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Old 01-16-2014, 09:37 PM   #1
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Default Yeast starter question

A beginner question, May not be worded right. Just looking for clarification.

Is the reason a starter is calculated to a certain volume amount because with that amount of sugars there is a set number of yeast cell that will grow and almost no alcohol will be produced?

Which is why you would step up a very large starter?

Than you in advance,
sfish

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Old 01-17-2014, 01:54 PM   #2
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Yes that is correct, except you will get alcohol in a starter. You are basically making a small batch of beer. It will be <4% ABV or so, but you will get alcohol. And it probably won't taste very good.

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Old 01-17-2014, 02:01 PM   #3
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The whole point of a starter is to allow the yeast to propagate under low stress conditions in order to acquire proper pitch rates. The volume (1L, 2L, etc) is to have an idea of amount of fermentable sugars available for cunsumption. You use a low gravity so as to not stress the yeast. Otherwise you could just do a high gravity one liter starter to acquire the rates as well but the yeast will be stressed and possibly cause off flavors.

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Old 01-17-2014, 03:30 PM   #4
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Originally Posted by mtnagel View Post
Yes that is correct, except you will get alcohol in a starter. You are basically making a small batch of beer. It will be <4% ABV or so, but you will get alcohol. And it probably won't taste very good.
If you're careful (and hop your starter), you can get a perfectly drinkable small batch out of it. For big starters, this can be worth it if you hate throwing out beer.
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Old 01-17-2014, 03:55 PM   #5
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Originally Posted by mtnagel View Post
Yes that is correct, except you will get alcohol in a starter. You are basically making a small batch of beer. It will be <4% ABV or so, but you will get alcohol. And it probably won't taste very good.
This may or may not be true depending on your starter method.


Basically the yeast make alcohol as part of the breakdown of glucose. It then in the presence of Oxygen break the alcohol down further for more energy. If you don't oxygenate the starter well (ie just make and let sit, put an airlock on, etc) then you will have residual alcohol.
If however, you put a loose (usually aluminum foil) cover on, and especially if you agitate it-either by hand or with a stir plate, then you will force CO2 out, increase absorbtion of atmospheric O2 and if left long enough, get the yeast to convert all the sugar carbon dioxide and water.


Back to the OP's original question.
Through experimentation, they've found the amount of yeast cells that are growable in a given volume -gravity combo of wort. They've also determined what is a good pitch rate. Under and over pitching create off flavors.
to make life easier, starters are usually figured at about 1.040 gravity - the 1.035 to 1.040 range giving the best growth rate vs gravity. This means you only have to calibrate the volume to get the desired yeast count for a starting batch.
In the case of step up starters, going from a vial to a 1L to a 2L will get you more cells than going from a vial to 2L. Off hand, I'd guess this is because you start with more in the 1L and give it a fresh jolt of wort to work off of - and more sugar in total, where as from vial to 2L, the yeast cells have to fuel themselves on less sugar and such - dang not explaining that well. Sorry, not enough caffine.

I strongly recommend reading "YEAST" by White and Zainasheff for more on this and pitch rates.
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Old 01-17-2014, 04:52 PM   #6
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Basically the yeast make alcohol as part of the breakdown of glucose. It then in the presence of Oxygen break the alcohol down further for more energy.
If you don't oxygenate the starter well (ie just make and let sit, put an airlock on, etc) then you will have residual alcohol.
If however, you put a loose (usually aluminum foil) cover on, and especially if you agitate it-either by hand or with a stir plate, then you will force CO2 out, increase absorbtion of atmospheric O2 and if left long enough, get the yeast to convert all the sugar carbon dioxide and water.
This isn't true. Yeast, in the presence of high concentrations of sugar (like you find in wort) engage in what is called the crabtree effect - a fancy name that simple means the yeast ferment sugar to alcohol despite the presence of oxygen. The only way you get yeast to fully metabolize sugar (e.g. turn sugar into CO2 and H2O), and get the resulting energy boost, is to carefully control sugar levels throughout the ferment, maintaining a sugar content equivalent to ~1.002 SG points. This is called batch-feed, and requires extremely expensive equipment (and reputably produces yeast with characteristics not amenable to making good beer).

What the addition of oxygen (via stirring) does is allow for the formation of unsaturated fatty acids and sterols, which are needed by yeast to then thrive in the low-oxygen environment of the ferment (these compounds stabilize yeast membranes). Stirring also reduces CO2 levels in the wort, which enables the fermentation of the sugars to go faster, accelerating the growth of the yeast in the starter. But the resulting "beer" is often unpleasant - high oxygenation also promotes ester formation, and while these are sometimes desired at low doses, they are pretty unpleasant at high concentrations.

As for the OP's question, the reason you step up a large starter is that you get more yeast from a stepped starter than you do from an equivalent volume single-step starter. As such you save $$$ in that you use less starter-wort and cheaper glassware. The yeast produced in multi-stage starters is also reputedly of higher quality, although I've not seen any proof that is true.

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Old 01-17-2014, 05:01 PM   #7
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This isn't true. Yeast, in the presence of high concentrations of sugar (like you find in wort) engage in what is called the crabtree effect - a fancy name that simple means the yeast ferment sugar to alcohol despite the presence of oxygen. The only way you get yeast to fully metabolize sugar (e.g. turn sugar into CO2 and H2O), and get the resulting energy boost, is to carefully control sugar levels throughout the ferment, maintaining a sugar content equivalent to ~1.002 SG points. This is called batch-feed, and requires extremely expensive equipment (and reputably produces yeast with characteristics not amenable to making good beer).
If this were true - that is that yeast will not reuptake the alcohol and consume it in the presence of O2 - then our water supply would be drinkable as alcohol.

While the yeast may preferentially make alcohol over complete conversion, when the sugar is gone, it will take back up hte alcohol and consume it away IF there is oxygen. Early in the starter (or beer) the yeast use the O2 to make more yeast through (basically the sterols are needed for cell walls). But once a density of yeast cells is obtained, the yeast use the O2 for respiration, not reproduction, and they use up the alcohol while doing it.
Again, in YEAST this is described, and I think even has a nice diagram or 2 showing what the authors mean.
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Old 01-19-2014, 04:38 PM   #8
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If this were true - that is that yeast will not reuptake the alcohol and consume it in the presence of O2 - then our water supply would be drinkable as alcohol.
Sorry, but you are completely wrong on this. The above comment doesn't even make sense - if 100% of the earth carbon were in the form of ethanol, you'd still only end up wit a fraction of a percent alcohol in our water (carbon is rare, compared to water, on earth). And you've mis-interpreted what was in Yeast as well...but what would I know - I'm only a microbiology prof with decades of experience working with yeast & other microorganisms

In nature yeast live in sugar-limited environments; skins of fruits, bark of trees, etc. Here they almost exclusively use the krebs cycle (oxidative respiration) in order to maximize the energy they extract from that limited supply of sugars. But while efficient, this process is slow. As such, when in a sugar-replete & oxidative environment, a lot of the sugar gets shunted through the glycolysis pathway, forming ethanol.

Ethanol itself is a very poor energy source for yeast as it produces both toxic by-products as well as excessive levels of NADPH; indeed, saccharomyces only effectively metabolizes ethanol in environments containing 1% or less ethanol. Above that the yeast growth decreases; above 4%, in the presence of oxygen, yeast simply become inert - just as if they were in an anoxic environment. Even in the low-ethanol ranges, catabolism of ethanol by yeast is limited; at 1% (optimal concentration for saccharomyces growth on ethanol) only about 1/4 of the ethanol is metabolized before the yeast enter dormancy. I'd also point out that achieving this sort of growth in-solution is nearly impossible; at room temperature the solubility of oxygen (from air) in water (~8mg/l) is on the lower-end of what is required for ethanol catabolism. Stir paltes typicallly don't get to that level; and instead provide 3-5mg/l O2. So without using pure O2, achieving that level of O2 is effectively impossible. In-lab, growth on ethanol is almost always done using plates.

In regards to starters, driving yeast through this metabolic pathway would be the last thing we would want to do - during this type of growth yeast upregulate a broad swath of genes specific to this form of metabolism, while turning off many of the genes required for proper beer fermentation.

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Old 01-19-2014, 07:06 PM   #9
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Haha, I love HBT. A beginner's question turns into a scientific debate! Regardless, interesting info either way :-)

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Old 01-20-2014, 01:29 PM   #10
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[QUOTE=aslander;5836560]Haha, I love HBT. A beginner's question turns into a scientific debate! Regardless, interesting info either way :-)

I thank you all. I feel my joining the forum is well wourth this education.

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