There are 4 growth phases in a yeast’s life cycle. There is no specific time period for each phase, rather each phase may overlap one another.
1. Lag Phase
In order for any of us to get things done we must have energy. And where do we get that energy from? Food. Maslow did know a thing or two about human needs. Well the same thing goes for yeast. In order for yeast to begin working, it must have energy. And yeast obtains their energy through their stored glycogen that is broken down into glucose. Now that the yeast has energy, the cell can begin reproduction. If the yeast has a low glycogen storage then the fermentation process will be longer. Glycogen serves as the yeast’s primary source of energy until its membrane becomes permeable to the sugars in the wort. During this stage the pH will be low because there is low level of oxygen and phosphate. The oxygen levels will be lowered since the yeast utilizes oxygen to make sterols and unsaturated fatty acids (both are important growth factors). These growth factors support the yeast’s cell membrane and its ability to adapt to its environment.
2. Growth Phase
Because the yeast prepared itself during the lag phase, it can finally begin rapidly multiplying its cells and begin consuming the sugars in the wort. The yeast reproduces asexually by forming a daughter cell that is the exact genetic copy of itself. The yeast uses the oxygen in the wort to oxidize acidic compounds. The wort will have a foamy layer due to the expulsion of carbon dioxide. The pH will also drop due to oxidization.
3. Fermentation Phase
This phase occurs once all the oxygen in the wort has been depleted. Carbon dioxide, ethanol, and beer flavors are produced during this stage. The yeast is in suspension and will remain in suspension for 3-7 days. The carbon dioxide bubbles you see are oxygen escaping the wort.
4. Sedimentation Phase
Yeast in this phase flocculates (clumps together) and settles to the bottom of the fermenter when fermentation ends. Since there is no more sugar and nitrogen in the wort, the yeast will begin to produce glycogen to prepare itself for “hibernation” as the glycogen will serve as its energy source in the lag phase.