Oxygenating Your Wort

Key points

  • Oxygen can have positive and negative effects on your wort
  • There is no "one size fits all" approach to oxygenating your wort
  • Add your oxygen based on the profile you want for your beer

 

At the time of this writing, I’m running experiments on low dissolved oxygen brewing, aka LODO and part of that includes taking a baseline measurement of exactly how much dissolved oxygen is being introduced at each relevant step. I’ll write a follow-up article when I’m finished, but it got me to thinking about the effects of oxygenating your wort

As most of my readers are home brewers, you actually have a significant advantage over commercial brewers in this respect. The issue is that dissolved oxygen concentration is difficult to measure across a large fermenter. Stratification happens and oxygen tends to come out of solution easily. There are mitigations (not solutions) in place for this, including adding oxygen in-line, placing the sensor at least 1 metre away from the oxygen port and recirculating the fermenter, but in the end, it’s a lot of educated guesswork – which is generally not what you want for something that is so important to the consistency of your fermentation.  I’ll also add that not all commercial breweries use pure oxygen – many use filtered air, which has its advantages and disadvantages, but makes oxygen concentration even more difficult to measure.

So let’s go over exactly what oxygen or the lack thereof does for your fermentation (and I’ll avoid getting overly technical this time).

To start, yeast use oxygen for a few things, but mostly for growing and reproducing. There are also non-oxygen based ways for yeast to grow and reproduce, but adding oxygen is kind of a guarantee that energy will be diverted away from producing ethanol and toward growth.

Growth = higher alcohols and esters.

Oxygen negates esters, leaving you with higher alcohols as a result. Now no-one wants a bunch of higher alcohols, so why would we add oxygen? This is where the ability to measure, or at least make an educated guess comes into play.

Adding just the right amount of oxygen will give you:

  • few higher alcohols
  • some esters
  • relatively quick fermentation
  • relatively quick acetaldehyde and diacetyl removal

In general, 8 ppm is where this falls for most home brewers and you can get here with just air. Anything below 4 ppm is too low. 

Adding too much oxygen will give you:

  • lots of higher alcohols
  • no or low esters
  • really fast fermentation
  • really fast initial acetaldehyde and diacetyl removal
  • yeast autolysis, which pollutes the beer with acetaldehyde and other off flavours

This tends to happen to homebrewers who get oxygen tanks and are unable to measure oxygen levels. There is very little written about the effects of too much oxygen, so they think that more=better.

Adding too little oxygen will give you

  • no to low higher alcohols
  • lots of esters
  • sluggish or stalled fermentation
  • slow or incomplete acetaldehyde and diacetyl removal
  • elevated levels of hydrogen sulfide and sulfite (rotten egg smell)

This happens when insufficient splashing is done in an effort to get more esters.

In the spirit of Relax, Don’t Worry, Have a Homebrew, there’s no way you’re getting to the “too much oxygen” level unless you have an oxygen tank. A vigorous shaking, or better, an aerator on the end of your hose to your fermenter will put you pretty much in the area you’re looking for if you’re making any standard gravity ale. I wouldn’t recommend this for a high gravity ale or a lager however.

If you’re using an oxygen tank, there are a couple of things to keep in mind:

  1. Don’t pump it up to full blast. You’ll just be venting oxygen into the atmosphere. You want to see the occasional bubble rise to the surface, not a “simmer” and certainly not a “boil”.
  2. Add oxygen from the bottom of your fermenter. If you have a conical, it’s easy to attach. If you have a stone and a flat-bottomed fermenter, move it around as you’re oxygenating. This will give you the best chance of mixing and avoiding pockets.
  3. Don’t add oxygen for more than 2 minutes for a 5 gallon (19L) batch.
  4. Oxygen should be proportional to the amount of yeast you’re pitching.

A helpful guide for what you're looking for in regards to oxygen is below

  • Estery ale or hefeweizen - Splashing + 0.75 million cells/mL/°P
  • Clean ale – 1 minute oxygen + 1 million cells/mL/°P
  • Lager or high gravity ale – 2 minutes oxygen and 1.5 million cells/mL/°P
  • High gravity lager – 2 minutes oxygen and 2-2.5 million cells/mL/°P. (Measure with a DO meter and add oxygen until you get to 15 ppm