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When making a starter, the intent is to make yeast. We use stirplates in part to continually oxidize the wort, enabling the yeast to multiply.

In this question about starter oxidation, one of the answers talks about lag transitioning to fermentation and quotes:

Fermentation (Wikipedia) says,

even in the presence of abundant oxygen, yeast cells greatly prefer fermentation to oxidative phosphorylation, as long as sugars are readily available for consumption (a phenomenon known as the Crabtree effect)

Which makes me wonder, "what really causes/encourages yeast to multiply?". If there's plenty of oxygen and sugar, why doesn't yeast just keep multiplying?

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2 Answers 2

The yeast want to multiply, that's their preferred activity. Given an optimal environment, the yeast will continue to propagate. The same is true for other microorganisms, like bacteria - given ideal conditions they would have taken over the world countless times over.

The key is sufficient resources and ideal conditions - something that is rare or short-lived in practice. The yeast alter the rate of propagation as dictated by the state of the environment, such as insufficient O2, amino acids, or other of raw materials needed to build new cells. As resources are used up, propagation rate decreases.

Which makes me wonder, "what really causes/encourages yeast to multiply?". If there's plenty of oxygen and sugar, why doesn't yeast just keep multiplying?

The yeast need more than oxygen and sugar to make new cells, but if the environment continues to provide favorable conditions and the resources needed, then the yeast will continue to propagate. But this is purely academic - in practice resources are limited and propagation must slow down eventually.

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This question is brought on by bad information of another post. The 'Crabtree effect' essentially states that yeast in high sugar will choose to undergo fermentation rather than aerobic respiration (using O2 to respire). However, this does not mean that they aren't using O2. On the contrary the yeast are actively dividing and use the O2 to make sterols which are used in membrane components, without which they couldn't survive. This greatly surprised me when I learned it as it goes against a lot of how mammalian cells work.

Why the Crabtree effect is advantageous for yeast I don't really have a good answer for that. But I think what you have to do is think about a yeast in the environment surrounded by a bunch of other organisms who want to eat the same food source. So what do you do? Slash and burn baby, which is essentially what the yeast does. They sacrifice optimal metabolism for optimal growth. If they use O2 for respiration there is less for sterol production and therefore growth. Note that after the sugar levels reach a certain point they switch back to aerobic respiration and actually clean-up some of the byproducts of their slash and burn strategy.

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I don't say that the yeast don't use O2, but that the amount is greatly reduced when the bulk of propagation is complete, and propagation slows. The yeast cannot propagate forever since resources are limited, and so O2 requirements will decrease. –  mdma May 1 '12 at 15:37
    
You are correct that O2 consumption is greatly decreased after propagation is complete. But the cells are not finished growing in lag phase (again semantics most likely) they continue to grow during exponential growth phase which is where most of the sugars are used up as the yeast grow and use O2. So they stop when resources are limiting as you say, whatever that resource is. –  Chris Plaisier May 2 '12 at 18:23

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