How does yeast attenuation work?

[BaconWizard]

Hey all.

Ok, so I get the general idea I think.

Say you have a brew with a potential alcohol of 10% abv, and a yeast that will happily tolerate 10% abv, and that attenuates at 80% (although it's actually a range depending on conditions, of course) then you can expect to actually get 8% abv and not 10%, with 2% sugars remaining.

That's how I understand it.

By the same understanding, if the potenatial alcohol was only 5% then at 80% attenuation, one is expecting 4% abv in reality. Correct?

Here's what I don't understand: In the latter case, the yeast is perfectly happy with the 4% alcohol it has created and could go much higher, to 10% if there were enough sugars. There remains in-fact just 1% sugars which have thus-far not been used. Why then, would the yeast not now get-to-work on this too, and end-up at almost 100% attenuation?

 

[Sadfield]

The wort contains a percentage of unfermentable sugars, regardless of potential abv. Also, alcohol tolerance, the 10% isn't what yeast ferments up to, its the level of alcohol the yeast can tolerate surviving in. What yeast is doing in making booze for us, is making its environment toxic to its competitors for sugar.

https://byo.com/article/whats-in-your-wort/

 

[Hazelwood Brewery]

Your wort is made up of many sugars. Some are easy to ferment others less easy and some are not fermentable at all with some yeasts.

The simple sugars like glucose are easy to ferment. You get more of these simple sugars by mashing at lower temperatures or by adding sugar into your wort. If you mash at higher temperatures you will get more of the complex sugars.

A yeast that has attenuation of 80% will ferment around 80% of the sugars so if your OG is 1.100 the FG could well be 1.020. If the OG is 1.040 the FG could well be 1.008. These would result in beers with ABV of 10.5% and 4.2% respectively. Alcohol is toxic to yeast so at some point the yeast will give up if you reach a critical level of toxicity for that yeast regardless of attenuation or gravity.

I will caveat this by saying the published attenuation can be quite different in the real world.

 

[dmtaylor]

Hey all.

Ok, so I get the general idea I think.

Say you have a brew with a potential alcohol of 10% abv, and a yeast that will happily tolerate 10% abv, and that attenuates at 80% (although it's actually a range depending on conditions, of course) then you can expect to actually get 8% abv and not 10%, with 2% sugars remaining.

That's how I understand it.

By the same understanding, if the potenatial alcohol was only 5% then at 80% attenuation, one is expecting 4% abv in reality. Correct?

Here's what I don't understand: In the latter case, the yeast is perfectly happy with the 4% alcohol it has created and could go much higher, to 10% if there were enough sugars. There remains in-fact just 1% sugars which have thus-far not been used. Why then, would the yeast not now get-to-work on this too, and end-up at almost 100% attenuation?

That is not how it works. Alcohol and attenuation are related, but not in that way. And your assumptions about 2% or 1% sugars are not correct. By your first example, I can actually back-calculate a wort that will achieve your goal of both 10% ABV at 80% attenuation. It turns out that this would occur at an OG of about 1.094. Basis: I know that ABV = (OG - FG) * 133 (approximately). And with apparent attenuation of 80%, the FG is about 1/5 of the OG (ignoring the 1.0 in front). If OG is 1.094, then FG is 94/5 = 19 or 1.019. And ABV = (OG - FG) * 133 = (1.094 - 1.019) * 133 = (0.075) * 133 = 10% ABV.

Hydrometers might provide a “potential alcohol” reading. However, this reading involves rough assumptions that are rarely if ever accurate in real life. And, they are based more on wine making than the making of homebrewed beer. Ignore any indications of “potential alcohol”, as it’s just confusing, and has little if any basis in reality.