attenuation - why does it stop?

[jigidyjim]

Something I never wondered about until today for some reason:

How come adding new yeast doesn't restart the attenuation %? For example, if yeast attenuates 75%, how come removing the trub and adding more yeast doesn't attenuate another 75%... lather rinse and repeat until you are at 1.000 gravity?

 

[wetzie]

I am not sure but wouldn't it be the lack of fermentable sugars?

 

[kxx]

I would imagine the % attenuation ranges are meant to estimate expected metabolism of sugars in your unfermented wort, ie; conversion of total fermentables, not conversion of leftovers....

 

[weirdboy]

It is usually lack of fermentable sugars, but it could also be the yeast strain, the level of alcohol in the beer, or other factors.

During fermentation the yeast are eating the sugars in the wort and turning them into CO2 + alcohol. If you take samples of the wort/beer before and throughout fermentation, you can literally taste the difference as the sweetness gradually goes away.

 

[jigidyjim]

Ok... so if I have an OG of 1.05 and a yeast with attenuation of 75%, I end up at 1.012.

Seems like you are all saying that the 1.012 is unfermentable sugars. But in that case, it would seem like the FG has nothing to do with yeast attenuation, and everything to do with how the mash is done. I don't think that's true, otherwise yeast attenuation rates wouldn't matter.... so it must be more complicated than that...

 

[Berock]

Yeah, but think about a big beer. If the OG is high (1.10), then if it only attenuates to 75%, that would leave 1.025 with some of the sugars could being fermentable.

 

[Yooper]

Ok... so if I have an OG of 1.05 and a yeast with attenuation of 75%, I end up at 1.012.

Seems like you are all saying that the 1.012 is unfermentable sugars. But in that case, it would seem like the FG has nothing to do with yeast attenuation, and everything to do with how the mash is done. I don't think that's true, otherwise yeast attenuation rates wouldn't matter.... so it must be more complicated than that...

You're WAY overthinking this. If you have a yeast that will attenuate 80% and one that will attenuate 75%, all other things being equal (amount of fermentable sugars, amount of non-fermentables, some mash temperature), the difference in attenuation will be 5%. Adding more yeast doesn't change the basic recipe. Beer is made up of several types of sugars (carbs). Some are long-chained and therefore less fermentable by ale yeast- some are simple sugars and thus more fermentable.

Most yeast strains will give you about the same FG, as the attenuation rates don't vary much, maybe 5% or so give or take. Some might average 68%, some might average 72%, but it really is recipe dependent.

 

[weirdboy]

Ok... so if I have an OG of 1.05 and a yeast with attenuation of 75%, I end up at 1.012.

Seems like you are all saying that the 1.012 is unfermentable sugars. But in that case, it would seem like the FG has nothing to do with yeast attenuation, and everything to do with how the mash is done. I don't think that's true, otherwise yeast attenuation rates wouldn't matter.... so it must be more complicated than that...

It's slightly more complicated because with larger beers especially you run into limits about how much the yeast can tolerate.

But essentially if you have a wort that is 1.050 OG it mashed at 152F for 60 minutes versus a wort at 1.100 mashed at 152F for 60 minutes, the bigger beer is going to have more (in absolute terms) unfermentable parts than the smaller one. It's just common sense.

 

[jigidyjim]

Ok I get what you all are saying, but to go back to my original question, let me rephrase this in more hypothetical terms to get to the thing I'm trying to think about:

Let's say I have a 100% fermentable sugar wort. And let's say the yeast survives in in 100% alcohol. If I have a 75% attenuator yeast to get my 1.05 down to 1.012, why can't I add more yeast at that point to go from 1.012 to 1.003? Or can I?

Thanks.

 

[McGarnigle]

What do you mean by 75% attenuator yeast?

 

[jigidyjim]

Some of my question is answered by this: https://www.homebrewtalk.com/wiki/i...ation#Yeast_Strain_Differences_in_Attenuation

but I still don't know/understand what would happen if more yeast was added after the original primary fermentation is done. it sounds like it would indeed start to ferment again?

 

[Yooper]

but I still don't know/understand what would happen if more yeast was added after the original primary fermentation is done. it sounds like it would indeed start to ferment again?

Turn it around. WHY would it start to ferment again? What is left in the beer that can be fermented by a second ale yeast? Either something is fermentable by ale yeast, or it is not. Adding more isn't going to change the composition of the substance.

The only reason it would ferment a bit more is if you used a more attenuative yeast. Say one yeast strain is 64%. Another is 75%. If you added the 75% strain first, adding a 64% attenuator later wouldn't do a thing. If you added the less attenuative yeast strain first, then perhaps adding the higher attenuator second might give you a couple of points more.

This ain't rocket science.

 

[Yooper]

Ok I get what you all are saying, but to go back to my original question, let me rephrase this in more hypothetical terms to get to the thing I'm trying to think about:

Let's say I have a 100% fermentable sugar wort. And let's say the yeast survives in in 100% alcohol. If I have a 75% attenuator yeast to get my 1.05 down to 1.012, why can't I add more yeast at that point to go from 1.012 to 1.003? Or can I?

Thanks.

Your question is flawed. Your 75% attenuative yeast is assuming that you have a wort. A sugar wash isn't a beer wort. In that case, sugar is nearly completely fermentable. Any yeast should take it dry. Probably to .990. You can add two yeast cells or 20000000000000000. Same results.

 

[Gameface]

Think about bottle conditioning. The yeast has finished attenuating the wort, but you add more consumable sugar and the same yeast that was finished "wakes up" and eats the newly added sugar. So you're going about it the wrong way. The yeast doesn't run out of ability to ferment more, it runs out of the food it needs to do it. Adding new yeast doesn't make unfermentable sugars fermentable again. You're just adding more yeast into a wort that has nothing left for it to eat.

 

[motobrewer]

nevermind....

 

[jigidyjim]

The only reason it would ferment a bit more is if you used a more attenuative yeast. Say one yeast strain is 64%. Another is 75%. If you added the 75% strain first, adding a 64% attenuator later wouldn't do a thing. If you added the less attenuative yeast strain first, then perhaps adding the higher attenuator second might give you a couple of points more.

Ah thanks Yooper - you've helped me get to the source of the confusion. Everyone seems to be saying "dude, you are an idiot, the yeast takes all of the fermentable sugars and then goes dormant", but clearly thats not true (the sugar part, the idiot part is debatable), since different yeasts will yield different results, hence the section in the wiki on why attenuation ratings exist.

So the remaining question I have is why would pitching a second, higher attenuating yeast "find more sugars", for lack of a better phrase. The wiki says a yeast will stop fermenting due to flocculation and getting stuck on the surface, but that wouldn't account for the new pitch case. The only other thing I can find on the wiki to explain it is "nutrient depletion"... so maybe the higher attenuating yeast can remain active with fewer nutrients left in the wort? I'm not really sure... any ideas on that?

The other new question I want to ask is what it means when ale yeast can only ferment maltotriose "to a third"... a third of what? Why does it stop? I'm guessing a more technically correct way to say it would be some balance between maltotriose and something else, otherwise I'm back to my original thought of why it doesn't just keep fermenting with new yeast.

Thanks.

 

[jigidyjim]

The other new question I want to ask is what it means when ale yeast can only ferment maltotriose "to a third"... a third of what? Why does it stop? I'm guessing a more technically correct way to say it would be some balance between maltotriose and something else, otherwise I'm back to my original thought of why it doesn't just keep fermenting with new yeast.

Or I suppose another explanation would be the byproduct of digesting maltotriose by an ale yeast is different than a lager yeast, where the ale yeast leaves behind unfermentable sugars and the lager yeast doesn't. Or something like that.

 

[motobrewer]

from another thread:

MY QUESTION:
What assumptions about wort fermentability does Wyeast make when determining the attenuation variability range for each paticular strain? For example, does the low end of the range account for an unusually non-fermentable wort, such as one created using 30% specialty/flaked/roasted grains, and mashed at temps around 156F? Or do the ranges assume a 'normal' wort and reflect only variability within the yeast itself.

WYEAST RESPONSE:
The reported attenuation numbers are based on tests completed years ago (standard media) but are, in my opinion, considered obsolete. Over the last year, I have been working to reduce the importance assigned to attenuation numbers. Wort and fermentation conditions are far more important when considering the expected level of attenuation. Most ale strains attenuate to the approximately the same level (+/- 1.5%). Lager and Wheat strains will attenuate a little less (-1.5%).

I am currently working on assigning new numbers to all of our strains

 

[weirdboy]

So the remaining question I have is why would pitching a second, higher attenuating yeast "find more sugars", for lack of a better phrase. The wiki says a yeast will stop fermenting due to flocculation and getting stuck on the surface, but that wouldn't account for the new pitch case. The only other thing I can find on the wiki to explain it is "nutrient depletion"... so maybe the higher attenuating yeast can remain active with fewer nutrients left in the wort? I'm not really sure... any ideas on that?

Some yeast strains are better at (or capable of) breaking down certain chains than others.

The other new question I want to ask is what it means when ale yeast can only ferment maltotriose "to a third"... a third of what? Why does it stop? I'm guessing a more technically correct way to say it would be some balance between maltotriose and something else, otherwise I'm back to my original thought of why it doesn't just keep fermenting with new yeast.

I believe this is an area of active research, but adding more of the same yeast is never going to be the answer. Active yeast is already abundant in the wort. One has to assume there is another piece, such as a catalyst, missing.

 

[jigidyjim]

I believe this is an area of active research, but adding more of the same yeast is never going to be the answer. Active yeast is already abundant in the wort. One has to assume there is another piece, such as a catalyst, missing.

Woah, interesting. Sometimes it's hard to remember that despite being made for thousands of years, there are still things to discover about how brewing works...

 

[kxx]

Woah, interesting. Sometimes it's hard to remember that despite being made for thousands of years, there are still things to discover about how brewing works...

The S. cerevisiae yeast genome was sequenced over a decade ago, and still researchers are trying to identify sequences in that ACTG soup may code for proteins( and other bio-regulatory factors) essential for a wide variety of yeast processes... most likely these include factors regulating metabolic efficiency.

It may be known to brewing microbiologists or geneticists the polymorphisms or mutations occurring between similar strains that may account for their differential response to certain environmental conditions -- such as wort composition.

The JZ and C White book on yeast shines some light on yeast biology.... it may be worth a read.