Feeding yeast with ethanol. . . whaat!?

I've been running 10L fermentations at work for secreted protein production. In trying to optimize fermentation for protein production I am comparing one with the standard glucose feed and one with 200 proof ethanol. Turns out S. cerevisiae will eat a wide variety of carbon sources, including its own product!

Of course this is a highly aerobic fermentation, and I'm looking for levels of secreted enzymes, not booze and yummy flavors.

If anybody is interested I'll keep you posted.

Wait, I am confused your feeding yeast with ethanol. Is that correct. Under conditions with high amounts of oxygen. I can't even comprehend the science involved.

Please extrapolate.

mccann51 said:

Please extrapolate.

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Or elaborate, even...

Cheers

If yeast are growing aerobically, they can metabolize ethanol and a variety of related small molecules through the TCA cycle and oxidative phosphorylation in the mitochondria. Probably a bit too technical but much more ATP (containing the energy cells need to make the things they need to grow and divide) can be obtained by metabolizing glucose all the way to CO2 and water. However, they can't do that without oxygen so they stop after breaking it down to ethanol and CO2. They can produce ethanol without any mitochondria at all. Blocking further metabolism is one reason, among many, why at endgame we don't want air getting into our fermentations.

Hope that's a little help.

So yeast eat sugar and metabolize it into CO2 and alcohol, but if the CO2 is removed and you run a lot of O2 thru the beer the yeast will turn around and eat the alcohol and metabolize it into water and what else?

day_trippr said:

Or elaborate, even...

Cheers

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Haha, yeah that works too!

kuntzbrewing said:

so yeast eat sugar and metabolize it into co2 and alcohol, but if the co2 is removed and you run a lot of o2 thru the beer the yeast will turn around and eat the alcohol and metabolize it into water and what else?

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co2

Brewitt said:

co2

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Ok, that makes sense

Is this anything like when puppies eat their own poop? I kid I kid this is pretty interesting not that we would want anything like this scenario on our fermentations but still interesting to know.

FuelshopMcgee said:

Is this anything like when puppies eat their own poop?

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Coprophagia: http://en.wikipedia.org/wiki/Coprophagia

Is this unique to a particular yeast strains or will they all do it under the correct conditions?

Who doesn't love making malt vinegar?

Ethanol -> acetaldehyde -> acetic acid
Ain't science grand

william_shakes_beer said:

Is this unique to a particular yeast strains or will they all do it under the correct conditions?

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There are strains that won't but I presume all of the brewing strains will.

Rushis said:

Who doesn't love making malt vinegar?

Ethanol -> acetaldehyde -> acetic acid
Ain't science grand

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Hopefully your yeast aren't doing this. If your malt is getting converted to vinegar, you have a contaminant or your brewing lambics.

Brewitt said:

Hopefully your yeast aren't doing this. If your malt is getting converted to vinegar, you have a contaminant or your brewing lambics.

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Or your fermenting in an aerobic enviroment

Rushis said:

Or your fermenting in an aerobic enviroment

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I don't think S. cerevisiae will produce significant amounts of acetic acid from ethanol under aerobic or anaerobic conditions. Do you know otherwise? I am aware that many other organisms do so.

Brewitt said:

I don't think S. cerevisiae will produce significant amounts of acetic acid from ethanol under aerobic or anaerobic conditions. Do you know otherwise? I am aware that many other organisms do so.

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I make no claims to be a microbiologist, nor a yeast expert. For what it's worth here are a few references with a nifty PDF containing a variety of quite large fancy words.

"Molecular Cloning, Characterization, and Potential Roles of Cytosolic and Mitochondrial Aldehyde Dehydrogenases in Ethanol Metabolism in Saccharomyces cerevisiae†"
J Bacteriol. 1998 February; 180(4): 822–830.

"Regulation of sugar and ethanol metabolism in Saccharomyces cerevisiae."
Crit Rev Biochem Mol Biol. 1990;25(4):245-80.

"Yeast metabolism"

Under reduced glucose conditions, some yeast (including sacc) can consume ethanol in an aerobic respiration pathway (ethanol dehydrogenase). This generates acetaldehyde which ideally would be converted to acetyl-CoA. Under aerobic conditions it is possible to convert the acetylaldehyde into acetic acid via a variety metabolic pathways (I vote for P450 just for kicks!). The amounts, heck if I know...

Old oxidized beer turns to vinegar (although probably not entirely the fault of yeast)

I stand corrected. I will have to read a bit more about this but I ran across several papers (not the ones you listed) measuring appreciable amounts of acetic acid accumulating in fermentations of both wine and beer. No time for more reading right now but there are apparently efficient enzymatic pathways to acetic acid that are more active in some strains than others.

Thanks for the motivation to look further.

sorry I left you guys hangin, been pretty busy. Well since I was comparing an ethanol feed to a glucose feed each had a specific feed rate. The glucose was being fed at a stepwise profile ending with a rate of 0.65ml/min, the ethanol was supposed to be fed at a rate of 0.15ml/min. Unfortunately I switched the pumps and thus the feed profiles, set myself back about a week in the fermentation schedule.

When I came in wednesday morning, after one reactor had been fed ethanol at much too high of a rate for almost a day, I pulled a sample and it was heavily flocculated. When the conditions get rough the yeast huddle together, just like at the end of our beer fermentations. Really interesting to see under the scope.