From a baking perspective, yeast is yeast whether you capture wild yeasts from the air or buy yeast at the grocery store. In French bread making, you can hold onto a levain -- I have not idea how to spell it --- which is essentially a lump of dough from the a previous bread-making session. But it is still a yeast-based leaven like any other. I must say, every time I have my hands in dough, which is often, I appreciate the Biblical use of leaven, breads and so forth as illustration.

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JWithnell
Member Bethel OPC
Virginia