There are different kinds of yeasts and that word is used as a bad one in the candida cleanse community certainly. The wild yeasts that cover every inch of our body in association with the acid mantle of the skin and in balance in most of our internal body help us tremendously. It is only when we
feed excessive sugar into the system does the sugar magnet candida come into play.
If you want to do a little experimenting, you might want to see if you can get your hands on some
raw milk. Pour a small portion into a sterile glass jar with a lid and sit it on your counter, out of the sun until the milk clabbers or separates. This is the way milk was consumed in many a Scottish traditional village because it is healthy for you and refrigeration was non-existant. I have tried it and it is VERY tart but my tummy found it fine.
If you then pour off the yellow liquid, this is live whey culture. You can use about 3 tablespoons whey to any jar of fruit/veg as the starter for fermentation.
You might also simply cut up the bananas, which have high fruit and leave them out for a few hours on a clean dish, then process them into the jar--this would take advantage of the wild yeasts in the air--they exist on the skin of the banana, but since you are removing them, you need to replace them somehow.
Good luck and let us know how it turns out. Kvass would change the taste and color me thinks and I never cared for rejuvelac at all. Didn't sit well with me.