[rainmaker]

In light of my recent thread regarding wlp565 and it's attenuation levels, it got me wondering about natural carb.

How does one determine the amount of gravity points a beer needs to drop to naturally carb? I believe some lambic breweries do this, but how is it calculated? If I want 3 volumes in my Saison, and I expect it to attenuate another 8 points, is there some formula to figure out what kind of co2 that will produce if bottled?

[Jimmy K]

Using an online gravity calculator, 1oz of sugar per gallon will raise the gravity by 0.002. This is probably better done with beer that is highly carbonated. The accuracy measuring such a small gravity change and knowing exactly what the FG will be would make it difficult for most low-medium carbonated beers. An error of 0.001 in reading the hydrometer or the actual FG would give you 50% higher or lower carbonation - not so good.
 
Unrelated to carbonation, you also wouldn't be allowing the beer to sit on yeast post-ferment. So the yeast couldn't clean up any diacetyl/acetylaldehyde or other off-flavors they've produced.

[erockrph]

The rule of thumb is that each gravity point nets you about 0.5 volumes of CO2. Generally you're looking at about 3 gravity points worth of priming sugar to carbonate a typical batch, depending on the temp of your beer as beer will hold on to some dissolved CO2 left over from fermentation.

Assuming your beer is at room temp, it would be at a little under 1 volume to start. Four gravity points would get you in your ballpark. Eight points requires a kevlar vest unless you're using champagne bottles.

[morticaixavier]

In light of my recent thread regarding wlp565 and it's attenuation levels, it got me wondering about natural carb.

How does one determine the amount of gravity points a beer needs to drop to naturally carb? I believe some lambic breweries do this, but how is it calculated? If I want 3 volumes in my Saison, and I expect it to attenuate another 8 points, is there some formula to figure out what kind of co2 that will produce if bottled?

the other half of your answer is the forced (or fast) ferment test or FFT. This takes a sample of the wort and using the same yeast as in the primary fermentation only more of it, and given perfect environment for the yeast to do it's thing, hopes to determine the absolute lowest gravity that beer with that yeast can achieve. This is your true (or as close as can be determined) FG. Now you know that if you bottle at that gravity + ~4 gravity points you are good to go.

[cornershot]

So assuming you bottle at just the right time during the primary fermentation to get just the right carbonation, would there be enough yeast in suspension at bottling to clean up the beer?