Stuck and sluggish fermentation

This is a winery problem rather than a consumer one, but it explains several things a drinker does encounter — volatile acidity, unexpected sweetness, and why nutrient additions are routine.

Yeast assimilable nitrogen is the most common limiting factor. Below roughly 150 mg/L, fermentations are more likely to be sluggish and to produce hydrogen sulfide.

Temperature bounds it at both ends: too cold and metabolism slows to a halt, too warm — generally above the mid-30s Celsius — and yeast cells are killed.

Very high initial sugar, as in botrytised or dried-grape musts, imposes osmotic stress before fermentation even begins, which is why those wines are the most prone to stopping.

Ethanol becomes toxic to the yeast as it accumulates, and medium-chain fatty acids produced during fermentation add to that toxicity.

The danger is what happens next. Sugar with no active yeast and little alcohol is an excellent medium for Brettanomyces, acetic acid bacteria and lactic acid bacteria, so restarting quickly is urgent.

Restarting is difficult because the conditions that stopped the first population will stop a second. Standard practice is to build an acclimatised starter culture and step it up gradually.