Yeast Flocculation

Brewing yeast flocculation is the most important process for the production of beer that causes the yeast cell to sediment to the bottom of the fermenter at the end of the fermentation. Thus, the brewing yeast can be harvested from the bottom of the fermenter and re-pitching (re-used) for the next batch.

Mechanisms of yeast flocculation

Yeast flocculation is a complicated process that is currently only partly understood. Yeast cells have tendency to adhere each other’s and to build up flocks. These flocks can then precipitate on the bottom of fermentation vessel or rise to the surface of beer. This biochemical mechanism requires the presence of at least two types of molecules on the yeast cell surface. One type is cell wall mannans which are produced by the gene products of the MNN genes and are present on the cell surface at all times. Cell wall mannoprotein seems to be important, higher mannan concentrations are connected to stronger flocculation. The other type is flocculins which are the gene products of the FLO (FLO means yeast with a dominant flocculation gene) genes, that are activated only after depletion of nutrients. The FLO bind to MNN on the surface of yeast cells leading to the cross binding of cells and ultimately the formation of flocs, each consisting of several cells.

Following things have been reported to have influence
• Flocculation is stronger in conditions, which are there at the end of fermentation (more
ethanol, less sugars, lower pH etc.)
• Mother cells have that feature; daughter cells create it after a period of time.
• Mitochondria activity seems to correlate with flocculation.
• Oxylipins on cell wall seem to form some binding sites.
• Mg, energy generation and cytoplasmic protein synthesis induces flocculation.
• Ca is important, same with hydrogen bonding (possibly these two together)

Lectin like adhesion
Lectins are plant proteins, which can react with carbohydrates causing precipitations. Lectins are very specific; e.g. concanavalin A makes bonds only with α-1,6-mannose. Probably today the most commonly accepted mechanism is called “Lectin Like Adhesion”
(LLA- mechanism). So, it is a reaction between some proteins in yeast cell to carbohydrates in another cell. Ca2+ changes the shape of this protein and so, in a way, activates it (possibly tiny amounts of Mn are also needed). Some role Ca has also in binding anionic groups of cell wall surface (prot-COO- etc.)

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