This Zymaster, Potrero Hill Sour-Mash IPA, begins in our distillery, where we prepare a 100% rye malt mash just as we would for our rye whiskey. After transferring it to a small open fermenter, we pitch the yeast and wait. Five days later, we mash it in the brewhouse, using a blend of pale barley malt and rye malt. Then we add the sour mash from the distillery, which lowers the pH of our brewery mash. By introducing sour mash before the boil instead of after it, we are able to create tartness in this brew without sourness.