At the end of the boil, the wort is set into a whirlpool. This practice of spinning the wort in a circular motion causes the solids and spent hops to pile up in the center of the kettle. These are the solids the brewer wants to remove from the wort.
What is a closed fermentation?
Closed fermentation is the practice of fermenting beer in a closed container, where the CO2 produced during fermentation is allowed to escape, but outside air is prevented from contacting the fermenting wort. Closed fermentation is the method most commonly used by homebrewers in the U.S. Some traditional British and Belgian breweries (and a few American microbreweries as well) use open fermentation, where the fermenters are open to the air.
Originally published in the Trivia Time section of our Craft Beer Club's Micro Brew News, Yazoo Brewing Company edition.