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What Makes Double Acting Baking Powder Double Acting?

I am reading your article about the baking powder.  Mine contains monocalcium phosphate, sodium bicarbonate and cornstarch.  Since the monocalcium phosphate is listed in your article as available for both the fast and slow acting components, is this particular brand I am using (Rumford, aluminum-free) considered 'Double'?  It bubbled up right away on mixing and had a decent rise with the heat.

Double acting baking powder is called that because it has two reactions, one at room temperature when the wet ingredients come into contact with it, and a second in the heat of the oven.  To get the double acting part, two different powdered acids are typically used -- one for each reaction.

Some people mistakenly think that because a batter increases in volume during cooking that this is means the baking powder they used is double acting.  While that may be true, batter made with a single action powder will also increase in volume while in the oven.  This is due to the thermal expansion of the carbon dioxide (CO2) bubbles in the batter.  Double acting only refers to the release of more carbon dioxide when heated and not to the expansion caused solely by heat.

Fast acting, or quick acting, or single action baking powder is usually made with only one acid that reacts when mixed with liquids.  In theory, it might be possible to make a baking powder that only has a delayed action, but I have never seen that.

You are correct as far as you go in noting the previous post on Baking Soda vs. Baking Powder lists monocalcium phosphate for both double acting and fast acting types, but you failed to notice that one is monocalcium phosphate monohydrate (Ca(H2PO4)2 • H2O), while the other is anhydrous monocalcium phosphate (Ca(H2PO4)2).  The difference is that little "H2O" at the start of the first one.  That is a water molecule attached to the monocalcium phosphate, which allows it to react once it is mixed with liquids.  The anhydrous version is less water soluble and won't react until it is heated.

In theory the manufacturer could be using both kinds, but chances are that what you have is baking powder made with sodium bicarbonate and monocalcium phosphate monohydrate, so it would be fast acting, not double acting.  The cornstarch, by the way, is just a filler to make up volume so that it can be used measure for measure with other kinds or brands.

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