Its so easy to make by just washing grapes and using the yeast on them with your own flour and water. Very easy!!!! The white film on grapes is natural yeastI tried that once and did not like it but don't remember why? There are some new options out there I have not tried or I can just go wild? I want to capture a specific sour flavor and may need to try various strains to find it. I have read quite a few recipes now and it would seam a small addition of instant yeast along with the sourdough starter will aid in getting the nooks and crannies I want in the muffins.