In dealing with a cell like yeast, it occurs to me that there are a lot of questions in my mind about what causes yeast to grow. All cells need the proper environment to grow. They need food, the correct temperature and the right fluid / gas exchange. Given the proper environment, cells will divide, consuming nutrients and producing a preferred product. For yeast, the product would be CO2 for baking, or alcohol in beer making. Different types of yeast you would assume would need different food and may change characteristics based on the environment.
We know in our work at Kairos Instruments, that hematopoietic stem cells for example are said to need a niche, a special place, to grow and divide to produce cells for our blood system. In the case of yeast, an environment without oxygen allows the cell to consume sugar and produce alcohol, but in the presence of oxygen, no or little alcohol is produced. So, environmental conditions effect the outcome of cell division. A question to be answered then is what nutrients and how much are needed to establish good growth.
A second issue is the temperature. Most lager yeasts need a colder temperature to work properly and most ales work best around room temperature. Changes in temperature affect the yeast efficiency and in many cases the flavor component will also be affected. The next issue in my mind is the amount of yeast present in the experiment. In many cell types, if the starter cell population is fewer and fewer, the division rate is affected. So, maybe they excrete something that the other cells need? Is that also true with yeast, or is there just a delay until there is enough division to get to a large enough population to consume all the sugar in the environment. The fluid / gas interface is also an important issue. In human cells, the gas environment needs to be held at 5% CO2 and possibly reduced oxygen as well, to produce the correct environment. For yeast, the presence of oxygen reduces the ethanol production, but not necessarily the CO2 production. You don’t have to have a reduced oxygen environment to bake bread! In any case, how much oxygen present is a question and what effect that has on the cells output is something to explore.
So, where to start. Its always, best to reduce the number of variables, so we started with one type of yeast in a well and just varied the amount of grain extract vs just sugar in water. In this experiment we looked at growth rate. We used 96 well plates, so we could look at various concentrations of extract and sugar, cell populations (number of cells in the view field) and different types of cells (different yeast strains). We looked at 10X magnification and then 20X magnification. That’s a typical experiment, we scanned the 96 well micro titer plate every 10 minutes for 3 to 5 days. In general we found that these cells need more than sugar to survive.