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Carbonating without additions

So I’m looking for a formula to help calculate the carbonation levels that result from capping (i.e., kegging) a fermenting beer at a certain gravity. For example, if a fermenting beer is at 1.020, 45 degrees, and I expect final gravity to be 1.012, what will the resulting carbonation level be if I keg it now? I’m sure it will also depend on headspace. All the brewing literature I’ve found only refers to either priming or force carbonating. I’m sure this info would have to come from professional brewing literature, as it’s not something most homebrewers would dabble in (not saying I will either, but I’d like to get a better feel for it.)

I have spoken to a professional brewer acquaintance who says that he caps it at 2-3.5 degrees Plato above the expected final, but that depends on the headspace and he is dealing with much larger volumes than 5 gallons. He is convinced though that it is the best way to carbonate. He thinks even tanked CO2 has its own flavor, so force carbonating lends a particular artificial flavor, which is what sparked my interest in this topic in the first place.

Thanks in advance to anyone who can help.

BTW, completely off topic, but I was able to tour the Weihenstephan brewery outside of Munich a couple weeks ago. I could post my notes and maybe a few pics in a separate thread if anybody has any interest.

Just off the top of my head, why not figure gravity as priming sugar. I could be totally wrong. Normally I will add between 4-8 oz of priming sugar to bottles which is between 2-4 gravity points. So maybe seal the keg around 1.016 for 4 vols CO2 or 1.014 for 2 vols CO2.

This is just me thinking. I have no experience carbonating the way you describe.

What you are proposing is a little dicey simply because you need to know exactly what your final gravity is going to be in order to know how much sugar is left. If you are reasonably comfortable with a recipe, then you can have a go at it like mvsawyer suggest. I’d cap at 1.015 for an expected FG of 1.012. A keg will hold 50-60psi and should have a relief valve that will blow so safety wouldn’t be a huge issue. not sure I’d do this with glass bottles.

Why are you interested in going this route anyway? Is time a factor and you don’t have bottled CO2 for force carbonating?

Well, it’s part scientific curiosity. I know one professional brewer who carbs all his beers this way, and I want to know how to reproduce it at home, regardless of whether I actually choose to carb this way. Also, this same brewer, whose judgement I trust, believes that even bottled CO2 has its own flavor that it imparts to beer. If true, I expect that kind of taste is way beyond the threshold of my palatte, but still… And yes, the same results could be had by priming with speise held back on brew day, but that seems like one more step than if I could simply calculate when to cap the fermentation.

Again, it’s mostly just curiosity, but if I could figure it out, it might be the simplest way of carbing.

It just occurred to me that you might want to look into something I’m experimenting with called pressurized fermentation. I made a variable pressure relief valve with gauge (spunding valve) and use this to run 7-15psi during fermentation. I can even dial up to 30psi towards the end and basically have carbed beer like you are after. Even if you run a very low pressure during most of the fermentation, you can set t at 30psi or whatever temp you want towards the end and not have to exactly time the last few gravity points.

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