FDA says it will launch an investigation of the safety of caramel coloring used in soda after a Consumer Reports study found varying levels of 4-methylimidazole in 12 brands of soda from five companies. The chemical is an impurity formed in some caramel coloring at low levels in the manufacturing process, and it is listed by the state of California as a carcinogen. Other studies of 4-methylimidazole's carcinogenic potential have not been conclusive.