Do you have to use dutch processed cocoa when it's called for?
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Usually yes, because the recipe is calibrated for it with regard to acid/alkaline balance. If you don't use Dutched, and just use Unsweetened Cocoa Powder, you will have to add baking soda to the recipe in order for it to come out right. The amount depends on the recipe and the amount of cocoa called form. Dutched in netural, Unsweetened is acidic, so you have to add an alkaline. Here is a reliable explanation:
Also, if it's called for as a decorative touch, such as coating truffles or dusting over a cake, it makes a huge difference in both taste and color.