Yeast growth is actually limited by volume, dissolved O2
, and the amount of carbon available to the cells (for those who do not known, sugar is carbon bound to water; hence, the name carbohydrate). Maximum cell density is maximum cell density. If volume had no bearing on cell count, then a culture would remain in the exponential phase for the length of a fermentation. Conversely, if a culture runs out of carbon before it reaches maximum cell density, it will never reach maximum cell density.
The interesting thing about dissolved O2
is that the load placed upon the medium is not solely dependent on the health of the cells when they are pitched. Different strains have different O2
requirements. A scientist named of Brian H. Kirsop outlined four classes of O2 demands by yeast strains.http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1974.tb03614.x/pdf
Class O1: Yeasts whose requirement is satisfied if wort is half saturated with air (4ppm dissolved O2
Class O2: Yeasts whose requirement is satisfied by air-saturated wort (8ppm is the maximum dissolved O2
from air at sea level)
Class O3: Yeasts whose requirement is satisfied by oxygen-saturated wort (40ppm dissolved O2
Class O4: Yeast whose need is not satisfied by oxygen-saturated wort (> 40ppm dissolved O2
I am currently working with an O3/04 yeast strain. I am almost certain that the strain is the John Smith culture, which is related to the Samuel Smith culture. I am willing to bet that most of the commercial yeast strains sold by Wyeast and White Labs are class O1 and class O2 strains, with easy to use strains such as 1056/WLP001 and 1098/WLP007 being class O1 strains.