Learning Lab: O2? Oh My!

In this Learning Lab, we tempt fate by inviting oxidation exposure and punishing our wort with hot-side aeration. Will it ruin our beer, or is the threat exaggerated?

Jester Goldman Jul 4, 2020 - 12 min read

Learning Lab: O2? Oh My! Primary Image

Last issue, we tortured our yeast for science. We flipped our best practices to the dark side and learned the effects of low pitching rates, oxygen starvation, and crazy temperatures. This time, we’re going to follow a similar path, but our goal is to better understand a very common flaw in homebrewing: oxidation.

Over the years, I’ve probably judged hundreds of homebrews that suffered from oxidation that sucked every spark out of the beer. You can start with a great recipe and the finest ingredients and still end up with a glass full of lifeless dreck, sabotaged by poor handling.

Oxygen is the basis of life, but it’s a double-edged blade. Our metabolisms rely on energy driven by oxygen’s ability to react with other chemicals; that’s also why we want to provide yeast with oxygen up front. But yeast cells aren’t too picky about finding a partner, and just as your best tools will rust away in dampness, oxygen can make your beer fall apart into distasteful side-products.

Recognizing Oxidation

The damage from oxidation is easy to spot in the flavor and aroma. Look for the distinctive character of cardboard or paper, or it can also turn vinous or offer earthy/musty notes. Some higher-alcohol, darker beers, such as barleywine or imperial stout, can get lucky and express it as sherry or raisin flavors, but brewers still need to be careful. Beyond those obvious descriptors, oxidation can just make the beer taste dull, blunting both hops bite and malt richness. Sometimes, it contributes to buttery diacetyl by breaking down acetaldehyde. There are also some less critical effects on appearance, such as making the beer darker. Often, this is accompanied by hazing that makes the beer look as dull as it tastes.


Most of the time, the problem comes from oxygen introduced during the brewing, fermenting, or packaging processes, but oxidation of ingredients can also have an impact. This usually comes from old ingredients that were poorly stored. Hops can lose their bitterness as alpha acids decay. Hop resins can break down into isovaleric acid, which comes across as cheesy or sweaty. Old malt and malt extract can produce a metallic tang or other related off-flavors.

Setting aside old ingredients, let’s take a look at our process to understand how oxygen can get in.

Hot! Hot! Hot!

During brewing, the concern is hot-side aeration (HSA). At higher temperatures, oxygen readily combines, so you want to keep air from interacting. This means treating the mash and the wort as gently as possible: Refrain from stirring a bunch of air into the grain bed, ensure that any recirculation smoothly flows back into the mash, avoid splashing during wort collection from the sparge, and let the wort cool fully before transferring it into your fermentor. Interestingly, the boil itself is fairly safe, because oxygen solubility drops off at those higher temperatures.

It’s worth pointing out that some more modern sources are skeptical about the threat of HSA. In particular, the team over at Brülosophy has run experiments looking at both the short- and long-term flavor impact of HSA, neither of which proved to be statistically meaningful. One possible explanation is that the boil itself might reduce the dissolved oxygen, keeping it from breaking free later in the process. Personally, I play it safe and avoid HSA, but it might be mere superstition. All the more reason for us to include this case in our experiment.

After the Fire

Once the wort is cooled and we’re ready to pitch, oxygen becomes our friend for a spell. It helps the yeast build strong cell walls during the reproductive phase of fermentation. It’s a short-lived respite, though. After primary fermentation, the yeast are done with oxygen, and it once again becomes our foe.

To some extent, a little oxidation is basically unavoidable. Even if you’re careful not to splash the beer when racking it off the trub, the surface is still exposed to the air. Bottling offers the same issue, made worse by more turbulent flow. The yeast will consume some of the oxygen in the headspace during carbonation, but all of these small exposures contribute to the relatively limited shelf life of most homebrewed beer. As a mead maker, I mitigate some of this by purging carboys with CO2 to limit the available oxygen, but it’s a challenge.


Whereas HSA may be controversial, there’s no question about sloppy handling after primary fermentation. Those off-flavors are no myth. The effect can also be exacerbated by high storage temperatures, which provide more energy to drive oxidation reactions.

Let’s Visit the Lab

In our experiment, we’ll test the effects of oxygen exposure on hot grain and wort, as well as post-primary fermentation. We’ll also take a look at the impact that storage temperature can have. Since color can be affected, our base recipe will be an all-grain pale ale. I recommend that you treat the control batch as gently as possible.

Base Recipe

Method: Mini-mash, brew-in-a-bag (BIAB)
Volume (after boil): 1 gallon (3.8 liters)
OG: 1.056
FG: 1.014
Grain: 2 lb (907 g) crushed 2-row malt
Hops: 0.25 oz (7.1 g) Cascade [7.0% AA] at whirlpool
Yeast: ½ packet of Safale US-05 American Ale Yeast

Each experiment should follow the same process, with variations on oxygen exposure.

Directions: Place the crushed malt into a nylon grain bag and set aside. Heat 3 quarts (2.8 liters) of water to 161°F (72°C). Remove the pot from the burner, then dip the bag of malt into the pot and swirl it around. Your goal is to make sure that all of the grain is thoroughly wet. Check the temperature of the liquid. We’re aiming for 150°F (66°C). You may need to heat it a little or add a splash or two of cool water. In either case, stir the bag in the pot to normalize the temperature. Once it reaches the target temperature, cover the pot and let the grain bag sit. Mash for about 60 minutes but check the temperature halfway through. If it has dropped more than a couple of degrees, put the pot back on the burner to kick the temperature back up to the appropriate target.

At the end of the hour, heat up another 2 quarts (1.9 l) of water to 170°F (77°C). Slowly lift the grain bag out over the pot. Gently pour the hot water over the grain and into the pot to rinse out a little more of the malt sugars. Top up the pot with another quart (0.9 l) of water. Boil the wort for one hour, then take the pot off the burner.


Stir the hops into the wort for a whirlpool addition. Cover and let sit for 20 minutes. Then chill the wort to 70°F (21°C), transfer into a gallon (3.8 l) jug, topping up with water if necessary. Pitch the yeast and put on the airlock.

Ferment each mini-batch at about 70°F (20°C). Once fermentation is complete, carefully bottle the batch with about 0.4 oz (12 g) total dissolved priming sugar.


Aside from the control batch, perform the following variations:

  • Grain exposure: Halfway through the mash, pull out the grain bag, open it, and stir the grain around a bit to aerate it. Then return it to the pot.
  • Hot transfer pre-boil: Immediately after the mash, vigorously transfer the wort to another pot before starting the boil.
  • Hot transfer post-boil: Before chilling the wort, vigorously transfer the wort to another pot.
  • Rough racking: After primary fermentation, roughly pour the beer into another gallon jug.
  • Rough bottling: Before bottling, roughly rack the beer into another jug.
  • Heat stress: For each batch, including the control, set aside one bottle and store at 80°F (27°C) or warmer.

Taste Testing

Give the beers two or three weeks to carbonate, then set up your taste tests. The well-treated control batch will be a good baseline. Compare it with each of the variations, including the heat-stressed bottles. Start out by looking for differences in appearance. Are any of the batches darker in color or hazier? How do they compare to one another?

Next, take a good sniff and appreciate the hop character in the control batch. How do the oxidized batches compare? Finally, move on to sipping each beer. Note the flavors you encounter. You won’t find any sherry, but do you pick up paper or cardboard? How about a musty cellar character?

This is a good time to assess the HSA effect. Alongside the control, does the difference seem obvious, or could it be mythical? If the flavor does stand out, contrast the expression of oxidation between the HSA batches and the post-fermentation batches.

Next, compare the normally carbonated bottles with their corresponding heat-stressed samples. Can you spot any differences? Are the effects any more pronounced? This may be especially interesting for the poorly treated control bottle.

Next Steps

There are a number of other variables to play with, but the most interesting one is to run the experiment again with a heavier base recipe. Stronger malt, especially if you use some melanoidin malt, could yield some of that sherry character. Higher hopping rates might emphasize the effect of oxygen on hop character.

It might also be fun to do a batch or two with aged ingredients: one with dried malt extract that’s been sitting in a bowl out on the counter for a week and another with hops that have been in an open bag at room temperature for a while.

Remember: the point is to recognize the effects of oxidation and to understand how it can damage your beer.

Photos: Matt Graves/