Why should you aerate the wort before pitching yeast but then try to avoid allowing oxygen into it later in the process? What is going on with the yeast that makes oxygen good at first but bad later?
It is because of the Pasteur Effect.
Yeast are facultative anaerobes, which means they will produce energy aerobically (respiration) when oxygen is present, and anaerobically, by fermentation, in the absence of oxygen. Aerobic metabolism produces about 20 times as much energy as anaerobic, but ethanol (alcohol) is not a by-product.
Having the yeast metabolizing aerobically at first is helpful for two key reasons:
- Yeast will grow in size and mass by storing fatty acids. These fatty acids store oxygen within the cell that may be used later. These make the yeast better at fermenting and increase alcohol tolerance.
- Yeast will reproduce faster to create a higher density population.
Once all the oxygen has been consumed from the wort, you will have a lot of hearty yeast, but little alcohol. However, with the oxygen gone, the yeast will begin using anaerobic metabolism, which produces alcohol (hooray)! Once this happens, you don't want the yeast getting access to any more oxygen from the environment, because they will switch back to aerobic metabolism and quit creating alcohol.
Additionally, presence of oxygen later can cause oxidation to create off-flavoring compounds (e.g., trans-2-nonenal that causes a paper/cardboard flavor.)
It has to do with the yeast lifecycle. Early on, you want yeast to reproduce very fast. This requires oxygen. Later on, you want yeast to more or less maintain itself, and specifically start living in oxygen-free mode, such that it stops giving off early-fermentation byproducts..
*Yeast is somewhat special in that it can operate in both an oxygenated environment and a non-oxygen environment- it just acts a bit different, depending on the case.
Although common practice, adding oxygen into the wort before pitching isn't always a good thing - getting the right amount can be tricky. Too much and the oxygen may stay in the beer causing premature staling. Too little and you may end up with a sluggish fermentation, under attenuation or a banana-easter bomb.
Oxygen is needed by the yeast to synthesize lipids for the cell walls. If lipids can be provided in the wort, then oxygen isn't needed, and the yeast can multiply without requiring oxygen. More details at http://www.homebrewtalk.com/f13/using-olive-oil-instead-oxygen-47872/