The ingredient, 4-methylimidazole (4-MEI) - a possible human carcinogen - is formed during the manufacture of some kinds of caramel colour. Caramel colour is a common ingredient in colas and other dark soft drinks."Soft drink consumers are being exposed to an avoidable and unnecessary cancer risk from an ingredient that is being added to these beverages simply for aesthetic purposes," said Keeve Nachman, senior author of the study.