I want to use a fruit juice in my next beer and I was wondering if the sugar content in the fruit juice say store bought pineapple juice would be enough to carbonate somewhat in place of a priming solution before bottling. Any help on this subject?
As far as I've read, fruit juices tend to be finicky, some people have success with adding as far as priming and a decent amount have gushing occur. Most doable process I've read is take a small amount of beer, blend and taste until the ratio is to your liking, scale up and add the juice directly after fermentation slows or even stagger between later days. Volatiles will be lost depending on fruit. Bottle as usual. This seems like the safest option for a homebrewer IMO.
You'll have to determine the sugar content of the fruit juice and calculate the amount required from there. For example, let's say a carbonation calculator says you need 4 ounces of table sugar for your (assumed) 5 gallon batch. And let's say you wanted to prime with apple juice, and the label on the container looks like the one at this link... http://nutritiondata.self.com/facts/fruits-and-fruit-juices/1822/2 The important data is the serving size (248 g, 1 cup) and the Sugars (24 g per serving). So you know that 8 fluid ounces (one cup) contains 24 g of sugars, or 0.846575 ounces by weight. Since you need 4 ounces of sugar, you divide 4 by 0.846575 and get 4.725. Now you know you need 4.725 servings of apple juice to get your 4 ounces of sugar. 4.725 servings x 8 fluid ounces per serving = 37.8 fluid ounces of apple juice needed to prime this batch. That's more than a quart. (Adding that much liquid 5 gallons of beer also changes the amount of sugar you'll need to reach "X" volumes of CO2, but it's not a big change.) If I were considering doing this, I'd probably balk at the volume of apple juice required (thus changing gravity and balance of the beer) and think about using a concentrate instead. Same basic process...determine how much sugar is in your contemplated priming sugar source and do the math. ETA: I see you asked about pineapple juice. Same process.
This is exactly how I prime a fermented beverage that we're not supposed to talk about, using frozen concentrate. I would think twice about using pineapple juice, though, as it contains a high level of acidity that may clash with the other flavors of your beer, and protein denaturing enzymes that might effect body and head retention.
