Seems like a reasonable question, jbaysurfer. If you're getting carbonation during 2nd ferment then you probably have enough residual sugar left over from your primary ferment to give you the fizz you're looking for. I start with 1 cup per gallon of tea & if I don't add the fruit juice at the start of 2nd ferment I don't get much fizz in just a coupla days. I would
get more fizz without fruit juice if I extended my 2nd ferment to maybe a week. Problem is that with that much extra time, the batch is continuing to get lower in pH & I don't like the taste as much.
One of my primary motivations for brewing KT at home is to control the sugar content. Commercially brewed KT and some local brewers at the farmer's market run a little sweet for my tastes. So to be clear, I'm only adding the fruit juice to kick up the carbonation a bit, hoping that most/all of the sugar from primary ferment and the fruit juice is gone by the time I drink my KT.
Here's the way I think of it, never having a true caloric analysis done on my final KT:
I start with 1 cup sugar in a gallon of brewed tea. That's equivalent to 770 calories into 16 (8oz) cups, or about 48 calories in a cup (8oz) of the sweet tea *before* primary ferment. I'm hoping almost all of that is gone after 10 days, eaten up by the yeast & bacteria.
I add 3-4oz of fruit juice per liter before 2nd ferment. The fruit juice I choose is 100% juice but not a concentrate, and it has about 90 cal in 4oz. So I start with 90 calories into a liter, which is only about 21 calories per 8oz cup.
Absolute worst case, no sugar eaten up at all, I'm in for 48 + 21 = about 70 calories per 8oz serving. But I believe almost all of the original 48 cal is gone after primary ferment, and want
to believe at least half of the fruit juice is eaten during carbonation, so I hope that I'm only getting 10 cal in an 8oz serving.
Amazing what I'll do to rationalize my KT habit