The Anheuser-Busch factory brews more beer than any other factory. To meet the daunting task of making every drop taste like the perfect Budweiser, Anheuser-Busch has used the same yeast strain since 1876. Crucial to its flavor, this secret ingredient is kept under lock and key at the 100-acre brewery. What are the steps, secrets and science behind their legendary concoction? Ultimate Factories provides answers when it enters the castle of "The King of Beers" at the Budweiser factory in St. Louis, Mo.