So, what concentration of dissolved oxygen would one be shooting for here? The concept of foam seems imprecise to me. Obviously we've all heard of the 8ppm limit of solubility in wort at room temperature. Is the foam continuously aerating it as it settles? Kai has done an experiment with an aquarium pump and sterile filter pushing air through a tube that rests just above the surface (to avoid foaming), and it seems like this would be similar.
There are few unknowns at this point. For example, what difference does it make if the culture is pitched before shaking? Does any advantage gained offset the exposure to physical stress? There have to be cells on the surface of the bubbles. Those cells, like a culture that is growing on solid media, are in an environment where the O2
level is 21 parts per hundred (air is 21% O2
), not 8 parts per million (0.0008% O2
). It takes several minutes for the foam to fall using this method. Chris White has mentioned that the yeast cells consume all of the dissolved O2
within thirty minutes of being pitched, which means that the cells in the foam are taking in O2
while the media is in a gas-foam state.
With that said, does being exposed to air make a difference? How many yeast strains need more than 8ppm dissolved O2
? Having just wrestled with a class O3 (40ppm O2
)/class O4 (> 40ppm O2
strain, I can honestly say that most of the strains available to home brewers are class 1 (4ppm dissolved O2
) or class 2 (8ppm dissolved O2
). A yeast strain with a high dissolved O2
requirement is a very different animal.