Hi MK, "commercial yeast" does refer to the type of yeast you can buy at the grocery store, as opposed to the yeast (and lactic acid bacteria) that make up your sourdough starter. Since most sourdough breads are baked at a relatively high baking temperature, adding steam to the initial part of the bake will not only provide for fuller expansion, but also deliver that golden/shiny crust you're looking for. In addition, you may find Maurizio Leo's article on Baking Sourdough at High-Altitude helpful.

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