Yeast, Levains, & Sour Dough
Yeast is the stuff that gives bread its flavor, structure, and rise. Regular commercial yeast is called "Active Dry" or "Instant or Quick Rise". Active Dry Yeast is the stuff your mother and grandmother used - it's the one in the little packets that had to be added to warm water or milk and activated. Today's Instant Yeasts just go right into your dough. These yeasts do a great job of quickly making bread rise but do almost nothing to unlock the flavors in the flour. 30 minnutes to an hour just isn't enough time. Having said that, these yeasts can be both useful and used in such a way as to improve their ability to create great bread.
What we here in the US call "Sour Dough" is a natural or wild yeast culture that you can easily create at home. The French call it a "Levain". It does wonderful things for your bread and works gently, over a much longer period, than commercial yeast. During this long fermentation of your dough the wild yeast is munching away at the flour unlocking hidden flavors, creating CO2 and alcohol, and transforming the proteins into structure that makes crust and a chewy interior.
This Natural or Wild Yeast will make an excellent loaf that is NOT sour. It actually takes a little extra time to get to that typical "San Francisco" sour dough taste. There is a great deal of mystery associated with sour dough starters but the reality is very straight forward. You can make a great starter at home without it having to come from an old woman who gave it to your grandmother 80 years ago!