I've never had it, but I'm intrigued by kvas and looking for authentic recipes. I have brewed beer and champagne-method hard ciders before, so I'm familiar with brewing
equipment. One of the recipes I've seen used plain yeast - do the authentic versions use different yeast? What is the final alcohol percent? Can you ferment it longer, or
should you even attempt that? Any help will be appreciated.
There is a recipe for kvas, along with good background information, in Anne Volokh's excellent "The Art of Russian Cuisine".
If you have access to Russian stores (e.g. Brighton Beach, Brooklyn), you can buy bottled kvas CONCENTRATE (avoid bottled kvas, which I find way too sweet and malty in a nasty, chemical way). The concentrate is a thick, black malty syrup that comes in a small brown bottle (or sometimes a jar) to which you add warm water, sugar and yeast and allow to ferment in a warm place for 18-20 hours. I have made it many times this way with very good results; you can control the sweetness by adjusting the amount of sugar.
I store the finished kvas in the fridge in glass bottles with those rubber-lined pastic caps attached with a locking wire closure; it lasts a day or two and then goes flat. In Russian the concentrate is called "kontsentrat kvasnovo susla". While perhaps less authentic than making it the old-fashioned way with dried bread etc., the concentrate is more convenient, consistent and the yields easily scalable. The bottle in my fridge was imported by a company called AMROS of Somerset, NJ. The phone number is 908-846-7755. Good luck.
OK, i'm spacing out on the exact name of the cookbook ("Nourishing Traditions"?). Anyway, it's published by the Weston A. Price Foundation. The info should be on their website. I can look all of this up, once i get home, if you can't find it. Send me an Email in that regard. The point is, the book has a recipe for kvas and a few other fermented drinks and foods. Definitely worth looking at.