Coca-Cola Co. and PepsiCo Inc. are changing the way they make the caramel colouring used in their soft drinks, in response to accusations that an ingredient in the colouring is a carcinogen.

The ingredient in question is called 4-methylimidazole. In 2011, authorities in California added 4-MI to a list of known carcinogens – the only state to have declared the chemical as cancer-causing.

State authorities said that unless the beverage companies changed their formulations, they would have to stick a "carcinogen" label to all their caramel-coloured drinks sold in the state.

The U.S. Center for Science in the Public Interest has long contended that 4-MI (also called 4-MEI) poses a danger to consumers. This week, they called again on the U.S. Food and Drug Administration to revoke its authorization for caramel colourings that contain the ingredient.

"When most people see ‘caramel colouring' on food labels, they likely interpret that quite literally and assume the ingredient is similar to what you might get by gently melting sugar in a saucepan," CSPI executive director Michael F. Jacobson said in the statement.

"The reality is quite different. Colorings made with the ammonia or ammonia-sulfite process contain carcinogens and don't belong in the food supply."

But Coca Cola and Pepsi have long said their drinks and their colourings are safe.

The American Beverage Association, the trade group that represents manufacturers of non-alcoholic beverages in the United States, issued a statement this week reiterating that the U.S. FDA, European Food Safety Authority and Health Canada all consider caramel colouring safe.

"California's listing was based on a single study in lab mice and rats. A person would need to drink more than 2,900 cans of cola every day for 70 years to reach the lowest dose levels mice received in the single study upon which California based its decision," the group said.

"This is nothing more than CSPI scare tactics, and their claims are outrageous. The science simply does not show that 4-MEI in foods or beverages is a threat to human health."

Despite their insistence that the ingredient is safe, Pepsi and Coke say they have begun to change the formulation of their colourings to comply with the law in California. The new formulation will contain lower levels of 4-MI.

The changes will be expanded nationally to streamline their manufacturing processes.

Late Thursday, Coca-Cola Canada released a statement to to say that it is confident that the 4-MEI levels in its products have never posed a health or safety risk.

"We do not agree with the State of California's Proposition 65 requiring a warning for some food products containing trace levels of 4-MEI. But the company did make the decision to ask its caramel suppliers to make the necessary manufacturing process modification to meet the requirement of California's Prop 65 and avoid having to place a scientifically unfounded warning on our products.

"It is important to note that while we have asked our caramel suppliers to modify their manufacturing processes, those modifications do not change our product. The caramel colour in all of our products has been, is and always will be safe."