Theory question: apparent attenuation

[Brewer393052]

Hi,

I'm reading about apparent attenuation and I was curious about the theory behind it.
Let's say I'm brewing a beer with an OG of 1.076, fermenting it with a dry yeast that has an apparent attenuation of 68-72%.
Does that mean that after fermentation, the yeast will have converted between 68 and 72% of the original 76 points of gravity? Ie. the beer will have a final gravity between 1.022 and 1.025?
Did I get that correctly?

Thanks

 

[DBhomebrew]

You got it

 

[BeerHolic]

 

[VikeMan]

Let's say I'm brewing a beer with an OG of 1.076, fermenting it with a dry yeast that has an apparent attenuation of 68-72%.
Does that mean that after fermentation, the yeast will have converted between 68 and 72% of the original 76 points of gravity? Ie. the beer will have a final gravity between 1.022 and 1.025?
Did I get that correctly?

Yes. Sort of. I say sort of, because of the words "converted between 68 and 72% of the original 76 points of gravity."

The FG reading doesn't mean that there are (in your example) 22 or 25 points worth of sugars/dextrins left. There are actually more that. i.e. there have not been 54 or 51 points worth of sugars converted. It's because there's a difference between Apparent Attenuation and Real Attenuation. Apparent attenuation is a function of two major things...

- the sugars attenuated
- the fact that alcohol is less dense than water

Because alcohol is less dense than water, it lowers the gravity further than just the absence of the fermented sugars would have. So some of the sugars that have "apparently" disappeared have been converted, but some are simply "hidden" (from a hydrometer reading perspective) by the impact of alcohol's low density. Thus the term "apparent" attenuation.

You can get a pretty good estimate of Real Attenuation by dividing Apparent Attenuation by 1.22.

Most times, there's no reason to know or care about Real Attenuation. But it can be important for estimating true remaining carbs (e.g. for a Keto diet), or for someone bottling, say, a Brett beer.