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."

  • Can you cite a source that the yeast remove all oxygen within 20-180 minutes? I highly doubt that. Jun 12 '18 at 16:26
  • @farmersteve: morebeer.com/articles/how_yeast_use_oxygen Jun 12 '18 at 16:28
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    I think the article answers all your questions. I don't know why you are asking these questions after I read the article. It's all spelled out there for you. I say we close this question and move to a biology stack exchange biology.stackexchange.com Jun 12 '18 at 17:29
  • Read my original question. The cited article states that the oxygen is removed from the wort within the first 30 minutes or so, and then goes on to discuss aerobic fermentation. This is contradictory: if the oxygen has been removed, aerobic fermentation can't take place. Hence my question: if no oxygen is left in the wort half an hour after pitching, aerobic fermentation can't exist in a brewing context. Jun 13 '18 at 12:20
  • You are getting into some serious low level Biology that nobody here knows anything about. Like I said take to the Biology stack Jun 13 '18 at 14:19

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.

  • Aerobic metabolism is a more efficient pathway but Saccharomyces prefers anaerobic fermentation anerobic over aerobic respiration (Crabtree effect) and will only switch to respiration in response to high oxygen and low sugar levels. Oxygen absorbed in the initial stage of the lag phase is used to synthesize cell building components; not to oxidize glucose. Higher levels of oxygenation (eg. in yeast starters) promote cell growth but inhibit fermentation (Pasteur effect). Jun 27 '18 at 9:33
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    The yeast will use its metabolic pathways dynamically and can change instantly based on what is available to that particular cell. At micro scale, if there is one oxygen molecule available to the cell, it will use that as the electron acceptor even in the presence of high glucose levels. If there is glucose over 5 degree plato then some of that will strategically be metabolized anaerobically even if high amounts of oxygen are available. That would be "aerobic fermentation" - aerobic conditions are present, but the fermentive metabolic pathway can still be used.
    – mattrices
    Jun 27 '18 at 13:12
  • Cell synthesis net eq for ammonia as N source : (1/5) CO2 + (1/20) HCO3 + (1/20) NH4 + H+ + e- ----> (1/20) C5H7O2N + (9/20) H2O C5H7O2N is the biomass chemical approximation, the O here comes from CO2 as O2 can't be saved for later.
    – mattrices
    Jun 27 '18 at 13:22

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.

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    So you are answering your own question? Is that even allowed? What's the point of asking it? Jun 13 '18 at 14:18
  • yes it is allowed, but not common. I have answered my own question before but usually after a week of research and not getting good answers.
    – jsolarski
    Jun 13 '18 at 16:50
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    The idea is to share knowledge here. I'm asking questions when I have any, and answering them when I can. If I happen to find the answer to my own question, why not share it? Jun 14 '18 at 9:03

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