New here too. Sounds delicious. Let me know how well it works, as I am also looking for a good from-scratch root beer recipe. Also, if you have a way of making it into a brewed (as in has alcohol from yeast) version, I would be super interested.
It should be a simple matter of adding a bit of yeast (I'm guessing about 1/4 tsp for this size recipe) and a couple tablespoons of extra sugar to keep the sweetness the same. How much you add depends on how fermented you want it to be.
I also read something about gently pastuerizing the bottles to stop fermentation. Although that was more for the exploding bottle problem, I imagine it could also come in handy to kill the yeast after the desired alcohol content is reached.
Yea I'm not exactly sure on the amounts of sugar and yeast that concentration of ABV would require. I just have experience with over fermented root beer while trying to get it to carbonate. I never tested it with a hydrometer (primarily because I don't have one) but it certainly tasted more alcoholic than I would let the kids drink.
Since beers are made from grain sugars I don't know what the conversion would be for cane sugar. I'm not a beer brewer as of yet, but I wonder what would happen if you added malted barley to the above recipe and let it ferment like a normal beer.
InkyDoug, have you brewed your first batch yet? How'd it turn out? How did you come up with your recipe? I would like to start experimenting with some brews but am not very good at determining what flavors would compliment the root beer and which would be bad...