When is the best time for a brewer to aerate the wort?

Aerating the wort after it has been chilled is critical because this is the optimal time to introduce oxygen into the wort to support yeast health and fermentation. When the wort is chilled, it is no longer boiling, which means that it is also less likely to drive off volatile compounds and flavors, potentially resulting in a cleaner and more stable fermentation environment.

Introducing oxygen while the wort is still hot, such as during boiling, can lead to oxidative reactions that may negatively affect the beer's flavor and can also lead to the development of undesirable off-flavors. Similarly, aerating before boiling is not practical, as the boiling process will not only sterilize the wort but also impede the viability of any bacteria or wild yeast that might be introduced alongside the oxygen.

Aeration just before bottling is also less effective because the yeast has already begun fermenting, and the introduction of oxygen at this stage can lead to oxidation and spoilage in the finished beer, which can result in staling and off-flavors. Thus, chilling the wort before aeration maximizes the chances of a healthy fermentation and ultimately contributes to a better quality finished beer.