Yeast are Aerobic! Yeast are Anaerobic!

Let me take a moment for a public service announcement.

So it has been years since I was asked, "are yeast aerobic or anaerobic?"  Granted, I've been desultory in the extreme about researching it, but at last I know the answer to this question.  And you can too.

This looks a lot like my first yeast sighting -
when I learned they were single cell critters
Ah... High School.

Experience leads one to believe it is both.  They are submerged in liquid when you proof* the little Saccharomyces sp., but the yeasties work fine in merely damp environments where they are exposed to air.

Look Ma!  Asexual reproduction.
Oh wait... no Ma. Or Pa.
(psst... occasionally yeast will go through
sexual reproduction, that's how variety is propagated.
Certain environmental conditions tend to trigger this.)
[I love the Wikipedia Creative Commons]

So now, at last, thanks to the research and consolidation skills of others, I know that yeast are both.  They are anaerobic during fermentation when they are engaged in chomping up sugars (of all sorts of descriptions) and pumping out carbon dioxide and ethanol (part of that "yeasty smell').  But when they are exposed to oxygen, they can go to aerobic respiration, and produce mainly CO2 and water.

Most yeast are facultative anaerobes.  That is, they can do either depending on the surroundings.  Interestingly it is not just the presence of oxygen that causes the switch.  The tipping point at which they shift to one or the other is something that can be bred for.  It is a complex chemical and energetic calculation dependent on chemical inputs and gene expression in a particular strain.  So this is why you get "fast fermenters" (bread yeast), "slow fermenters" (brewer's yeast), and then variety in the slow fermenters such as "top cropping" (grows on the surface of the liquid) and "bottom cropping."  And then there are yeast that prefer different temperatures, pH's, sugars or mixes of sugars and produce different byproduct odorant molecules†.  It's wild!

There are a some yeast that are only aerobic (obligate aerobes), but none are only anaerobic (obligate anaerobes) like some bacteria.  Clostridium botulinum bacteria - the bug responsible for both Botulism and Bo-Tox (short for botulinum toxin in case you were wondering what these people are injecting into their foreheads) is a perfect example of an obligate anaerobe.  It dies when exposed to oxygen - and thus can only be found in improperly sterilized canned goods, sausages and other oxygen free environments.

The kitchen yeast in its
domestic habitat.
Docile and approachable.
(Thanks King Arthur Flour)

And now, back to our irregularly scheduled programming.

*  Stir them into some warm sugar water and see if they are still alive - by seeing if they start burping out bubbles.

† Odorant molecules - stuff that is smellable.  Watch out for musings on the musings of Hervé This in this space.