beginners questions: in some recipes, why is a small amt of IDY added to dough even when a natural starter is used?

=== i tried a recipe today that i really liked and it called for a small amount of yeast in addition to the natural starter. i was just wondering if it is to simply quicken the fermentation process? i thought yeast would overpower the natural bacteria? ===

I add bakers yeast to sourdough when I have a tight schedule and otherwise cannot afford a long or failed rising cycle.  It does result in less total sourness and a somewhat different flavor, but I put the whole grains in the overnight build so they are pretty well sour-ized by the time I get to the morning rising cycles.

I think that is basically why Hamelman's recipes, for example, include bakers yeast:  commercial bakers need to have a reasonably exact production schedule.

sPh

I started as a purist but quickly discovered I didn't have time for long rises and potential failures,as the previous response indicated. I came to understand why housewives in the 1920's and 30's would save their pennies so they could purchase a cake of yeast. It must have seemed a miracle to have consistent and fast rises.

I occasionally have a sourdough-only baking fest but I like to use 1/4-1/2 tsp of instant yeast for a 1-2 loaf recipe simply so I can get more baking done in a day as I bake on Saturdays. I bake all our bread for lunches and breakfasts.

My sourdough culture is strictly wild-gathered and not started from a commercial yeast-it adds such wonderful flavor to everything you make with it.