That bitter, faintly tinny flavor in a homemade muffin, biscuit, or scone may not come from the baker's technique. It can come from the baking powder itself. When a recipe calls for a larger amount of baking powder — as quick-rise treats like muffins and biscuits typically do — the aluminum in some baking powders can leave a metallic taste in the finished product.
Aluminum-free baking powders, including Rumford and Bob's Red Mill, are widely available and cost about the same as conventional versions. Bakers who prefer can also make their own aluminum-free baking powder rather than buy it.
The issue is not limited to one recipe or brand. It applies to any baked good that relies on a high proportion of baking powder for lift, which is why the off-flavor tends to show up in quick breads and similar items rather than in recipes using smaller amounts.
Editorial Notes