RE: Your original question on sourdough yeast...
“Sourdough Starter
Prior to commercial yeast production, bakers relied on starters to leaven their breads. Early starters were simple hut magical mixtures of flour and liquid (water, potato broth, milk) left to capture wild yeasts from the air and then ferment. Only a portion of the starter was used at a time. The rest was kept for later use, replenished periodically with additional flour and liquid so that the magic could continue. Today, starters are generally fortified with yeast to provide consistency and reliability; they are prized for the unique, sour flavors they impart. Prepared dry cultures are often used commercially to give bread a sourdough’ flavor without requiring the time and space necessary to develop and maintain an active starter. “
http://www.victoriapacking.com/yeastinfo.html