Chapter 4

Brewers Yeast

Yeast Life Cycle

The life cycle of yeast is activated from dormancy when it is added (pitched) to the wort. Yeast growth follows four phases, which are somewhat arbitrary because all of the phases may overlap in time: 1) the lag period, 2) the growth phase, 3) the fermentation phase, and 4) the sedimentation phase.

Lag Phase

Reproduction is the first great priority upon pitching, and the yeast will not do anything else until food reserves are built up. This stage is marked by a drop in pH because of the utilization of phosphate and a reduction in oxygen. Glycogen, an intracellular carbohydrate reserve, is essential as an energy source for cell activity since wort sugars are not assimilated early in the lag phase. Stored glycogen is broken down into glucose, which is utilized by the yeast cell for reproduction – the cell’s first concern. Low glycogen levels produce abnormal levels of vicinal diketones (especially diacetyl) and result in longer fermentations.

Growth Phase

The growth phase, often referred to as the respiration phase, follows the lag phase once sufficient glycogen reserves are built up within the yeast. This phase is evident by the covering of foam on the wort surface due to the liberated carbon dioxide.

Fermentation Phase

The fermentation phase quickly follows the growth phase when the oxygen supply has been depleted. Fermentation is an anaerobic process. In fact, any remaining oxygen in the wort is "scrubbed," i.e. stripped out of solution by the carbon dioxide bubbles produced by the yeast.

Sedimentation Phase

The sedimentation phase is the process through which yeast flocculates and settles to the bottom of the fermenter following fermentation. The yeast begins to undergo a process that will preserve its life as it readies itself for dormancy, by producing a substance called glycogen.

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