Centrifugation is the application of radial forces upon an object by moving it in a circle. The object can be gaseous, liquid, or solid. The centrifugal forces increase with an object’s density. They also increase with the speed with which the object moves in a circle. The radial force pushes objects toward the outer border of the circle being transited, just as we feel ourselves pressed to the door of a car as it rounds a tight curve.
In a brewery centrifuge, which may also be called a separator or a decanter, these same principles are applied to a liquid—wort or beer. The liquid is laden with various types of suspended particulate, including yeast, trub, and hop residue, each with a different density. When a centrifuge spins the liquid within a round chamber, at a given rotation speed, eventually the heavier components of the liquid, notably trub and yeast, because they experience greater centrifugal forces, will move closer to the chamber’s outer wall. Meanwhile, the lightest components, including water and alcohol, will stay closer to the center axis of the rotation chamber. This type of separation is aided further by the fact that the circumference of the circle traveled by the liquid closest to the center is much smaller than the one traveled by the particles along the outer wall. This even further reinforces the separation of particles in the liquid according to their density. In terms of physics, the power of the centrifugal forces is governed by an equation called Stokes’ law, and centrifuges in a brewery are constructed to generate truly enormous rotational forces that may be several thousand times the size of the earth’s gravitational forces. See stokes’ law. Such devices also allow for the draining of the clear, separated liquid from the rotation chamber, while leaving behind the layers of particulate along the wall of the chamber. This is the principle of a functioning centrifuge in practice. Centrifuges in the brewery are employed in several ways. A wort centrifuge can separate hot trub from clear wort even more efficiently than a whirlpool, which is actually a type of low-tech centrifuge. See whirlpool. A beer centrifuge employed before packaging can reduce the beer’s turbidity by separating many or most of the yeast cells from the clear beer. This is particularly useful if the beer is to be packaged unfiltered, but the yeast does not flocculate sufficiently to the bottom of the fermenter. It is possible to link the centrifuge to an optical sensor that measures the turbidity of the beer; properly used, this setup can be used to adjust haze or yeast cell counts in the final package. Likewise, a centrifuge can remove some of the yeast between the end of primary fermentation and the beginning of secondary fermentation or before lagering. If the brewery uses fining agents, these, too, can be removed via centrifuge. See finings and kettle finings. Another favorite location for the use of centrifuges is right before an in-line filter. See filtration. Because the centrifuge removes the bulk of the particulate, the filter medium can perform its function for much longer intervals before it needs to be serviced. A completely different purpose of a centrifuge is beer reclamation. When excess or spent yeast slurry is purged out of the bottom of a fermenter, the liquid part of the slurry is of course perfectly drinkable beer. Some breweries consider it worthwhile to centrifuge the slurry and introduce the reclaimed beer back into the beer flow. Finally, beer can be centrifuged to remove any residual cold break right before packaging. This eliminates, among other substances, some of the protein-type of materials that are directly and indirectly involved in haze formation as well as beer oxidation. See colloidal haze and oxidation.
The centrifuge is a useful piece of equipment for a brewery, but they are expensive. As a result, it is unusual to see a centrifuge in a brewery producing much less than 118,000 hl (100,000 US barrels) per year.