Cocoa powder is made from the hard disk of powder that is left after pressing, which expels about half the excess cocoa butter from cocoa beans, which are about 50% fat. The disk is grated into a fine powder containing 20-22% fat. Now you can understand why most low-fat chocolate recipes call for cocoa powder.
Dutch processed cocoa powder is natural cocoa powder that has been treated with an alkalizing agent such as baking soda or potassium carbonate. This alteration of PH levels reduces the acidity of the cocoa, rendering its chocolate flavour more mild and less bitter. Dutched cocoa is also more reddish in colour compared to natural cocoa, which is more brown. Dutched cocoa is also less lumpy and more soluble than natural cocoa.
Some artisan companies in the United States don’t Dutch-process their cocoa as they claim their cocoa beans don’t need to be acid-neutralized. Most supermarket brands of cocoa powder in America, such as Hershey’s and Nestlé, are natural cocoa powders.
Dutched cocoa is said to be high in antioxidants. Wikipedia says:
Compared to other processes, Dutch process chocolate contains lower amounts of flavonols (antioxidants). The effect this has on health is disputed. Professor Dr. Irmgard Bitsch of the Institut für Ernährungswissenschaft, Justus-Liebig-University Giessen claims that the reduction of antioxidants due to the process is not significant and enough polyphenols and procyanids remain in the cocoa. One study determined that 60% of natural cocoa’s original antioxidants were destroyed by even light dutching, and 90% were destroyed by heavy dutching.However, natural cocoa has such high levels of antioxidants that even a 60% reduction leaves it high on the list of antioxidant-rich foods.