Priming a Keg - non standard method

Here's a link to a search that I did of the forum. Maybe there is something in either thread that will help. I know this topic has been discussed in this forum and calculations were a part of the discussions, but I didn't look thru these two threads to see what is there.
https://www.beeradvocate.com/commun...t&t=post&o=date&g=1&c[title_only]=1&c[node]=8

 

I think I recall some thread or other on priming a keg with sugar that made me think it would be useful to keep a moderate positive pressure in the at the start, as some kegs (used kegs?) will leak under low, gradually accumulating pressure provided by fermentation. Higher pressures help set the seal. Maybe something to keep in mind...

 

I never dug into the “easy” way to calculate what you need… But when I wanted to bottle condition exclusively with maple syrup (from my aunt) I did a little experiment to find out how much to use. Worked flawlessly and bottle carbed to a perfect level as desired (actually, that was in 2015 and we just cracked one of those open last night!). Probably the “long” method, so hopefully somebody has a shorter way to go about this for you… I just never dug into it.

Set your target CO2 volume (3.5vol’s)… Now, let’s say 12oz is enough to fill a hydrometer tube… Using a carbonation calculator, this will require .13oz of corn sugar.

Place solution into hydrometer tube and take gravity. Let’s call it 1.016. You can discard this, now.

Get another 12oz of water and add fruit concentrate (by mass) until you reach 1.016. Let’s call it .08oz the end result was (yes, you will need an accurate scale!).

Use that ratio now to reach desired carb level… If your carbonating 5gal to 3.5vol, that would traditionally require 7oz corn sugar (at 66F) to carbonate. Since the fruit extract was 61.5% that of corn sugar by mass (.08/.13 * 100), just adjust by that ratio… So instead of 7oz, add in (7oz * .6153) = 4.31 oz of fruit puree.

Obviously the numbers will be different… but process is the same. At least how I went about it. Again, the end result worked perfectly, not over or under carbonated.

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Now thinking about this in more of a calculative sense, I never looked into it really as I just did my experiment and never had to do it since… But I think it may be as simple as comparing sugar densities of solution.

1 gal water + 1 lb sugar = 1.046 SG (rule of thumb)
Same as adding 1oz of sugar into 8oz of water.

This results in a density of .037789 lb/in^3 (SG x .036127 to convert SG to density [lb/in^3])

The puree solution, assuming 1.050, would have a density of .037933 lb/in^3

Meaning the puree has a higher sugar concentration… .037789/.037933 * 100 = 99.62% (the sugar density of the sugar solution vs. puree)…

So it takes ~ 7.0oz of sugar to carbonate 5-gal of beer to 3.5 volumes at 66F.

7oz * .9962 = 6.97oz puree to carbonate to 3.5 volumes (you need less of it, due to the higher sugar concentration). But as you can see here, you could almost treat them identical as they are so close… 7oz should do it with either sugar or puree.

Anybody agree with this logic?

 

I used to carb my kegs with priming sugar often. As @pweis909 mentions, you must keep the keg under some pressure for a couple of days or else the keg will not seal.

I never took the time to fine tune carbonation with this method. I would calculate my priming sugar addition lower than I wanted, expecting the 5 lbs of pressure for 2 days to contribute additional carbonation. Not an exact science but it worked well enough.

 

Carbonation aside, I'm questioning your use of puree in the finished beer. It won't look good, and it will feel odd in the mouth. In fact, all of that fruit pulp may even alter the flavor of the beer in a way that you'd rather not have it go. Just my 2 cents.

 

I totally overlooked the puree angle. I would worry about solid bits clogging up lines and valves. I've recently had problems with hops coming out of a hop sack and clogging things (confirmed when I had to re-rack) , and a bit of cranberry clogging another keg (haven't confirmed yet, but I am afraid I'll need to take measures if I want to drink more cider). You really want to keep solid bits from clogging tubes and valves on your keg.