At what stage is dry hopping best performed?

Dry hopping is best performed when fermentation is actively taking place, specifically during the peak of fermentation when CO2 production has slowed but is still occurring. At this stage, known as the "active fermentation" phase, the yeast is still in motion, and the moderate temperatures and CO2 levels help to enhance the extraction of hop oils.

Adding hops at this point can contribute to a variety of desirable flavors and aromas without introducing excessive bitterness, which tends to happen with early kettle additions. The presence of yeast during this time can also enhance the biotransformation of hop compounds, potentially leading to more complex and nuanced hop flavors and aromas in the finished beer.

The other stages mentioned do not facilitate the same benefits as dry hopping during fermentation. Dry hopping prior to fermentation may lead to a loss of aroma and flavor as the hops do not have the same interaction with the yeast. Adding hops after filtration would not allow for the intended aromatic contributions since many volatile hop compounds would not be as effectively captured. Lastly, dry hopping during the cooling process does not utilize the active yeast fermentation to its full advantage, leading to a less dynamic extraction of flavors and aromas.