I went to the 4R website to print out the formulae for their various barrel strength bourbons. Yeast strains play such a critical role in theirfinal products that a Cowdery kinda question occured to me: In the sourmash method, consistency is provided by backsetting some of existing mash into the next vat--just the way sourdough bread is self perpetuating. Even so, master distillers and yeast makers (their old title) maintain a library of jug or dry yeast. Is that yeast kept for "insurance" against the loss of a key component or is yeast also added to the sourmash? Why isn't the sourmash sufficient on its own?