Flocculation is the tendency of yeast cells to aggregate together, forming a multicellular mass and sedimenting rapidly from the suspended medium or rising to the surface. Yeast flocculation is a complex phenomenon occurring in brewer’s yeast under various conditions near the end of the fermentation process. It is a matter of fundamental importance to brewery fermentations.

Flocculation involves the interaction of cell wall proteins of one cell adhering to a receptor site of another cell. Lager yeast strains aggregate into large groups, rapidly sedimenting to the bottom of the fermenter, while ale strains may rise to the surface to form a thick top layer. Flocculation is a reversible process wherein yeast cropped from either the top or bottom of a fermenter and re-pitched into a sugar-rich solution will become disassociated and re-suspended in the wort.

Yeast specialists and brewers often categorize yeast flocculation behavior as being “high,” “medium,” or “low.” High flocculation type strains start to flocculate the earliest, which can leave behind some unfermented sugars or unwanted flavor compounds such as diacetyl. See diacetyl. Some highly flocculant strains require recirculation of the beer late in the fermentation to re-suspend the yeast, allowing the fermentation to finish to the brewer’s satisfaction. Highly flocculant strains do have advantages—namely, they can produce a brighter beer with less suspended yeast, making filtration easier. Producers of cask-conditioned beers will often desire this quality as well, as they want the yeast to drop quickly to the bottom of the cask when fermentation is complete.

Medium flocculation type strains tend to be some of the more commonly used strains and will start to flocculate out as sugars become less abundant. Often they need help in order to flocculate, and this can be accomplished by cooling the fermenter to lower temperatures.

Finally, low flocculation strains are those that stay in suspension well after fermentation has ended. These strains tend to be wheat beer strains in which low flocculation is desirable, as the finished beers are meant to be hazy with yeast.

In working breweries, yeast used over a number of generations can be selected in a manner that may increase or decrease a yeast strain’s flocculative tendencies. In a fermenter with a cone bottom, yeast collected from the bottom of the cone will be the yeast that flocculated early, while yeast collected from the top layer of sediment will be yeast that flocculated last. By continuously selecting for these tendencies, many brewing strains can be “trained” to perform in a manner that better fits the brewery’s needs.