Flotation Tank is a vessel used to separate cold trub from cold wort in the fermentation cellar. See trub. The flotation method relies on sterile, compressed air—between 30 and 70 l (1014 and 2367 oz) of air per hectoliter (26.4 gals)—being percolated into the wort at the beginning of fermentation. The air injection, usually using a ceramic or metal carbonating stone, is best performed within roughly 6 to 8 hours after pitching. Importantly, the yeast must be given enough time before the start of flotation to absorb trace nutrients, especially sterols, from the trub material. However, some brewers perform flotation prior to pitching yeast. As the flotation air is dispersed in the wort, it not only aerates the wort and thus supplies the needed oxygen for the yeast to get ready for its reproductive and metabolic cycles but also forms small bubbles in the wort, to which cold-trub particles cling. As the bubbles float to the surface, they drag the attached trub particles with them. At the top of the wort, the bubbles coalesce into a brown, firm, and compact layer of foam. As time passes, this head of foam may even dry out and become crusty. Once the beer starts fermenting, however, the liquid is racked out from under the trub-laden foam head, leaving the unwanted sediment behind. If the flotation tank is open, the foam layer may also be carefully skimmed off by hand. See skimming.

Although the exact tank shape—horizontal or vertical—for floatation is secondary, tank geometry is important. The depth of the tank’s head space must be at least 30% to 50% of the beer’s depth. This means that cylindroconical tanks are often too slender for effective flotation. Such tanks would cause an accelerated updraft as the bubbles made their way up the long rise, leading to inhomogeneous flotation at the surface. In cylindroconical tanks, the foam head is also more likely to collapse into the beer during racking, defeating the purpose of the operation. Flotation is most effective if the wort depth does not exceed 4 m (approximately 12 feet). A second, fresh batch of beer may by pumped in underneath an already floatation-clarified wort, in which case the combined worth depth should not exceed 6 or 7 meters (18 to 21 feet). The air bubbles passing through the bottom batch will continue to rise through the top batch and scrub even more trub out of that batch. To keep the surface foaming from becoming too vigorous during flotation—which could prevent the head of foam from firming up properly—the tank can be put under slight pressure, up to about 0.5 to 0.8 bar (roughly 7 to 12 psi). After the beer has been racked, the tank must receive a thorough cleaning, often manually, to remove all crusted-on foam residues.

A key reason for removing cold trub is the trace residues of undesirable compounds it contains. These include varying amounts of fatty acids, polyphenols, undegraded carbohydrates and proteins, and even heavy metals, such as copper and iron, which can take part in oxidation reactions. Reducing these residues can improve the beer’s shelf life. However, the practice of cold trub is controversial, with many brewers believing that it can strip wort of nutrients necessary for good fermentations and yeast health. Flotation remains popular in Germany and some parts of Europe, but the technique has become rare in the United States.