Yeast need oxygen to synthesize sterols and unsaturated fatty acids for cell membrane biosynthesis. Without aeration, fermentations tend to be underattenuated because oxygen availability is a limiting factor for yeast growththe yeast stop budding when sterol levels become depleted. Higher gravity worts need more yeast for proper fermentation, and thus need more oxygen, but the higher gravity makes it more difficult to dissolve oxygen in the first place. Boiling the wort drives out the dissolved oxygen normally present, so aeration of some sort is needed prior to fermentation. Proper aeration of the wort can be accomplished several ways:
shaking the container, e.g. the starter jar
pouring the cooled wort into the fermenter so it splashes,
using a bronze or stainless steel airstone with an aquarium air pump and using it to bubble air into the fermenter for an hour.
For the beginning brewer, I recommend the simplest methods of shaking the starter and pouring/shaking the wort. This method is especially effective if you are doing a partial boil and adding water to the fermenter to make up the total volume. Instead of shaking the wort, you can shake the water.
Pour the water into the fermenter and cover it tightly. The fermenter should be about half full.
Now pick it up, sit down in a chair and place the fermenter on your knees. Shake it vigorously for several minutes to aerate it well.
Now you can pour your cooled wort to the fermenter and not worry about trying to shake the entire five gallons.
The last method mentioned above works well and saves you from lifting the heavy fermenter. This popular method uses an airpump and airstone to bubble air into the fermenter. The only precaution you need to take, other than sanitizing the airstone and hose, is to be sure that the air going into the fermenter is not carrying any mold spores or dust-borne bacteria. To guard against contamination, a filter is used in-line to prevent airborne contamination from reaching the wort. One type is a sterile medical syringe filter and these can be purchased at hospital pharmacies or a your local brewshop. An alternative, build-it-yourself bacterial filter is a tube filled with moist cotton balls. See Figure 41. The cotton should be changed after each use.
Figure 41 - Aeration System
Here is an example of an aquarium air pump using an airstone and a microbial filter for aeration. The filter is a HEPA (medical) syringe filter or alternatively one can be made from a plastic tube, moistened cotton, and rubber stoppers. The moist cotton provides the filtering action and should be thrown away after each use.
Briggs, D.E., Hough, J.S., Stevens, R., Young, T.W., Malting and Brewing Science, Vol. 2, Aspen Publishers, Gaithersburg, Maryland, 1999.
Heggert, H.M., Margaritis, A., Pilkington, H., Stewert, R.J., Dowhanick, T.M., Russel, I., Factors Affecting Yeast Viability and Vitality Characteristics: A Review MBAA Technical Quarterly, Vol. 36, No. 4, 1999.