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.
Normally, we associate spinning with a tendency for things to move outward, like when you ride the big swing carousel at an amusement park. What is colloquially called centrifugal force (but is really just your momentum attempting to keep you moving in a straight line) pushes you toward the perimeter as the carousel spins faster and faster, while a centripetal tension force within the swing’s chain keeps you from flying off and landing in the corn-dog stand.
In a rotating volume of liquid, however, there is a solid boundary (the kettle wall) beyond which liquid cannot move. Furthermore, friction at the solid-liquid interface (the wall and the bottom of the kettle) slows the spinning liquid more effectively at the edges than the liquid slows itself in the middle. The resulting pressure difference (the higher the speed, the lower the pressure) induces a secondary circulation that flows down along the wall, then inward along the bottom of the kettle toward the center, up the central axis of rotation, and finally, when it nears the surface, back toward the wall. (Attention science nerds! This is classic Ekman flow.)
Thus, as wort spins within the kettle, there’s a second, weaker flow pattern under the surface that we barely notice unless we’re specifically looking for it. When we stop forcing the liquid to spin, that secondary circulation pulls light particles such as hops and trub toward the center of the kettle, where they deposit themselves neatly into a little cone.
Applications
The reason the whirlpool was invented in the first place was to push sediment toward the center of the kettle after the boil. Once all of the trub is collected in a nice compact cone, a dip tube near the perimeter can cleanly drain clear wort out of the kettle, through the chiller, and into the fermentor. It’s a great way to promote wort clarity and keep a plate chiller from getting clogged.
However, craft brewers have long used the whirlpool as an additional place to introduce hops. Why? Well, the whirlpool is conducted while the wort is still hot—remember, it hits the chiller afterward. The whirlpool might last for 15 to 30 minutes, so crafty brewers decided to steep extra hops in this spinning vat of hot wort to extract additional flavor, aroma, and even bitterness. It’s sort of like an extended hopback.
DIY Whirlpool
So how do you execute a whirlpool at home? Well, the pros actually pump wort from the base of the kettle and reinject it near the surface of the wort. It’s pumped in tangentially to the kettle wall at high speed, which sets up the spinning pattern, and once the whirlpool gets going, it keeps going until they shut off the pump and wait for it to settle down.
At home, there are a few ways to do this. Some retailers now sell special-built kettles that feature a tangential inlet near the top, for the express purpose of pumping in wort from the bottom and creating the desired effect. You can do the same thing without such an inlet if you just place the outlet of the pump’s hose beneath the surface of the wort, although you’ll want some way of securing the hose in place.
The cheapest option, though, is to just grab the biggest spoon you’ve got and give that kettle of wort a good stir. I actually like to do this with an immersion wort chiller so that the wort cools down during the whirlpool. This method is only limited by your arm’s stamina. If you want to use this time to introduce additional hops, add them when you start stirring or pumping so that they have contact with the hot wort while you get it started.
Regardless of how you set up your whirlpool, it’s a good idea to keep it moving for at least a couple of minutes to ensure that the entire volume is moving and that the hops particles and protein trub are nicely suspended throughout the wort. Then, when you’re ready to stop, just cut off the pump (or stop stirring) and let the whole thing sit, undisturbed for at least 15 minutes—preferably 30—to let the trub settle out.
If you chill your wort as it whirlpools, it’s a good idea to have a sanitized lid or some sanitized aluminum foil on hand to cover the pot while everything settles. If, however, you whirlpool first and then chill after the trub cone has developed, it’s best to leave the kettle uncovered so that dimethyl sulfide (DMS) precursor s-methyl methionine (SMM) and other volatile compounds don’t get trapped within. No need to worry about contamination as long as the wort remains hot enough to kill nasties (above 180°F/82°C).
Hop Stand
If all of this bother with spinning wort isn’t your cup of tea, you can still extract additional hops character after the boil by conducting what is known as a hop stand. This is super simple and requires very little effort on your part. All you do is have your whirlpool hops ready to go when you cut the flame on the boil and then add them to the hot wort. Then you just wait for the amount of time specified in the recipe. That’s it.
So, if you decide to brew up Matt Gallagher’s Less Thinking IPA, and you don’t want to actually bother with the whirlpool, you can just add the 1.5 oz (43 g) of Amarillo and 0.5 oz (14 g) of Centennial at knockout and then wait—anywhere from 10 minutes to an hour is fine. The longer you wait, though, the more IBUs you’ll get out of the hops, so if you perform a hop stand using a high alpha-acid variety in a recipe of your own design, you might want to take your bittering charge down a notch or two to account for those extra iso-alpha acids.
Other Considerations
The whirlpool (or hop stand) is typically conducted just off the boil, so a little shy of 212°F (100°C) in most locales at or near sea level. That’s high enough a temperature to extract bitterness from any whirlpool hops you add and to make infection rather unlikely.
But if it’s only flavor and aroma you want, you could always cool the wort to about 150°F (66°C) first and then conduct your whirlpool or hop stand. At this temperature, you’ll extract delicate hops oils without isomerizing alpha acids. Contamination is more of a concern at these temperatures, though, so be extra vigilant with your sanitation routine.
Whether you pump, stir, or stand, the period immediately following the boil is a great opportunity to bring in hops character and clarify wort on its way from the kettle to the fermentor. And when you’re brewing a great American IPA, clarity and hops are the name of the game.