Calculating ABV with after back sweetening

Ok so I am trying to make homemade wine coolers. There isn't a whole lot of info out there about the process, so I apologize for my questions if they seem like nonsense...

I started out with some store bought juice (mixture of various kinds...) and I added some dry champagne yeast. It has been bubbling away for the past two weeks, and it has only dropped from 1.042 (OG) to 1.030. So that puts it at about 1.6% ABV. I am wanting the final product to be about 3.5%-4%.

I plan on adding juice after it is done fermenting to kind of take the wine tasting edge off (more of a normal wine cooler taste... so really juicy! ) After adding the juice, I want to bottle it and let it carb. I don't think it should take too long to carb (It wouldn't surprise me if it only took 24 hrs...) and then I am going to pasteurize it on the stove top in a big pot of water to stop the yeast from fermenting it dry and causing bottle bombs.

I guess I have two main questions...

#1: Is it normal for champagne yeast to move so slowly? I have it in a room that sits at about 68*F-70*F...

#2: How do I calculate the ABV after I add the juice back in right before bottling? I realize I should let it ferment almost dry, if not all the way dry since I will be "watering" it back down with juice. So lets just say hypothetically it ferments down to an even 5% ABV... How much juice do I add back in to make it around 3.5%? I know it won't be exact because some additional alcohol (not much though) will be produced through the carbonation period... I would probably use the same juice concoction to back sweeten with so it would be very close to 1.042 juice going back in...

Hopefully this makes sense, and I have a feeling that it is going to be difficult to approximate... But I would love to hear anyone's thoughts, ideas, or suggestions!

Thanks Guys and Gals!!!

That does seem like a slow fermentation for Champagne yeast at those temps. I wonder if there were any preservatives in the juices that might have slowed it down?

As for ABV, carbonation usually only consumes about .004 (I think) of gravity. So I think that would be a negligible amount to worry about.

Personally, I'd ferment to 5-6% and then just sweeten to your preferred taste and not worry about hitting 3.5% exactly. That said, I'm sure there is someone on here that can do the math for you exactly if you know the exact gravity of the "juice concoction" that you are going to backsweeten with.

Here's a link to a recent thread we just had on this topic.
https://www.homebrewtalk.com/f32/backsweetenings-impact-abv-395642/


What you need to do is average the ABV of your fermented product and your backsweetening liquid. Here's the equation:
(Volume1 X ABV1) + (Volume2) / (Volume1 + Volume2)

I knew someone could provide the formula. haha

I'm an art major...if it tastes good I'm usually ok with whatever ABV is ends up at

Thanks for the posts... Ill try to figure out the math... Definitely not my strong subject! but always up for a challenge... I thought about adding juice until it taste good, but it's for my mom so it would probably end up somewhere like .5% ABV... Lol!

LeBreton said:

Here's a link to a recent thread we just had on this topic.
https://www.homebrewtalk.com/f32/backsweetenings-impact-abv-395642/


What you need to do is average the ABV of your fermented product and your backsweetening liquid. Here's the equation:
(Volume1 X ABV1) + (Volume2) / (Volume1 + Volume2)

Click to expand...

Slightly incorrect, use this instead:

(Starting volume X ABV)/(Total volume after back sweetening)