Final gravity after bottling?

[Shiggity]

I found https://www.homebrewtalk.com/showthread.php?t=148693 in an attempt to answer my question but it was so old (2009) that I figured the good-citizen thing was not to add to it, but my question is this:

Doesn't the FG go down during the time in the bottle? In the referenced thread, ajf says "Either the fermentation has finished or it hasn't."

But my understanding of the stoichiometry is that yeast simply converts sugar into alcohol and carbon dioxide. If it's making CO2 in the bottle, it's also making alcohol, isn't it? Then the "fermentation" isn't technically "finished," is it? Is it just that the additional alcohol content is negligible here, or am I missing something?

My concern of course is that I haven't gotten an accurate ABV.

 

[day_trippr]

If bottle conditioning is employed there is alcohol produced by virtue of the priming sugars being digested, and that will drop the FG by a small amount.

The scope of "fermentation" is generally considered to cover the original wort, not packaging-related processes (eg: when I keg the same recipe others bottle condition, my fermentation was definitely complete, and doesn't gain ABV through conditioning).

As well, when bottling beer, it's generally prudent to wait until successive SG checks demonstrate "fermentation" has completed, to avoid grenades...

Cheers!

 

[Shiggity]

Thanks for the reply. If I measure the FG from the bottle-conditioned beer just for the heck of it, will that yield an accurate result, or will the CO2 affect the gravity?

 

[Ogri]

The CO2 will cause a hydrometer to sit higher in the sample tube, giving a higher gravity reading. If you shook/swirled the sample enough to get all the CO2 to dissipate, then measured, you'd get a more accurate reading.

 

[flars]

The increase in ABV due to the priming sugar being converted by the yeast can be estimated using this calculator.
http://www.brewersfriend.com/extract-ogfg/

Enter the volume of beer and the amount of priming sugar in pounds. The priming sugar is entered as DME. Conversion is estimated using general attenuation of the yeast instead of 100% conversion of pure sugar. Not an exact estimate of ABV but in the ball park.