations presents two significant barriers to yeast activity, making it difficult for fermentation to continue successfully. Understanding these factors is crucial in winemaking practices to ensure that fermentation completes as intended.