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: (1) the lag period, (2) the growth phase, (3) the fermentation phase, and (4) the sedimentation phase. During the lag phase, the cell count does not change. During the growth or exponential phase, cell division occurs, resulting in a rapid increase in total cell count. During the fermentation phase, the rate of cell division is the same as the rate of cell death; thus, total cell count reaches a plateau. Finally, during the sedimentation phase, cell death prevails, and the total cell count declines.
The Lag Phase
The lag phase starts when the yeast is pitched into the aerated wort. During this phase, the yeast cells acclimate to the environment, assimilate or absorb nutrients, and prepare for cellular division. There is little to no fermentation during the lag phase. This stage is marked by a drop in pH because of the utilization of phosphate and a reduction in oxygen.
The 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. During this phase, yeast cells are growing logarithmically. Growth of yeast cells (sexual reproduction) is by budding, which is a form of cell division. The absolute amount of yeast growth is roughly proportional to the nutrients available in the wort. Commercially, one to three doublings in the cell number will occur. It is during the growth phase that most of the beer flavor, alcohol, carbon dioxide, and heat are produced.
The 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.
The Sedimentation Phase
The sedimentation phase is the process through which yeast flocculates and settles to the bottom of the fermenter following fermentation. During this phase, the specific gravity of the beer approaches its terminal point, and the yeast will begin to flocculate.
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