Many people believe that a starter started with commercial yeast will eventually be "taken over" by wild yeast. This is a good thing, and the quality/predictability of the resultant bread should improve as this happens, since commercial yeast isn't really designed to be used that way. Until that process is completed, the starter is in a state of transition toward a desired end-state where wild yeast and bacteria maintain a balanced, stable, symbiotic relationship. It follows that an evolving starter will produce breads with differing characteristics as the nature of the starter changes. Only a stable starter will produce consistent results.
It can actually take quite a while for a starter (even one that began without commercial yeast) to reach a good balance of microbiological life. This is one of the reasons why many bakers use cultures from long-established starters (why go through all the time and trouble to nurse your starter to a balanced state when there are lots of already-balanced starter cultures out there with proven characteristics?).
-Sam
There was an experiment done by a Dutch group: baker's yeast didn't survive more than two refreshments of a sourdough culture. I think that it's the acetate that kills the yeast as it is less acetate tolerant than sourdough yeasts.
-Michael
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This page was last updated on Tue, Aug 3, 2010 at 7:52:59 PM.