Beer FG vs. fast ferment FG

[p_p]

Simple question ...

In theory, should the FG of a beer match that of a sample of wort fermented warm and in the presence of oxygen (i.e. put back in the stir plate, etc.)?

thanks

 

[Dmoney1911]

I would think they would vary. In the presence of oxygen, yeast produces water and CO2, not alcohol. Since ethanol has a gravity about 75-80% that of water, your final gravities will be different. Not to mention anytime you split a batch the final gravity could vary a little bit, because the environment for the yeast is caused by many factors that make it difficult to perfectly replicate results

 

[p_p]

Thanks for your post.

Based on your comment, I would expect the beer to finish at a lower FG than the fast ferment sample considering the latter contains more water and less alcohol, however I usually see the opposite. Not by much, but I see the beer finishing higher.

Perhaps, as you said, there are way too many other factors which cannot be considered. Having said that though, I take the fast ferment sample from innoculated wort as opposed to split the wort and inoculate separately.

Either way, a more existential question comes to mind, what (TF) is fast ferment useful for if you cannot tell by it the exact FG your beer will reach.

I've heard interviews with pro brewers who claim to practice the use of fast fermented samples and rely on the results. How can they be wrong? I must be doing it wrong myself.

I have been very careful in the way the fast fermented sample of the beer I am brewing now has been treated and measured. As soon as the beer finishes, I will post the results back (and probably end up eating my words if the two end on the dot )

Cheers

 

[rkzi]

Interesting. I thought that FFT should finish at the same gravity as the beer itself, and this has been my experience the few times i have made FFTs. In my last batch i however found 2 points higher FG on the beer (used Danstar London ESB yeast) than in the FFT and thus used less priming sugar. My FFTs are just some wort taken to a sanitized mason jar after pitching, so pitching rate and yeast strain are the same.

Yeast will produce ethanol even in aerobic conditions if there's enough sugar available (IIRC the limit is SG 1.008). Google crabtree effect if you dont believe me. But in aerobic conditions also ethanol gets eaten after the sugars get depleted.

 

[p_p]

Well, the beer I have in the fermenter has reached FG and it happens to be the exactly the same I got from the Fast Ferment sample which finished 2 days ago ...

In this occasion though, I did not put the sample in the stir plate but instead kept it warm in a sealed container and gave it a swirl now and then.

So until I come across more info on the subject, I am going to conclude that, if done correctly, a Fast Ferment sample should result in a very close if not exact same FG as the beer ... I shall continue fast fermenting samples if I believe there is gain in knowing the FG of the beer in advance

 

[rkzi]

I think they can be useful if working with new or difficult yeasts. Some instructions for them though seem a bit weird. Many of them tell you to overpitch and use whatever yeast you have in hand, although they all have different attenuation ranges.