Stressed Yeast (Why It Happens, Common Signs, & Off-Flavors)

Just like any living organism, yeast can become stressed. But what exactly does it mean for a microorganism to be stressed, and how does this stress affect your beer?

When yeast are continually overworked in a difficult environment, they take longer to metabolize sugar into alcohol and reproduce, and can even die from the stress. This can result in stuck fermentation and green apple, muddy, buttery, or sulphuric off-flavors from decomposing yeast cells.

Read on to learn more about the types of environments that can cause yeast to become stressed, and how to tell if your yeast is healthy or not.

What is stressed yeast?

All living things expend energy to survive and proliferate and can become increasingly stressed as the conditions become increasingly difficult. 

Yeast can become stressed when their environment forces them to use up more energy for their basic metabolic functions. Things like temperature, oxygen stress, and high ethanol content can make it harder for yeast to survive, causing stuck fermentation and off-flavors associated with dead yeast.

Watch out for some of these common stressors that could impact the quality of your yeast:

  • Ethanol Stress – High levels of ethanol can put increased strain on yeast. As the alcohol content increases, the environment becomes more and more difficult, leading to overworked yeast.
  • Osmotic Stress – When you pitch yeast into a high gravity wort, the yeast’s cell walls can burst, resulting in a lot of dead yeast. As a result there will be fewer yeast left to ferment your beer, leading to the same type of stress as underpitching.
  • Temperature Stress – When yeast is exposed to extreme temperature fluctuations, yeast can have difficulty maintaining consistent fermentation. In extreme cases, this temperature fluctuation can cause some cell death, though the more likely result is stuck or incomplete fermentation.
  • Oxygen Stress – New yeast cells require oxygen in order to make the sterols and unsaturated fatty acids it needs to build a strong cell membrane. When yeast multiply too quickly, they can use up the available oxygen too quickly, resulting in cells with weak cell walls. This is a major reason why fast fermentation and overpitching can often result in off-flavors. 
  • Underpitching – When too few yeast are pitched, they can become overworked in the process of alcohol fermentation. When too much stress is put on the yeast in this way, it can result in stuck fermentation.
  • Shear Stress – When yeast are continually agitated, their cell walls can become damaged by the liquid moving against them. While shaking a stuck batch can suspend yeast for a more complete fermentation, frequent movement will result in stressed yeast.

What happens if yeast is stressed?

When yeast is stressed, the vitality and viability of the microorganisms are threatened. 

When yeast is stressed, the decreased vitality and viability can result in an under-fermented beer filled with unpleasant flavors or aromas. 

Vitality refers to metabolic activity; yeast with low vitality will ferment and multiply slowly, resulting in stuck fermentation or incomplete fermentation.

Viability is the ratio of living yeast to dead yeast in your beer. A high ratio of dead yeast will result in off-flavors and compound the stress on the remaining yeast.

What are the signs of stressed yeast?

While there are a number of laboratory tests that will give you exact figures on the viability and vitality of your yeast, you shouldn’t need them to identify stressed yeast in a homebrew setting.

You should suspect stressed yeast if you notice any off-smelling aromas from your beer, or if you notice that your fermentation is stuck.

If there your airlock is not bubbling early on in the brewing process, it is very likely that your fermentation is arrested because of stressed yeast.

What does stressed yeast smell like?

While stressed yeast are responsible for a wide number of off-flavors, there are a couple of common aromas to watch out for during the brewing process.

If your beer has a sulphury, rotten-eggs smell, you can be sure that your yeast is not fermenting at a healthy speed. In addition, muddy and savory aromas are indicative of a high number of dead yeast in your beer.

The most common stressed yeast off-flavors

In addition to the smells listed above, there is a wide range of off-flavors that can arise in your beer as a result of stressed yeast.

These flavors are developed by incomplete fermentation, along with dead or unhealthy yeast:

  • Green apple flavor
  • Muddy, umami flavors
  • Buttery or butterscotch flavors
  • Fusel alcohols
  • Sulphuric, rotten eggs flavors.

If you want to avoid these unpleasant flavors, make sure to keep your yeast healthy by pitching the right amount of yeast, keeping it at a stable temperature, and taking care to reduce stress on your yeast when working with high gravity wort or high alcohol contents.

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