Whirlpooling is a common practice in North American breweries. It is a post-boil technique, the original purpose for which was to separate trub (mostly hops particles and precipitated proteins) from wort. But in a turn of events that should not be at all surprising to anyone even vaguely familiar with American craft beer, brewers have now adopted the whirlpool as yet another opportunity to introduce hops into the brewing process.
If you’d like to do a whirlpool at home, it helps to first know a little bit about tea leaves.
Mechanics
The whirlpool relies on what physicists call the “tea leaf paradox.” It’s the phenomenon by which light particles, such as tea leaves or trub, collect in the center of a stirred vessel of liquid and settle to the bottom in a neat little pile after the stirring stops.