Fermentation - Aeration is Good, Oxidation is Bad
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 growth—the 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
- Ppouring 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 distiller, 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. 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 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. The cotton should be changed after each use.
Aeration is Good, Oxidation is Bad
The yeast is the most significant factor in determining the quality of a fermentation. Oxygen can be the most significant factor in determining the quality of the yeast. Oxygen is both your friend and your enemy. It is important to understand when which is which.
You should not aerate when the wort is hot, or even warm. Aeration of hot wort will cause the oxygen to chemically bind to various wort compounds. Over time, these compounds will break down, freeing atomic oxygen back into the beer where it can oxidize the alcohols and hop compounds producing off-flavors and aromas like wet cardboard or sherry-like flavors. The generally accepted temperature cutoff for preventing hot wort oxidation is 80°F.
Oxidation of your wort can happen in several ways. The first is by splashing or aerating the wort while it is hot. Other beginning-brewing books advocate pouring the hot wort after the boil into cold water in the fermenter to cool it and add oxygen for the yeast. Unfortunately the wort may still be hot enough to oxidize when it picks up oxygen from the splashing. Pouring it down the side of the bucket to minimize splashing doesn't really help either since this increases the surface area of the wort exposed to the air. Thus it is important to cool the wort rapidly to below 80°F to prevent oxidation, and then aerate it to provide the dissolved oxygen that the yeast need. Cooling rapidly between 90 and 140°F is important because this temperature region is ideal for bacterial growth to establish itself in the wort.
In addition, if oxygen is introduced after primary fermentation has started, it may cause the yeast to produce more of the early fermentation byproducts, like diacetyl. However, some strains of yeast respond very well to "open" fermentations (where the fermenter is open to the air) without producing off-flavors. But even for those yeast strains, aeration or even exposure to oxygen after fermentation is complete can lead to staling of the beer.
To summarize, you want to pitch a sufficient amount of healthy yeast, preferably grown in a starter that matches your intended fermentation conditions. You want to cool the wort to fermentation temperature and then aerate the wort to provide the oxygen that the yeast need to grow and reproduce. Then you want to protect the wash from oxygen once the fermentation is complete to prevent oxidation and staling.