Learning Lab: Accidentally on Purpose

In this Learning Lab, we intentionally make things harder for our yeast cells—to better understand what can go wrong, and how to recognize what happened.

Jester Goldman Apr 25, 2020 - 12 min read

Learning Lab: Accidentally on Purpose Primary Image

The Learning Lab series started out looking at ingredients and has now moved on to the brewing process. For the fermentation step, the two most obvious variables are the yeast strains themselves and fermentation temperature. When we covered yeast before (December/January 2019), we presented an experiment comparing multiple strains using the same base recipe. Fermentation temperature has a major impact on your beer’s flavor, but many homebrewers haven’t invested in a controlled fermentation setup. If you do have one, it’s certainly worth looking at the effect that a few degrees can have, especially with German Weizen or Saison strains.

But this article is going in a different direction. Normally, you want to follow best practices to make the tastiest beer possible. You treat your yeast cells gently, so they’ll be as happy and healthy as possible. Instead, we’re going to intentionally make things harder for them to see how they cope with stress. This is likely to create some problem beers. While that might seem like a waste of time and money, the goal is to better understand what can go wrong and how to recognize what happened.

Breaking the Rules

Let’s start by reviewing fermentation best practices. The first step is to pitch enough yeast up front. This reduces the reproductive lag stage, so the yeast cells get ramped up for alcohol production and also supports them against any bacterial competition. Good wort aeration also helps this process.

Temperature, of course, is probably the most important element. From the moment we pitch to the time we get ready to package the beer, our guidelines around temperature help us protect the yeast. If the wort is too hot, that can decimate the starting yeast population, but if it’s too cold, that can shock the yeast, too. Ultimately, we want to ease the yeast into the environment they’ll be working in. We generally choose our fermentation temperature to match the yeast strain. If we stray too cold, the yeast will struggle or go dormant. Warmer temperatures kick the yeast into high gear, which usually drives higher ester production and can also lead to fusel alcohols.


We also recognize that yeast can be sensitive to wild temperature swings. Instability through the course of fermentation can disrupt the yeast and create off-flavors.

Finally, if you’ve brewed stronger beers, you know that yeast can also need some careful handling. Osmotic pressure caused by a high starting gravity can slow the yeast down, and they may not be up for the long slog to chew their way through all the sugar. That stress can compound the other effects.

If that’s how to nurture our yeast, let’s step into the Upside Down and identify our worst practices for torturing them:
Underpitching: Let’s make sure they start out understaffed for the job at hand.
No aeration: They need oxygen to grow? Let them do without.
Mismatched pitching temperature: Why worry? They’ll warm up or cool down eventually.
Uncomfortable temperatures: Do they think they’re Goldilocks? They’ll just have to accept too hot or too cold.
Temperature instability: They can get used to anything... unless it never stays that way.

Control Batch

Before we give over to our dark sides, let’s create a control batch based on an old-ale recipe. We’re intentionally using a high-gravity recipe to highlight yeast-stress issues.


Volume (after boil): 1 gallon (3.8 liters)
OG: 1.080
FG: 1.020
IBUs: 51
ABV: 7.89%

Control Batch Recipe

Grain: 2 lb (907 g) light dry-malt extract (DME); 0.25 lb (113 g) Crystal malt (80L)
Hops: 0.125 oz (3.5 g) Amarillo [8.6% AA] at 60 minutes; 0.25 oz (7.1 g) Amarillo [8.6% AA] at 30 minutes; 0.125 oz (3.5 g) Amarillo [8.6% AA] at 5 minutes. Yeast: ½ package (5.75 g) SafAle S-04 English Ale yeast. Bottling: 0.8 oz (23 g) total dissolved priming sugar for bottling
Directions: Fill your pot with 1 gallon (3.8 l) of water plus the makeup for evaporation loss. Put the crushed crystal malt in a small nylon grain bag and place the bag in the water. Turn the heat to medium and allow the crushed grain to steep, stirring occasionally. After about 20 minutes or when the temperature hits 165°F (74°C), pull the grain bag out of the water. Remove the pot from the heat and dissolve the DME into the hot water. Return the pot to the heat and boil for 60 minutes following the hops schedule. Chill to 65–70°F (18–21°C). Splash the chilled wort vigorously to oxygenate. Hydrate the yeast in warm water before pitching, then let ferment at 65–70°F (18–21°C). Bottle as usual.
If you need to substitute a different hops strain, aim for about 50 IBUs in a 1-gallon (3.8 l) batch. You could also use a different yeast, but dry yeast is easier to measure out. I recommend a top fermenting ale yeast.


I’ll outline a half dozen experimental batches, but feel free to try your own variations. Each one follows more or less the same recipe but reduces the pitch rate. The fermentation process may also vary.

Victim 1—Very Low Pitch

For this batch, reduce the yeast to 1 g, but still hydrate it. This gives us a severely underpitched batch.


Victim 2—Low Pitch, Low Oxygen.

Reduce the yeast to 2 g and don’t oxygenate the wort before pitching.

Victim 3—Low Pitch, Low Oxygen, Osmotic Stress.

Reduce the yeast to 2 g and pitch directly into the wort (no hydration step). Don’t oxygenate the wort before pitching.

Victim 4—Low Pitch, Low Oxygen, Hot Pitch.

Reduce the yeast to 2 g, don’t oxygenate, and pitch at a higher temperature: when you chill the wort, cool it to only about 85°F (29°C), then pitch the yeast. The main fermentation will still be at the cooler control temperature.

Victim 5—Low Pitch, Low Oxygen, Hot Ferment

Reduce the yeast to 2 g, don’t oxygenate, and ferment at a higher temperature. You can put the fermentor on or next to a heating vent or keep it above your stove in the kitchen. We want to aim for a fermenttion temperature higher than 80°F (27°C), if possible.


Victim 6—Low Pitch, Low Oxygen, Variable Ferment

Reduce the yeast to 2 g and don’t oxygenate before pitching. This batch will be the most labor intensive. Start out fermentation at a higher temperature. After a half day, put the fermentor in a cool location for a while. You could take it into your basement for a few hours or even put it in the refrigerator for 20 minutes or so. Then go back to the heat to start the cycle again.


For this Learning Lab, you should start making your observations from the start of the experiment. Track the lag time for each batch: how long did it take before you saw signs of fermentation such as airlock bubbles or foam? You should also note how long it took each one to complete and how vigorous the fermentation was.

Once each of the beers has been bottled and given time to carbonate, it’s time to sit down for the sampling session. I’d recommend inviting a friend to join you in the evaluation. If you do, let him/her offer observations without biasing him/her with your knowledge of the victims and the control. Follow the usual evaluation steps, taking stock of the aromas and flavors you find, along with the mouthfeel and body. You might also see whether there are appearance differences, such as haze or poorer head retention.

You might find that some of the batches are sweeter than the others, which is a sign that the yeast didn’t finish or that they were less efficient. There’s also the potential for sour notes or other signs of infection. When the yeast are handicapped, it’s easy for bacteria to get a foothold.


You should expect variance in the ester levels, with the hotter fermentation yielding more fruitiness. You can note the intensity, but also pay attention to the nature of the esters. Some may be more desirable or fit better with the beer than others. You might also pick up on some phenol (spicy, smoky, plastic), acetaldehyde (green apple, latex paint), or diacetyl (butterscotch) characteristics.

Also be on the lookout for harsh alcoholic character, perhaps with some solvent-like notes. This is most likely to turn up in the hot-fermentation batch. Note that several of these beers may have some noticeable alcohol because we are aiming at almost 8 percent ABV. Without a fair amount of aging, the alcohol might not be smoothly integrated in with the beer. You should be able to distinguish this from the harsh bite of fusel alcohols.

As a final step, take the general measure of each sample. The control batch may well turn out to be the favorite, but get a sense of each one on its own merits. Even if you can’t directly identify a specific element, it’s useful to see how clearly you can distinguish the beers.


It’s not normal to flagrantly violate standard brewing wisdom. You may have cringed at this affront to the brewing gods. But don’t get hung up on the idea that you made flawed beer because the primary goal of this Lab was to map out the connections between bad process and mediocre results.
That said, some of the victims may have thrived despite their mistreatment. That, too, is useful information. Not every crime is punished. One thing to keep in mind is that different yeast strains have different sensitivities. Keep your notes on hand from this experiment, then run it again with a different yeast.

Photos: Matt Graves/