Prior to the evolution of commercially available baking powders and yeasts during the 19th century, yeast starters were the leaveners used in breadmaking. Such starters are a simple mixture of flour, water, sugar and yeast. (At one time, airborne yeast was the only source used, but today convenient commercially packaged baker's yeast is more common.) This batter is set aside in a warm place until the yeast ferments and the mixture is foamy. A portion of the starterusually about 2 cupsis removed and used as the base and leavener for some bread recipes. Once fermented, yeast startersthe most famous of which is sourdough startercan be kept going in the right environment for years simply by adding equal parts flour and water. Herman starter is a colloquialism (of unknown origin) for a honey- or sugar-sweetened starter used primarily for sweet breads. Starter should be refrigerated and can be stored this way indefinitely as long as it's replenished every 2 weeks. Before using or replenishing, it should be brought to room temperature. If a starter turns orange or pink and develops an unpleasantly acrid odor, undesirable bacteria have invaded it and the mixture must be discarded. Two cups of the foamy starter mixture can be substituted for each package of yeast called for in a recipe.