What makes the yeast stop fermenting in low abv ale?

Hi, I'm trying to make a session bitter ale, with an OG of 1.045 and a FG of 1.015... my question is, what will make the yeast stop where I want them to? I'd assume that 3.9% abv isn't going to stop the yeast (white labs english ale), and it will still have some residiual sugars left in the beer, so how do I get it to stop at 1.015? Do I simply take hydrometer readings until I get to 1.015 and then benzoate?

Low ABV beers do tend to finish on the low side but mostly it's affected by the fermentability of the wort and the attenuation of the yeast strain you use. If you brew all grain then it's as simple as mashing a little higher, I'd say in the 157F range and using a yeast that tops out at 70% attenuation. If you do extract try and research the fermentability of different brands of extract out there. Picking the brand with the lowest is about the best you can do, as well as selecting your yeast for low attenuation.

Why do you want it to stop at 1.015? You could also do a cooler fermentation for a slower, more easily monitored fermentation that would allow you to chill it down when your target FG is reached. This would take a bit of monitoring though.

FG depends largely upon the amount of non-fermentables in the beer. If you do extract brewing, there are some non- fermentables in the extract. If you brew all- grain, a higher mash temperature gives you more non-fermentables. In ether case, using crystal malts in your grain bill (as a steeping grain for extract brewing) will add more non-fermentables and increase your final gravity.

You don't want to stop the yeast or kill them, because you will need them for bottle conditioning.

So the recipe is the key. If you are new to brewing, you might consider a top quality kit. Cheers!

1.045og to 1.015fg is 67% attenuation
white lab English ale yeast wlp 002 lists it's as 63-70%
so you should be pretty close
I don't think it's a good idea to try and force the yeast to
stop attenuating. If the gravity comes out too high or to low
adjust your mash temps next time you brew that recipe.

hmm, I see... interesting, I just assumed all yeasts ate 100% of the sugar until they passed out from the booze

also is there a rule of thumb for deciding how long to leave the beer in the carboy before bottling? do you just monitor until FG stops going down, or should it be left to age for awhile at a stalled FG before bottling?

3-4 weeks is a good policy.

If you are looking for a higher FG, Muntons regular dry yeast is one of the lowest-attenuating yeasts around. It's better with simple sugars than complex malts.

If you want a 3.9% Bitter, start at 1.036-1.038.

badmajon said:

hmm, I see... interesting, I just assumed all yeasts ate 100% of the sugar until they passed out from the booze

also is there a rule of thumb for deciding how long to leave the beer in the carboy before bottling? do you just monitor until FG stops going down, or should it be left to age for awhile at a stalled FG before bottling?

Click to expand...

Not all sugars are alike. Lactose is an example of a sugar that doesn't ferment out. Steeping crystal malt will also add non-fermenting sugars to your wort (and make the resulting beer sweeter).

+1 on David's timeline suggestion.