Attenuation percentage vs alcohol tolerance.

[Culinarytracker]

My brain is having a hard time picturing how both of these factors play together.

Does a yeast really stop fermenting after it's attenuation percentage, even if its under it's alcohol tolerance?

Say we have a yeast that has an 80% attenuation, and alcohol tolerance of 10%.

If I mix up some light DME at three different OG levels, one where 80% attenuation would result in an abv of 5%, 10%, and 12%.

I get that the yeast will just stop at 10%, and leave the residual sugars unfermented in the 12% potential batch..... but why would the yeast just stop fermenting the other 20% in the first batch????

 

[day_trippr]

I expect the ultimate governance is alcohol tolerance...

 

[Golddiggie]

Also keep in mind that those tolerance numbers are not hard lines. I've had ale yeast go beyond what's published before. It was only .5% (12.5% vs the listed 12%) but if the batch had more fermentable sugars it could have gone even higher. It won't go to something like 18%, but it could hit 14% if you treat it right.

 

[bracconiere]

i'd 'guess' it's how smert they are at digesting different sugars? not all sugars in wort are dextrose, or glucose, they're tag teamed...i think, anyway? some yeast are better at using more for energy?

 

[Golddiggie]

IME, it also depends on what you did to the wort before you pitched the yeast. Did you infuse enough O2 for the yeast to replicate in a way that they can handle the higher ABV% that they'll be swimming in later? How about nutrients and such? Bigger beers is where using pure O2 to oxygenate plays a much larger factor in things. Even though it is a factor at lower ABV levels, the higher you go the more important it is. Just using the 'shake/stir/shimmy' method with atmospheric oxygen just won't cut it.

 

[Qhrumphf]

It's a more complicated question than that. Neither the attenuation figure nor alcohol tolerance figure stated for a yeast are black and white numbers. They're more average or typical numbers, but can be pushed outside of those ranges. Attenuation can be higher or lower regularly. And below a given yeasts alcohol tolerance, you'll see attenuation range be the stronger influence. Once you start pushing the limits of a strain's alcohol tolerance, you might see that take the stronger influence.

But as I said, neither are hard and fast figures. It's not that the yeast will hit a magic number and just "stop".

Are we we talking about beer wort? Then we have a complex, and variable profile of sugars depending on the way it's brewed, with a portion of its sugars unfermentable by most domesticated yeasts. A strain stating 72-75% apparent attenuation could see sixties or even lower if brewed to ve less fermentatable (high mash temp, lactose, etc), or in the 80s if brewed to be more fermentable (low mash temp, dextrose, etc). How the fermentation is managed will also make a difference (pitching rate, nutrition, oxygenation, temp control, etc).

And then, if you know what you're doing, you can push a given strain well above it's listed alcohol tolerance. I've gotten Chico (US-05, 1056, 001, or another variant of) to about 15% ABV (typically stated to stop at 12%) by babying it with oxygen, nutrients, and staggering sugar additions.

Then if we start talking wine/mead/cider must, then we have a much simpler sugar profile. If adequate nutrition is there, most yeasts will ferment these pretty completely, and you'll likely see the alcohol tolerance much more plainly in the driver's seat.

 

[Qhrumphf]

If you were to make wort solely with DME, you'd probably see attenuation around the average for a given strain, (assuming you provide for proper yeast health, poor pitching rate and poor oxygenation when pitching often lead to underattenuation). With extract you lose much of your ability to control fermentability apart from using added sugars (maltodextrin and lactose for example are highly unfermentable while sucrose and dextrose are highly fermentatable). What defines a yeast's attenuation level is the ability to ferment more complex sugars.