Carbonating sugar-equivalent quantity of juice?

[Cider Wraith]

For folks that are versed in bottle carbonating and know how to do the calculations kindly looking for input. Have a few two-and-a-half gallon cider batches completing flat-dry primary in five gallon corny kegs. Hoping to transfer to five gallon corny servers and at the same time add some additional quantity of off-the-shelf apple juice to give enough fermentables to get the server PSI up into the range 12 ~ 18 PSI.

This would be different from bottle carbonating because of adding apple juice instead of sugar, although couldn't the sugar content be calculated by someone who knew how to do it, and because of the greater headspace, about two-and-a-half gallons. Because the kegs can tolerate tremendous pressure no danger of overdoing it, just wanting to see if I can start out somewhere in the ballpark. Maybe somewhere shy of a quart of fresh apple? Thanks to any kind folks that can do the math or would otherwise care to take a stab at it

 

[VikeMan]

Look at the nutritional info for your apple juice. Note the following:

Serving Size (probably in fluid ounces)
Sugar (will be stated in grams)

Figure out how much sugar you want, in grams, using one of the many online calculators or your brewing software. Divide that number by your juice's Sugar grams per serving number. The answer is the number of servings of juice to add. Multiply that by the Serving Size to get the number of fluid ounces to add.

The above may be "good enough," but because the juice will also add volume, you may want to take the "fluid ounces to add" answer from above, add that to your beer volume, and iterate throuigh the whole thing one more time.

 

[Cider Wraith]

Thanks for the reply - ok, I'm working on it:

Serving size, 8 ounces
Total sugars (per serving) 27 grams

So, 3.375 grams sugar per ounce juice

At about 65 F 15 PSI is 1.7 Vol

Assuming apple based sugar is roughly equally fermentable as table sugar

To produce 1.7 Vols in 2.5 gallons requires about 30 grams sugar

To get those 30 grams need 30 x 3.375 = 101.25 ounces.

And again, like you wrote, that isn't taking into account the additional beverage

So just under a gallon of new apple juice? Seems high, does my work look approximately like it? Thanks -

 

[VikeMan]

At about 65 F 15 PSI is 1.7 Vol

Are you planning to serve this at 68F and 15 PSI? The answer may be important, because it seems like you are "backing into" the volumes of CO2 you want/need (1.7 volumes), rather than choosing the CO2 level you want by preference.

Serving size, 8 ounces
Total sugars (per serving) 27 grams

To get those 30 grams need 30 x 3.375 = 101.25 ounces.

Not exactly. Here's a shortcut. If you have 27 grams of sugar in 8 fluid ounces of juice...

8 Fl Ounces Juice
------------------- x 30 Grams Sugar = 8.9 Fl Ounces Juice
27 Grams Sugar

 

[Cider Wraith]

Ha, I'm laughing, did I make a whopping mistake? I've got to start over again lol

The reason for mid-teens PSI is experimentation seems to point to pressurizing fermenting into about that area, when refrigerated, drops down to about 8 ~ 10 PSI which seems pretty good

 

[VikeMan]

The reason for mid-teens PSI is experimentation seems to point to pressurizing fermenting into about that area, when refrigerated, drops down to about 8 ~ 10 PSI which seems pretty good

I don't know how you measured, but a keg at 15.1 PSI at 68F (1.7 volumes of CO2) drops to about 2.8 PSI at 38F.

If you want it to settle at (say) about 9 PSI at (say) 38F, you'd need ~2.3 volumes of CO2. At 68F, that's a little over 24 PSI.

 

[Cider Wraith]

"If'n it was a snake it woulda' bit me" ... right there on the label, an 8 ounce serving gives 27 grams of sugars and I was guesstamating about 30 were needed so just throw in about ten or twelve ounces of new juice.

I don't know how you measured, but a keg at 15.1 PSI at 68F (1.7 volumes of CO2) drops to about 2.8 PSI at 38F.

If you want it to settle at (say) about 9 PSI at (say) 38F, you'd need ~2.3 volumes of CO2. At 68F, that's a little over 24 PSI.

Thanks again for the insightful replies. I might experiment with about sixteen new ounces of juice and go from there. And honestly, it doesn't have to be apple juice and if I added something like pom and it didn't fully ferment I'd have some residual pom sweetness.

You've answered and actually corrected my question, much appreciated.

Edit - and ah yes, I appreciate that the sugar content of pom is probably different