I was wondering what the purpose of the baking soda is in these rolls. I've never added soda to a yeast bread recipe.

Thanks, boulangere! I add baking soda to bread dough made with buttermilk for the same reason. ;o)

Baking soda reacts with the acid in the sour cream, creating carbon dioxide, which expands in the heat of the oven, and adds some additional leavening. As important is the fact that, by neutralizing the acid, a more hospitable environment is created for the yeast, which in this case does not thrive in an acidic environment. You get the benefit of using sour cream for its richness and tenderizing effect, without its acid interfering with the yeast.

Baking soda (sodium bicarbonate or bicarbonate of soda) is a leavenor. It helps your biscuits rise higher and be lighter by releasing carbon dioxide gas once moistened in a batter. It is used in recipes that contain acidic ingredients to reduce the acidity while maintaining the flavor. Too much baking soda will, however, give your baked goods a somewhat soapy flavor and a course, open crumb.