Yeast Nutritional Needs
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. During racking to a secondary fermenter or to the bottling bucket, it is very important to prevent gurgling or splashing. Keep the siphon flowing smoothly by placing the outlet of the siphon hose below the surface of the rising beer. Decrease the difference in height between the two containers when you begin. This will slow the siphon rate at first and prevent turbulence and aeration until the outlet is beneath the surface.
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 beer from oxygen once the fermentation is complete to prevent oxidation and staling.
In the next couple chapters, I will walk you through brewing a batch, and we will apply the principles we have discussed.