Stuck fermentation

Lore & Background

The most common causes of a stuck fermentation include an excessively high temperature killing off the yeast, or a must deficient in the nitrogen food source needed for yeast to thrive. Nitrogen is vital for yeast growth and is usually provided by the grapes themselves. Grapes grown in nitrogen-poor soils or varieties like Chardonnay and Riesling, which naturally have low nitrogen-to-sugar ratios, are at greater risk. Overripe grapes with high sugar levels can also cause stuck fermentation, as yeast cannot reproduce in environments with 16-18% ABV, and multiple stressors may halt fermentation even before that alcohol level is reached.

Reader's Guide

A stuck fermentation is a significant concern in winemaking because it can ruin a batch through bacterial spoilage and oxidation. The difficulty of restarting fermentation is compounded by an inhibitor released by dying yeast cells. Winemakers employ several preventive measures, such as adding nitrogen in the form of diammonium phosphate, using cultured yeast strains with high alcohol and temperature tolerance, and controlling fermentation temperature with modern equipment like stainless steel tanks with cooling jackets. Each of these approaches can subtly or dramatically affect the resulting flavors and quality of the wine. The risk of stuck fermentation is lower for red wines, which are often fermented with their skins in open containers or oak barrels, providing oxygen and grape solids for yeast to feed on.

Did You Know?

• A stuck fermentation is different from an arrested fermentation, which is intentionally stopped by the winemaker.
• Yeast dies when temperatures near 104°F (40°C), risking a stuck fermentation.
• Grape varieties like Chardonnay and Riesling are naturally prone to low nitrogen-to-sugar ratios, increasing stuck fermentation risk.
• A chemical compound released by dying yeast cells inhibits the growth of future yeast cells in the batch.