The more I read about yeast and fermentation, the more conflicting, confusing (and possibly incorrect) statements I come across.
I have read in several bits of literature that yeast requires oxygen for biosynthesis (i.e. cell growth) and prefers to use it for this purpose, to which end it absorbs oxygen shortly after pitching (i.e. during the lag phase and before fermentation begins). Many sources indicate that almost all the dissolved oxygen is absorbed by the yeast within 30-180 minutes after pitching and is thereby removed from the wort.
Which begs the question if there is such a thing as "aerobic fermentation" in brewing. I mean, if the oxygen is being removed from the wort, aerobic fermentation (fermentation in an environment that exposes the yeast to oxygen, which not necessarily means the yeast will use oxygen for fermentation) is impossible without adding more oxygen after the lag phase (which, to the best of my knowledge, is neither recommended nor common practice).
As I understand it, yeast absorbs oxygen and nutrients from the wort during the lag phase and uses it for the synthesis of ATP, sterols and fatty acids which are stored in the cells. Then, following the lag phase, anaerobic fermentation starts, during which stage most cell growth takes place.
- Does aerobic fermentation has a place in this? If so, when and how, and where does the oxygen come from?
- Does the yeast use sugar for biosynthesis (i.e. cell formation)?
- What about anaerobic cell growth? I remember reading somewhere that yeast can multiply anaerobically, but the cells produced differ from those synthesized aerobically.
"One fool can ask more questions than ten wise men can answer."