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.
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.
The growth phase, often referred to as the respiration phase, follows the lag phase once sufficient reserves are built up within the yeast. This phase is evident from the covering of foam on the wort surface due to the liberated carbon dioxide. In this phase, the yeast cells use the oxygen in the wort to oxidize a variety of acid compounds, resulting in a significant drop in pH. In this connection, some yeast strains will result in a much greater fall in pH than others within the same fermenting wort.
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. This phase is characterized by reduction of wort gravity and the production of carbon dioxide, ethanol, and beer flavors. During this time period, yeast is mostly in suspension, allowing itself dispersal and maximum contact with the beer wort to quickly convert fermentables. Most beer yeasts will remain in suspension from 3 to 7 days, after which flocculation and sedimentation will commence.
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. Glycogen is necessary for cell maintenance during dormancy and, as mentioned, is an energy source during the lag phase of fermentation.
Click on the following topics for more information on brewer's yeast.