These two terms, aeration and oxygenation, are fairly similar processes and most often referenced as beneficial processes in brewing. For a long time they were considered a must for homebrewers, but these days they aren’t deemed as significant and are factors that can be tinkered with depending on the situation and the brewer’s desired outcome for the beer.
When most brewers use the term aeration, it is in reference to the process of forcing air into brewer’s wort after the boiling and cooling processes are complete, but prior to the commencement of active fermentation. A better term for this process would be cold-side aeration and is extremely beneficial in many brewing situations. There are many ways homebrewers can go about aerating their wort: Vigorously shaking the fermenter, using a mechanical device like a mixing paddle, dumping wort back and forth between sanitized buckets, a spray aerator, a venturi system, or an aquarium pump with carbonation stone, just to name a few. At this stage of the process brewers want their yeast to have some dissolved oxygen (DO) available for healthy growth. Generally when folks discuss a healthy level of dissolved oxygen for yeast growth, the most often amount cited is approximately 8–12 ppm. Aerating room temperature wort means that brewers can at best achieve ~8 ppm dissolved oxygen . . . so there is no such thing as over-aerating wort. I personally use an aquarium pump with carbonation stone and let it run for 10 minutes (or until my bucket fills with foam) as I am pitching the yeast.
There is another use of this term brewers will use that has negative connotations: Hot-side aeration or HSA. It is not a term most homebrewers should care about, but it refers to aeration of hot wort, generally in the mash. This can lead to premature staling of beer and a faster loss of malt and hop aroma in the finished beer. The reason most homebrewers need not care about the term is because most of us consume our beers relatively fresh and it takes a lot of aeration to have an effect.
Oxygenation is similar to aeration with one major difference . . . brewers use pure oxygen instead of air to provide the yeast this nutrient for growth. This shortens the time required for this step since a batch can be oxygenated in one minute to 12 ppm. Brewers will use compressed oxygen to push the air through a carbonation stone. There are some drawbacks to oxygenating wort. The first is that you need to pay for the oxygen, either small bottles purchased at a hardware store or a refillable bottle. If you go the route of a refillable bottle, you will also need an oxygen regulator for the tank. The second drawback is that it is hard to control how much oxygen you are putting into the wort unless you have a way to gauge the oxygen’s flow, and too much oxygen can damage your yeast cells.
Using Dry Yeast
A segment needs to be specially dedicated here to those brewers that use dry strains of yeast. That is because most strains of dry yeast have been grown in an environment that has been specifically designed to provide the yeast cells with a reserve of glycogen (the compounds yeast construct from proper oxygen) to grow and thrive in wort to ferment a beer properly. So any brewers using a fresh pitch of a dry strain of yeast (at the proper pitch rate) don’t need an aeration/oxygenation step in their brew day procedures. Simply chill to the proper pitch temperature, sprinkle the yeast, and the yeast should do the rest for you.
One area that truly advocates for the use of oxygenation over aeration is when fermenting wort that is high-gravity. Those big double IPAs, barleywines, doppelbocks, and imperial stouts will be much better served if the brewer opts for a direct injection of pure oxygen over air since high-gravity wort has a lower saturation point of oxygen. So instead of one minute of forced oxygen through a carbonation stone in most standard-strength beer, a high-gravity beer might need two minutes of pure oxygen for proper yeast growth and health. For those aerating, a second round of aeration 12 hours later can also serve this purpose.
How Much Oxygen?
Finally we get to the part where a brewer can tinker with their level of dissolved oxygen (DO) to produce variances in their beer. I’ll start by saying that if you are using liquid yeast or re-pitching a yeast, you’ll want some dissolved oxygen for growth. It has been shown that the range of 0–10 ppm of dissolved oxygen in the wort is where the biggest effects are seen. Once above that level, changes can be a bit more complicated. I would recommend at least 4 ppm if you want to have a more expressive ester profile. If you are looking for a cleaner profile, then 8–10 ppm should be your goal. Unfortunately there is no way to test for levels unless you have a DO meter, which start around $200 (USD). The next best thing to do is to play around, if you want more esters, maybe do 30 seconds of oxygenation instead of one minute. Record your techniques and you can eventually come up with a game plan for different beers.