is a device for continuous beer fermentation or processing. See continuous fermentation. In conventional batch fermentation, yeast ferments wort while it is in mobile suspension. In immobilized yeast reactors, by contrast, yeast is kept stationary, while wort passes by it. Beer can emerge from this fermentation essentially clear and yeast free. There are two common types of yeast immobilizers. In a fluidized bed reactor, porous glass beads harbor the yeast and float through the beer, whereas in a loop reactor, a fixed, porous silicon carbide (or other ceramic) cartridge contains the immobilized yeast. Because wort flows continuously past the yeast cells, the rate of fermentation is not dependent on the distribution of live yeast cells throughout the fermenter. And attenuation can be rapid, with fermentations requiring half the time they do in conventional batch processing. There are drawbacks, including an elevated level of vicinal diketones and their precursors in the finished beer. See diacetyl, vicinal diketones. These can be reduced by raising the beer temperature to convert the precursors to diacetyl and then reprocessing the beer through the reactor again.

In the commercial world, although much research has been done, immobilized yeast reactors have not been made efficient enough to supplant normal batch fermentation. Much research, therefore, concentrates on ethanol production. However, some large breweries do use immobilized yeast reactors for fast diacetyl reduction. During the summer months, when demand is high and tank space is at a premium, some large European breweries process green beer by heating it to force diacetyl from precursors and then using the reactor to absorb the diacetyl. By this method, diacetyl reduction that would normally take weeks is achieved within hours, allowing the breweries to produce acceptable “lager” beer within 14 days from brewing. Immobilized yeast reactors can also be used to conduct partial fermentations used to make certain types of low-alcohol beers.