sourdough bread is leaven bread (in French, pain au levain, see below), where the fermentation is secured by a combination of lactobacilli (see lactic acid), i.e. acid-producing bacteria, and wild yeasts. Unlike yeasted bread dependent on the alcoholic fermentation of strains of Saccharomyces cerevisiae (brewer’s or baker’s yeast), sourdough undergoes a lactic fermentation, thanks to the lactobacilli, while the wild yeasts step in as raising agents. The greater flavour of sourdoughs, currently much preferred by bread enthusiasts, over regular breads is thanks to the lactic fermentation. Satisfactory performance, however, requires that the activities of the yeasts and the lactobacilli be held in balance through skilful manipulation of the starter dough. Anyone can make a starter by leaving flour and water in the warm and allowing it to ferment over a period of days. During this time it will capture the yeasts and bacteria that are present everywhere and fermentation will commence. Each place has its own population of yeasts and bacteria, and this is a reason for the variation in flavour between breads of one region and another. It has been established that San Francisco sourdough, among the most celebrated types, is the product of the yeast Candida milleri and Lactobacillus sanfrancisco. By contrast, a German rye sourdough can be reconstituted on the basis of the yeast Candida krusei and Lactobacillus brevis var lindneri. These bacterial cultures can be freeze dried and marketed.
