This was in the current issue of Chef Magazine in an article about muffins, coincidentally written by the very instructor, Paula Figone, that I referred to above. I took a gingerbread muffin recipe last week and made it two ways, once with soda, and once with powder. It collapsed with the soda, but got nice and dark. Next time, with just powder, it didn't get as dark, didn't collapse, and didn't taste as good. And this might be why....
"Why do some muffins contain both baking soda and baking powder?
Baking powder is the main chemical leavener in muffins. It contains both baking soda and acid, with cornstarch to absorb moisture.The baking soda and acid react in the presence of heat and water, producing carbon dioxide gas for leavening.
Some muffins contain an acid ingredient, like fruit
or fruit juice, cultured dairy products or honey.
Those that do typically contain added baking soda,
to react with the acid ingredient. This provides some leavening,
and if more is needed, which is typically the case,
baking powder is also added.
It's important to neutralize the acid in muffin batter
for more reasons than for leavening. Acid batters
bake white, which is fine for white cake, but too
stark for muffins. And acid batters bake up bland-tasting
because the same chemical reactions that
cause browning in baked goods provide flavor. So
for the most appealing muffin, add a small amount
of baking soda to neutralize the acid ingredients.
You might wonder what happens if too much baking
soda is added. Have you ever seen a green ring
around the berries in a muffin? Or too much browning
too quickly? How about a salty, chemical after-taste?
If you notice any of these problems, reduce
the amount of baking soda and try again." June Chef Magazine