Yeast Flocculation
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Yeast flocculation is an important process for the production of beer that causes the yeast to sediment to the bottom of the fermenter at the end of the fermentation. Thus, the yeast can be harvested from the bottom of the fermenter and used for the next fermentation, while the beer may be matured without the need of a centrifugation step.

Ideally, brewing yeast does not flocculate at the beginning of the fermentation, but only after all the nutrients have been used up. However, depending on the conditions, the yeast may initiate flocculation either too early or too late, leading to either improper fermentation or the need of centrifugation, respectively. In order to improve the control of flocculation during beer production the genetic mechanisms of flocculation are being studied.

Yeast flocculation is a complicated process that is currently only partly understood. It requires the presence of at least two types of molecules on the yeast cell surface. One type is mannans (carbohydrate chains), which are produced by the gene products of the MNN genes and are present on the cell surface at all times. The other type is flocculins (sugar binding proteins), which are the gene products of the FLO genes, that are activated only after depletion of nutrients. The flocculins bind to mannans on the surface of neighboring cells leading to the cross binding of cells and ultimately the formation of flocs, each consisting of several cells. Due to the reduced surface to volume ratio of the aggregated cells, the flocs sediment much faster relative to the free cells.

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