Homemade bread kvass with rye flour sourdough

Homemade kvass in the heat is a real salvation! Recently, there has been a lot of talk about the dangers of yeast (purchased), they attribute to them a lot of harmful effects of consumption, etc. Whether this is true or not is not completely clear. It should be noted that yeast appeared in our country not so long ago, and before that we managed without them. Why not take advantage of the experience that has not yet been lost and cook, for example, kvass without yeast. I will immediately make a reservation that kvass is a product of fermentation, and this process cannot do without yeast. But it can do without the purchased ones.