Correct me if i'm wrong
Yeast has 3 phases:
1. anaerobic - reproduce
2. aerobic - eat
3. cold – sleep
No, yeast cells normally go through three major phases. The first phase is the called the lag phase. During the lag phase, the yeast cells adapt to their new environment and prepare to bud. The next phase is called the log or exponential phase. During the exponential phase, yeast cells are budding (multiplying) like crazy. The final phase is called the stationary phase. Depending on whose work one reads, each of the major phases has one or more sub-phases.
The thing that confuses most people is that yeast cells have two separate metabolic pathways. The first metabolic pathway is called the respirative (aerobic) metabolic pathway. The respirative metabolic pathway is extremely efficient. It basically converts sugar to water and carbon dioxide gas. The second metabolic pathway is called the fermentative (anaerobic) metabolic pathway. It is the pathway through which sugar is converted primarily to ethanol (which is a carbon-based compound) and carbon dioxide gas. The fermentative metabolic pathway is not anywhere near as efficient as the respirative metabolic pathway.
Many older home brewing texts erroneously refer to the growth phase as respiration. Brewing yeast cells do not respire in beer wort due to something known as the Crabtree Effect. The Crabtree Effect states that yeast cells will favor fermentation over respiration when subjected to dissolved glucose levels above the Crabtree threshold, and they will do so even in the presence of dissolved oxygen (O2). The Crabtree threshold is around 0.3% weight/volume (w/v).
To put things into context, the extract from the average mash contains roughly 14% glucose, which means that wort with a specific gravity above 1.008 contains a glucose level above the Crabtree threshold.
Here's the math:
A 1.008 solution is a 2% sugar weight/weight (w/w) solution, which is the same thing as w/v when dealing with a solute dissolved into water because 1ml of water weighs one gram. Of that 2%, only 14% is glucose; hence, 0.02 x 0.14 = 0.0028, or 0.28% glucose w/v, which is below the Crabtree threshold.
As all beer and batch yeast propagation worts have specific gravities above 1.008, yeast biomass growth in brewing is fermentative. Now, the metabolic pathways in yeast cells are a little on the leaky side. What yeast cells do while there is still O2 in solution is shunt a small percentage of the glucose being consumed to the respirative metabolic pathway for the synthesization of ergosterol and unsaturated fatty acids (UFA). These compounds make cell membranes more pliable, which, in turn, make passage of nutrients and waste products in and out of the cells easier.
With this knowledge, it's better to keep a yeast starter barely covered to stop contamination, but allow breathing.
I've seen a lot of people use airlocks on their flasks
I am sorry to inform you, but the no airlock argument is also for the most part home brewer pseudo-science. CO2 is heavier than air. Plus, the culture is under positive pressure; hence, little to no O2 makes it into solution from the atmosphere after CO2 production begins, especially in an Erlenmeyer flask. The only way to ensure that a culture receives a continuous supply of O2 is to use forced aeration throughout propagation.
With that said, there is a way to propagate yeast respiratively, but it requires a hi-tech piece of equipment known as a bioreactor. This type of propagation differs from how brewers propagate yeast. Brewers use batch propagation. Respirative propagation is a continuous process in which nutrient and O2 are continuously added to the medium while yeast cells are continuously drawn off. A bioreactor makes this process possible because it can hold the dissolved glucose level in a steady state below the Crabtree threshold. Lallemand and Lesaffre (the parent company of Fermentis) use this type of propagation to produce dry yeast cultures. Respirative growth is more efficient than fermentative growth; hence, more yeast cells can be produced using the same amount of carbon (sugar is basically carbon bound to water). Respirative growth also has the added advantage of continuous ergosterol and UFA production; hence, the yeast cells that are produced via the process do not need to undergo ergosterol and UFA replenishment after being pitched. As mentioned above, yeast cells use the O2 that is solution at the beginning of fermentation to synthesize ergosterol and UFAs via the respirative metabolic pathway. Pitching fully-charged yeast cells basically eliminates the need to aerate one’s wort.
For anyone who is interested in learning more about Lallemand’s yeast propagation process. Here’s a link a to short paper that describes how they propagate baker’s yeast in layman's terms. Baker’s yeast and brewer’s yeast are the same yeast species; namely, Saccharomyces cerevisiae (S. cerevisiae). The strains only differ in the application at which they excel.http://www.lallemand.com/BakerYeastNA/eng/PDFs/LBU%20PDF%20FILES/1_9YPROD.PDF
“Growing via respiration is important because it is about eighteen times as efficient as fermentation at converting sugar into yeast. The tendency of yeast to grow via respiration when large amounts of oxygen are present is known as the Pasteur effect. The tendency of yeast to grow via fermentation when high levels of sugar are present is known as the Crabtree effect. The combination of Pasteur and Crabtree effects in Saccharomyces cerevisiae is the reason commercial bakers yeast fermentations use high aeration and incremental feeding to maintain high oxygen and low sugar levels throughout the process.”