The only kind of yeast used when making authentic sourdough. Don't misinterpret that; there are many varieties of wild yeast, each with its own characteristics. Several different kinds of wild yeast can be found in a single starter. Unlike commercial yeast that you get at your local supermarket, wild yeast prefers an acidic environment. It has to, to survive the acids produced by the lactobacilli. (see lactobacilli: see wooden spoons)