Treating cocoa with alkaline salts to reduce acidity is called the Dutch process.
The method was developed by Dutch chocolate maker Coenraad Johannes van Houten in the nineteenth century. Alkaline treatment darkens cocoa, softens some of its sharp acidity and can make it disperse more easily in water.
Dutch-process cocoa is used in drinks, biscuits, cakes and other products. Its colour can range from light brown to very dark brown, depending on the treatment. It does not have exactly the same chemical behaviour as natural cocoa powder, so recipes may specify one type or the other.
The Dutch process is different from conching, which refines chocolate’s texture and flavour, and from fermentation, which happens to cacao beans after harvesting. Those stages solve different manufacturing problems.