Yeast cycle - Fermentation

It's not an extremely useful or practical question, but i was wondering if someone can explain to me how exactly do Yeast "know" when to stop their fermentation phase?

** Do they stop once a given level of ABV has been reached?
If so, why are there some known cases of yeast unintentionally killing themselves through too much produced alcohol?

** Do they stop once a given concentration of any residual sugar has been reached?
If so then if he were to make a table sugar fermented beverage, would they stop once a certain amount of residual sugar was achieved? I always thought beer yeast fermented refined sugar almost entirely.

Excuses if any barbarities have been said.

They don't 'know'. Yeast continues fermentation until the fermentable sugars are gone or they die in the waste products, alcohol being the main one.

Yeast don't know anything. A typical beer yeast is tolerant of alcohol levels up to 8-9%. In a normal ale, alcohol tolerance isn't really an issue. You can kind of control it by the attenuation properties of a yeast. Some yeast consume a lot of stuff really fast, then drop out of suspension. I'd say these are typically English strains. They are highly flocculent, but relatively low attenuators. Once again, they dont really know, they have been "bread" for this purpose. A yeast like 1056 has a higher attenuation rate. It has been "bread" to ferment more.

You are right. Yeast tend to go after simpler sugars first. Thats why its generally not recommended to use a lot of simple sugar during the start of fermentation. The idea is the yeast will use up all the simple sugar, then get lazy and not go after maltose, maltriose and some of the longer chained sugars.

Fermentability of your wort is also a huge factor. This is determined by your mash, or more specifically your saccharification rest. The enzymes responsible for breaking down starches into fermentable sugars work in the 140*F to 160*F range. This is why most single infusion procedures concentrate in the 150-160 range. A cooler mash will produce a more fermentable wort, while a hotter one will create more long chained sugars and dextrins that yeast can't get to.

So theres two semi-conflicting answers there. Because on the one hand you have yeasts only stopping until they consume all of the fermentable sugars but on the other you have them consuming variable ammount of sugars based on their attenuation rates.

I am aware the attenuation rates play a part, but what is confusing me is this:
If you have a table sugar wort and a malt wort will they ferment the same X% (X being the attenuation rate) of sugars with the same yeast?

This was really my question, since the other one was addressed.

David and I said almost the same thing. I went a little further trying to explain how fermentability of wort can affect attenuation. David was 100% right. Yeast will consume all fermentable sugars until they are gone (flocculate to the bottom), or die in their waste (alcohol). Notice that he said FERMENTABLE sugar, not simple sugar. Yeast consume simple sugars, as well as maltose. Once the sugars start getting bigger than maltose, yeast have a problem.

Endovelico said:

If you have a table sugar wort and a malt wort will they ferment the same X% (X being the attenuation rate) of sugars with the same yeast?

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The simple answer is no. In theory, a solution of 100% simple sugars will ferment completely (between .998 and 1.001 ish). Very dry. I've never done experiments with yeast strains and simple sugars. You could do some searching and find the tolerance of different yeast strains.

A malt based wort will never ferment out the same as an all simple sugar solution. There are other substances in wort that yeast cannot consume. That is what gives beer body and mouth-feel.

Hope this helps

I appreciated it you trying to explain the basics to me in both your posts, but it really wasn't my question at all. David touched on it, but theres still some doubts.

First of all when some yeast is labeled "75% Attenuation" what is it refering to? Obviously not only fermentables since we have established that they eat up all of the fermentables...

So its taking into account different types of sugar, most likely. Having said that, why exactly do they ferment 75% and not 80%? I am aware its an average, but WHY 75% or 80%? Does it mean they tire out USUALLY around that mark? Die (obviously not)? Just stop fermenting at 25% or 20% sugar solution? Or something else? This is what im trying to get at. They stop, but why do they stop after attenuating a given ammount.

The reason i ask is because i thought i read somewhere awhile ago, that yeast stopped fermenting based on the concentration of sugar in the solution. Kinda like when you take a bath you add just enough hot/cold water to not end up with an overly warm or cold water. Thats what i meant by quoting "know", kinda like saying "feel" but it would have worked out wierd.

This podcast (brew strong on attenuation) could probably answer your question better.

Cool, i'll check it out if i don't find answers here. BasicBrewing probably has something on it aswell.

I believe that the ABV% is much more important than the amount of residual sugars as to when yeast cells finally "give up". Basically, yeast rely on enzymes throughout each stage of fermentation. The last stage, the Zymase reduction basically converts all of the other enzymes into ethyl alcohol. With the reduction of the enzymes and the new presence of the alcohol, the yeast cannot ferment any further and will start breaking down over time in the alcohol. Not sure if this is really part of the "original question", but that's what I think causes fermentations to stop.

Thats what i was looking for, very interesting. Thank you very much.

I believe that the ABV% is much more important than the amount of residual sugars as to when yeast cells finally "give up".

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Help me with this example.

I am using one yeast strain for three different worts with an alcohol tolerance of over 5% ABV.
All have the same starting gravity of 1.040 and not enough fermentable sugars to reach the alcohol tolerance limit of the yeast.
All fermentation temps are ideal.
The yeast will now produce three different beers with three different alcohol levels.
How does the yeast know when to stop if only the alcohol concentration plays a role?

Wort one has a lot of fermentable sugars
Wort two has 25% less than #1
Wort three has 50% less than #1

Thanks,
ClaudiusB

VERY interesting. Maybe the residual sugars do play a big part. Do you have the FG for all of the solutions?

The key here is that you said each beer contains differing levels of fermentable sugars. Since this is a low gravity ale, alcohol doesn't play a role in stopping the yeast. The yeast will simply consume all of the fermentable sugars in each wort. What will be left behind are the unfermentables in each wort. Your FG will be different also. Wort 1 will have the lowest, and 3 the highest.

The key here is that you said each beer contains differing levels of fermentable sugars. Since this is a low gravity ale, alcohol doesn't play a role in stopping the yeast.

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Correct, this is the reason I sad "Not enough fermentable sugars to reach the alcohol tolerance limit of the yeast" to show other factors may play a role too.

low gravity ale

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Wrong, I am a 99% lager brewer

Cheers,
ClaudiusB

Is it safe to assume that the atenuation rate only comes into play in moderate sized brews (1.040+)? In small brews the Zymase catalyst effect doesn't get to fade out, probably.

ClaudiusB said:

Help me with this example.

I am using one yeast strain for three different worts with an alcohol tolerance of over 5% ABV.
All have the same starting gravity of 1.040 and not enough fermentable sugars to reach the alcohol tolerance limit of the yeast.
All fermentation temps are ideal.
The yeast will now produce three different beers with three different alcohol levels.
How does the yeast know when to stop if only the alcohol concentration plays a role?

Wort one has a lot of fermentable sugars
Wort two has 25% less than #1
Wort three has 50% less than #1

Thanks,
ClaudiusB

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It is far from only the alcohol concentration. The pitching rate, oxygenation, rousing regimen, temperature, fermentability of the wort, nutrient levels, Ph and more all effect the behavior of the yeast. It is a multivariant problem. The apparent attenuation is something the yeast sellers test with a standard wort and allows you to compare yeast strains against each other. You can get a cal ale yeast, for example, to ferment a wort nearly 100% if you add a certain enzyme (I'd have to look it up if you want the specific enzyme) which will cleave almost all complex starches into simple sugars for the yeast to consume.

dantodd said:

You can get a cal ale yeast, for example, to ferment a wort nearly 100% if you add a certain enzyme (I'd have to look it up if you want the specific enzyme) which will cleave almost all complex starches into simple sugars for the yeast to consume.

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I believe that would be amalayse enzyme which you can buy at most LHBSs. Beano (alpha galactosidase) also does the same thing. Just for the record, I have used it and would never recommend any one use it. Close to 100% attenuation in a beer is just not good.