Is aerobic fermentation in beer brewing a myth?

Question

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.

My questions:

1. Does aerobic fermentation has a place in this? If so, when and how, and where does the oxygen come from?
2. Does the yeast use sugar for biosynthesis (i.e. cell formation)?
3. 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."

Answer 1

1) Does aerobic fermentation has a place in this? If so, when and how, and where does the oxygen come from?

Aerobic metabolism is a more efficient pathway to convert the glucose to cellular energy and therefore is always taken when O2 is present which is only during the initial phase. The time it takes until oxygen depletion occurs depends on population, headspace, temp, mixing, etc.

2) Does the yeast use sugar for biosynthesis (i.e. cell formation)? What about anaerobic cell growth?

The cell depends on sugar for all its energy. Some energy goes to synthesis of metabolic end products some goes to biomass generation; the ratio and end products of synthesis depend on envronmental conditions such as the level of O2 or glucose presence.

Biomass growth will occur in both aerobic and anaerobic conditions, but it will occur at a higher rate when oxygen is present - which is why stir plates are used to create yeast starters. Ethanol synthesis begins regardless when >5 degree plato is reached to inhibit the growth of other organisms.

3)I remember reading somewhere that yeast can multiply anaerobically, but the cells produced differ from those synthesized aerobically.

No. They have just changed which metabolic pathways are being used to convert the glucose into biomass and ethanol.

Answer 2

I've since consulted with a local brewing academic. He is of the opinion that under optimal conditions the oxygen is, to all intents and purposes, indeed absorbed and thereby removed from the wort completely within 20-30 minutes after pitching.

What muddles the field a little is the fact that oxygen can be (and usually is) stored within the cells before it is being used for biomass synthesis, most of which takes place during the high growth phase that follows the lag phase, the start of which essentially coincides with the initial phase of the primary fermentation.

However, the cited article defines 'aerobic fermentation' as fermentation in an aerobic environment. But after the yeast has absorbed practically all oxygen (shortly after pitching) such an environment no longer exists.

As a result, aerobic fermentation only takes place when yeast cells commence fermentation almost immediately after pitching. Which, in home and craft brewing, does not occur to any appreciable degree. I know of at least one industrial brewery where yeast is being cropped, checked and repitched in under 12 hours, and in that case aerobic fermentation MAY occur to some meaningful degree, because the yeast will start the fermentation almost as soon as it hits the wort. But in all other cases aerobic fermentation within a brewing context is pretty much a non-existent phenomenon.