Wort Aeration
“The yeast is the most significant factor in determining the quality of fermentation, and oxygen can be the most significant factor in determining the quality of the yeast.” — How to Brew, John Palmer.
What John Palmer is referring to in the quote above is wort aeration — the practice of adding oxygen to the cooled wort immediately prior to the start of fermentation. The presence of oxygen in your fermentating wort is essential for yeast vitality and growth. The presence of oxygen allows yeast to produce lipids that help build cell membranes that are needed for healthy yeast. A lack of adequate oxygen prevents proper yeast growth and results in underattenuated wort or beer. This is especially important for higher gravity beers where a high yeast cell count is necessary to convert all of the sugars to alcohol because oxygen is less soluble in high gravity (high sugar) worts.
The rate of oxygen you want in your wort immediately prior to pitching your yeast varies by style, yeast strain, fermentation conditions, and other factors, but you will generally want 8 to 16 mg/L (ppm) oxygen in your wort. If you are unable to get to those rates with the equipment you have, remember it is important to add some oxygen rather than skip this important step all together.
So how do you get that oxygen into your wort? There are a few different ways. The most economical is shaking, stirring, whipping, or splashing the wort in your fermenter, or dumping it from one vessel to another causing the wort to be well disturbed. This doesn’t require any additional equipment, although it is also the most labor-intensive method.
The second method is using a stainless steel or bronze aeration stone hooked up to an aquarium pump. With this setup, air is forced through microscopic pores in the stone for 15 to 30 minutes to aerate the wort. Another method that can save time and energy is adding a piece of hard plastic or stainless steel tube near the end or middle of your racking hose and puncturing it with needle-size holes. As the wort flows through the hose it will draw air in, creating a Venturi-style aerator. A further explanation and directions to build an in-line aerator was published in the “Homebrew Hacks” feature in the March-April 2015 BYO.
Because air is only about 21 percent oxygen, all of these methods that incorporate mixing air with wort will max out with a wort around 8 ppm oxygen. If you want a higher rate of oxygen, the only way to get there is by adding pure oxygen directly to your wort. Oxygen tanks can be purchased from welding supply stores, hardware stores and even some homebrew shops. One method is adding oxygen to the headspace of your carboy, capping it, and then shaking vigorously for 30 seconds or so. Or, you can attach an oxygen tank to an aeration stone and instead of releasing air as described earlier, the stone will deliver tiny bubbles of pure oxygen into your wort, bringing it to a suitable level within a minute or so. You do not want to let it go too long, as there are potential negative consequences with over-oxidating the wort: Like a loss of desired aromas and stalled fermentations.
Aeration can be done immediately before adding your yeast or right after, but you don’t want to aerate your wort until it is cooled to fermentation temperature. Aerating hot wort can lead to unwanted color pick-up and decreased solubility.
