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 Why do I need to aerate my wort? What does the aeration system do?

Why do I need to aerate my wort? What does the aeration system do?

Aerating the Wort

You may be scratching your head here, wondering, “I thought oxygen was bad for beer? Oxygenation and all that...why would I want to inject it into my beer?” While you are correct that oxygen in beer is undesirable, there is one point (and only one point) in the brewing process where it is desirable. This is when the post-boil wort has been chilled down to fermentation temperature, but before the yeast has been pitched into it.

Oxygen dissolves into wort as a function of temperature and specific gravity. As such, the colder and less concentrated the wort, the more oxygen will be able to enter into solution. All the bubbling and splashing that occurs during the boil drives most of the oxygen out of solution because the wort is too hot while boiling. Therefore, oxygen must be replenished after the wort is cool and able to retain the oxygen in solution again.

Oxygen is essential for yeast growth and reproduction. Yeast must grow and reproduce first, before actually fermenting the wort to make beer. Yeast needs oxygen to synthesize the material for expanding cell walls; namely sterols and fatty acids. Overlooking proper wort aeration can lead to problems such as long lag times before the start of fermentation, stuck or incomplete fermentation, or excessive ester (fruit flavor) production, any of which would produce less than desired results. Now that we understand the “when” and “why” of wort aeration, let’s examine how homebrewers can supply oxygen to chilled wort to meet the needs of their yeast.


After the wort has been chilled and transferred to the primary fermenter, there are a number of methods that can be employed to agitate the wort to introduce oxygen. If your wort is in a glass carboy, you can cover the mouth of the carboy with a loose fitting cap, get a firm grip and rock the carboy back and forth to slosh the wort around inside. Care must be taken to support the carboy on a cushioned surface and to maintain a secure hold on the carboy at all times while agitating.

If your wort will be fermented in a bucket or other vessel with a wide open top, a stainless steel whisk borrowed from the kitchen can be sanitized and used to whip the wort until it has at least a couple inches (several centimeters) of foam on top. If you don’t think your arm will last long enough to manually whisk the wort, you can sanitize a (new) paint stirrer, attach it to an electric drill and agitate the wort accordingly. A word of caution if using an electric drill and paint stirrer: be sure the drill is connected to a ground-fault protected receptacle, take care not to damage the fermenting vessel (or yourself), and avoid splashing the wort out of the fermenter.

Regardless if your wort is in either a carboy or bucket, if you need to add cold water to make up the volume of wort to the desired level, splashing or spraying the cold water into the wort can also increase the amount of oxygen that will go into solution.

Agitation is the simplest and least expensive method for aerating wort, but involves a little more time and elbow grease than splashing or injection. For many homebrewers, a good vigorous shaking of your “ale pail” before you pitch your yeast will do the trick. But if you’re brewing an Imperial Stout or Barleywine (or any other higher-gravity recipe), you might want to consider aerating your wort using more efficient means.


Another opportunity to add oxygen to your batch of homebrew is when the chilled wort is being transferred from the kettle to the fermenter. If the transfer is taking place through some type of hose or tubing, attaching a tool such as the Fighter Jet™ Fly Sparge Sprayer to the end of the tubing will cause the wort to spray and splash as it enters the fermenter. Alternately, if you have two sanitized vessels, you can transfer the wort between the two (at least five or six times) with as much splashing as practical. Pouring the wort through a sanitized wire mesh strainer can also increase splashing during the wort transfer process. As with agitation, splashing should result in at least a couple inches of foam on the wort surface. Be sure to brace the container receiving the wort so it cannot move and cause spilled wort. Also, be certain that containers and strainers are thoroughly sanitized before use. Splashing is a relatively simple and inexpensive approach to oxygenation, but if accomplished by pouring wort between vessels it may lend itself to contamination from airborne microbes.

Wort Aeration

One of the more effective methods of wort aeration is to employ the use of a filter and diffusion stone to aerate your wort. While you are not injecting pure oxygen into the wort, you will achieve the aeration necessary for insuring that your yeast gets a great start, and will ferment more completely. Midwest’s Aeration System comes with everything you need to aerate your wort. Instead of diffusing pure oxygen into your wort, this system uses filtered air. Comes with pump, sanitary filter, tubing, and Stainless Steel 2 micron diffusion stone. This system will never run out of oxygen. The down side is you need to let the system run for 30 - 120 minutes, depending on the original gravity of your wort. Contamination is not an issue, but make sure to check on the progress as it can create a lot of foam. While you are brewing, this system can also be used to pre-fill fermenters with sanitary air. The filter should never get wet, so don’t try and sanitize it by dunking it in sanitizer. If you want to clean the outside of it, use Alpet D2. To store the filter, use a little tinfoil on the “In” of the filter and store in a zip-lock bag.

Aeration of cooled, post-boil wort is essential for proper fermentation by brewing yeast. To be successful, homebrewers must pay heed to this important step in the brewing process by selecting and implementing a successful wort aeration technique that fits their equipment and budget. Oxygen is critical for the growth of brewing yeast … and happy yeast makes for happy homebrewers!

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