A Tale of Two Loaves

Today, I baked two loaves of San Francisco sourdough bread from a recipe on http://www.thefreshloaf.com .  I've made this recipe before with some success.  Today's bake, however, was very interesting.  First the loaves feel heavy - not light and airy.  Hmmm, underproofed, overproofed?  Then while baking the first loaf, the crust never browned.  It looks pale and anemic.  So, for the next loaf I kept the oven at 500F for at least 20 minutes into the bake, browner loaf, still kinda heavy but better color. 

See for yourself --