In terms of fermentation, aeration is also important but only in the early stages (first 6-24 hours). Aeration in later stages can oxidize beer constituents and lead to the development of off-flavors. Since aeration sets the stage for maltose fermentation and alcohol tolerance, it is easy to envision why insufficient aeration could lead to stuck fermentations or incomplete fermentations. Incomplete fermentations can be manifested as either high finishing gravities or the production of off-flavors especially diacetyl, acetaldehyde, and hydrogen sulfide. Insufficient aeration is also associated with excessive ester formation. The profound effect of aeration on yeast is further illustrated in studies where yeast from a poorly aerated beer was repitched into aerated wort and still did not perform well. Thus insufficient aeration can have a long-lasting effect on yeast.
In general, it is difficult for homebrewers to achieve sufficient oxygen levels. The levels of oxygen necessary for optimal fermentation vary depending on the yeast strain. Ale strains usually need between 8-12 part per million (ppm) while lager strains require slightly higher amounts (10-15 ppm). At atmospheric pressure the maximum level of dissolved oxygen in wort is approximately 8 ppm and the saturation level decreases further as the gravity of the wort increases. Thus unless special steps are taken to introduce air or oxygen into the wort, it is difficult for homebrewers to achieve adequate aeration. Recent studies have shown that oxygenation is by far more efficient than aeration. Injection of oxygen through a 2 micron diffusing stone can actually supersaturate the wort with 10-12 ppm of dissolved oxygen being reached in 5 gallons of wort by a single 60 second blast of oxygen!