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A simple mixture of flour and water--a 6,000-year-old discovery in
ancient Egypt--is all it takes to create sourdough starter. Left at room
temperature, wild yeasts and the bacteria, Lactobacillus, from the
grain and the air, jumpstart the process of fermentation, which gives the
dough its sour flavor. After "feeding" it with more flour and water for
about a week, the starter can be used in place of yeast in bread and
other recipes. The starter may be kept alive for an indeterminate
period--many trace back over 160 years.