What Causes Stuck Fermentation?

According to Google, fermentation is “the chemical breakdown of a substance by bacteria, yeasts, or other microorganisms, typically involving effervescence and the giving off of heat.”. Basically, it is a process in which a substance (like sugar for spirits production) is broken down into a simpler substance.

For beer, wine and spirits production, yeast is used to facilitate this “breakdown”.

Yeast is a “microscopic fungus consisting of single cells that reproduce by budding and are capable of converting sugar into alcohol and carbon dioxide”. For this conversion to occur, there are certain conditions that must be met. And for optimal conversion to occur, the brewer will try to create an optimal environment for yeast to thrive and remain healthy. The healthier the yeast can remain, the more efficient they are at producing alcohol. If the environmental conditions surrounding the yeast are less than optimal, then the yeast health can degrade - with the end result being less attenuation and even a stuck fermentation altogether.

Fermentation Stopped Early, But Why?

Some of the most common problems with a stuck fermentation are:

  • Distilling equipmentTemperature. Yeast strains have an optimal temperature that allows the yeast to deliver the most desirable outcome. The most common yeast is Saccharomyces cerevisiae (Pronounced: Saccarow-miceese sahro-vis-ee-ae). Optimal working temperatures for this strain range from 90.14° F (32.3° C) to a maximum of 113.72° F (45.4° C). Deviating away from these recommended temperatures for this yeast strain may very well result in a stuck fermentation.
  • Nutrients. Much like human beings, yeast cannot remain healthy for the duration on a diet of sugar alone. Like humans, overconsumption of sugar will basically make yeast sick. This unhealthy state will produce undesirable byproducts and ultimately stall or create a stuck fermentation. In order for yeast to reproduce and continue to ferment, yeast needs nutrients like amino acids, fatty acids, nitrogen, and other vitamins.
  • pH. Potential hydrogen (pH) refers the acidity of a substance. Yeast in nature feed on sugars found in fruits. Average pH levels in fruit can range from 4.5 to 5.5. Therefore, over time, yeast has developed a preference for very similar pH ranges. Deviation from this average range can affect the rate at which yeast will thrive.
  • Too Much Sugar. When fermenting fruit, grains or molasses (including cane juice), the resulting sugar content can be considered too low for those individuals looking to maximize yield. A common way to boost potential yield is to add sugar prior to fermentation. However, adding too much sugar can result in an inhibiting effect on yeast's ability to finish fermentation. Potential alcohol that exceeds 12 or 13% can affect the health of yeast at least. At worst it can stall the fermentation.
  • Yeast Was Never Viable. It is not uncommon for yeast simply be too old. Maybe it was packaged up ages ago and parked on the shelf for too long.
  • Fermentation May Be Complete. On the opposite end of the spectrum, the brewer may not realize that fermentation would work off to completion so quickly. For this reason, take a starting gravity measurement and a final gravity measurement, so that the brewer can better understand that fermentation has stopped because there is no more sugar remaining for the yeast to feed on. If a starting gravity measurement is not taken, a good trick is to simply taste the fermented liquid to check for sweetness. If the liquid is sweet, fermentation is not complete. If the liquid is dry like a dry wine, then fermentation is likely finished.
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