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Old 05-17-2014, 12:49 AM   #1
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Default Does the liquid from a yeast starter have alcohol in it?

What the topic says. If I pour off the liquid from my yeast starter, it looks, smells, and tastes like beer, but is there any actual alcohol in it? My (rudimentary) understanding is that yeast produce alcohol when they're in an O2 deprived environment, and since starters are O2 rich, does that mean all their effort is going to multiplying rather than producing booze?

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Old 05-17-2014, 12:51 AM   #2
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With my recollection of basic microbiology, yes, you are correct - aerobic respiration does not produce ethanol....

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Old 05-17-2014, 01:30 AM   #3
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Eh I might be wrong but if thats so why do we aerate wort after it cools down But before yeast pitching?

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Old 05-17-2014, 01:36 AM   #4
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Originally Posted by reese9885 View Post
Eh I might be wrong but if thats so why do we aerate wort after it cools down But before yeast pitching?
It should be oxygen rich beer (alcohol and all) it wont keep long and will become oxygenated if not pitched in to a new wort or consumed if you wish to drink all that suspended yeast!
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Old 05-17-2014, 01:55 AM   #5
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Eh I might be wrong but if thats so why do we aerate wort after it cools down But before yeast pitching?

You oxygenate the wort for the benefit of the yeast. After then initial growth & reproduction stages the yeast uses up the O2 and goes into anaerobic respiration. It is at this stage that ethanol & CO2 are produced.


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Old 05-18-2014, 05:19 PM   #6
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This brings up some common misconceptions. In a typical yeast starter (10% DME), the yeast never undergo aerobic respiration in the sense of using oxygen as a terminal electron acceptor to produce ATP. This is due to the so-called "Crabtree effect." When sugar concentrations are beyond a threshold level, yeast will ferment the sugars to carbon dioxide and alcohol, and produce ATP via substrate level phosphorylation, even in the presence of oxygen. This doesn't mean oxygen is not required for yeast cell growth, however. The oxygen is used by the yeast for production of sterols. In the brewing world, yeast never aerobically respire. So to answer the OP's question, yes, there is alcohol produced in a yeast starter.

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Old 05-18-2014, 06:18 PM   #7
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What Ishaner said, the high sugar concentration of the wort causes yeast to use the anaerobic metabolic pathways. Aerobic respiration would result in more growth but unfortunately the Crabtree effect kicks in

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Old 05-18-2014, 11:44 PM   #8
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Quote:
Originally Posted by lshaner View Post
This brings up some common misconceptions. In a typical yeast starter (10% DME), the yeast never undergo aerobic respiration in the sense of using oxygen as a terminal electron acceptor to produce ATP. This is due to the so-called "Crabtree effect." When sugar concentrations are beyond a threshold level, yeast will ferment the sugars to carbon dioxide and alcohol, and produce ATP via substrate level phosphorylation, even in the presence of oxygen. This doesn't mean oxygen is not required for yeast cell growth, however. The oxygen is used by the yeast for production of sterols. In the brewing world, yeast never aerobically respire. So to answer the OP's question, yes, there is alcohol produced in a yeast starter.
Awesome answer, thank you. But if oxygen isn't needed for the yeasty beasties to do their job, why do we aerate the wort before we pitch the yeast? (In other words, what are sterols and why do we want them?)
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Old 05-19-2014, 12:03 AM   #9
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Awesome answer, thank you. But if oxygen isn't needed for the yeasty beasties to do their job, why do we aerate the wort before we pitch the yeast? (In other words, what are sterols and why do we want them?)
When oxygen is present there will be some areobic respiration however, the primary metabolic pathway will be anaerobic due to the Crabtree effect as mentioned. Oxygen is important in the initial stages of fermentation because it is required for sterol production. Sterols aid in cell membrane permeability and flexibility which aid the yeasts ability to regulate nutrient and by products in and out of the cell.

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Old 05-19-2014, 12:34 AM   #10
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The liquid from a yeast starter is unhopped beer.

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