The new craze for chocolate brought with it a thriving slave market, as between the early 1600s and late 1800s, the laborious and slow processing of the cacao bean was manual. Cacao plantations spread, as the English, Dutch, and French colonized and planted. With the depletion of Mesoamerican workers, largely to disease, cacao production was often the work of poor wage laborers and African slaves.
Several types of chocolate can be distinguished. Milk chocolate is sweet chocolate that additionally contains milk powder or condensed milk. In the U.K. and Ireland, milk chocolate must contain a minimum of 20% total dry cocoa solids; in the rest of the European Union, the minimum is 25%. "White chocolate" contains cocoa butter, sugar, and milk, but no cocoa solids.
Chocolate and cocoa are under preliminary research to determine if consumption affects the risk of certain cardiovascular diseases or cognitive abilities. Chocolate may be a factor for heartburn in some people because one of its constituents, theobromine, may affect the oesophageal sphincter muscle, hence permitting stomach acidic contents to enter into the oesophagus. Theobromine is also toxic to some animals unable to metabolize it (see theobromine poisoning).
Chocolate and cocoa contain moderate to high amounts of oxalate, which may increase risk for kidney stones. During cultivation and production, chocolate may absorb lead from the environment, but the total amounts typically eaten are less than the tolerable daily limit for lead consumption, according to the World Health Organization.