A while back, I purchased a “Spike Your Juice” kit from ThinkGeek.com. This kit contains a plastic and rubber airlock device and several small packets of yeast.
To use the kit, you pick a 64-ounce bottle of a fruit juice you like (one that does not require refrigeration in the store) and open it up. You drop the yeast into the juice, then put the stopper on top of it. The yeast sets about turning the sugar in the juice into alcohol for you. After a couple of days, the beverage is ready for consumption. The longer you leave the yeast working, the higher the alcohol content will be.
After two days, you’ll typically find that the juice tastes a little sweet, with some elements of the “raw” version of the fruit in the flavor (e.g., green under-ripe apples in apple juice). After three or more days, the juice will have a very “dry” flavor to it with virtually no sweetness. With experimentation, it’s possible to find a balance of time and flavor that you like.
The makers claim that your alcohol content can reach “as high as 14%” in two days. I’ve never made an attempt to measure it (don’t own the equipment) but I can tell you that some experiments have yielded more potent juice than others. Some left me feeling the effects after a glass or two, while others I was able to drink most of the bottle and not notice much. Your experience may vary.
The Spike Your Juice kit [Amazon affiliate link] isn’t cheap. For $20 you get one stopper, six packets of yeast, and some labels (which I never used). This will be enough to allow you to make six 64-ounce batches, but only one at a time since you have only the one stopper.
If you’re looking to experiment with this “on the cheap” you may want to check out the WonderHowTo site’s tutorial, showing both the Spike Your Juice kit and their own DIY version using just the airlock (two of which will cost $7.49 on Amazon as of this writing) and champagne yeast ($6.06 for 10 packets which should make many batches).