The Center for Science in the Public Interest, a consumer advocacy group, in February filed a petition with the US Food and Drug Administration to ban the use of ammonia-sulfite caramel coloring. A spokesman for the Food and Drug Administration said the petition is being reviewed.

Coca-Cola Co. and PepsiCo Inc. are changing the way they make the caramel coloring used in their sodas as a result of a California law that mandates drinks containing a certain level of carcinogens bear a cancer warning label. The companies said the changes will be expanded nationally to streamline their manufacturing processes. The changes have already been made for drinks sold in California. Coca-Cola and PepsiCo account for almost 90 per cent of the soda market, according to industry tracker Beverage Digest.

The American Beverage Association, which represents the broader industry, said its member companies will continue to use caramel coloring in certain products but that adjustments were made to meet California's new standard. "Consumers will notice no difference in our products and have no reason at all for any health concerns," the association said in a statement.

A representative for Coca-Cola, Diana Garza-Ciarlante, said the company directed its caramel suppliers to modify their manufacturing processes to reduce the levels of the chemical 4-methylimidazole, which can be formed during the cooking process and as a result may be found in trace amounts in many foods.

"While we believe that there is no public health risk that justifies any such change, we did ask our caramel suppliers to take this step so that our products would not be subject to the requirement of a scientifically unfounded warning," Garza-Giarlante said in an email. The Center for Science in the Public Interest, a consumer advocacy group, in February filed a petition with the US Food and Drug Administration to ban the use of ammonia-sulfite caramel coloring. A spokesman for the Food and Drug Administration said the petition is being reviewed.

But he noted that a consumer would have to drink more than 1,000 cans of soda a day to reach the doses administered that have shown links to cancer in rodents. The American Beverage Association noted that California added the coloring to its list of carcinogens with no studies showing that it causes cancer in humans. It noted that the listing was based on a single study in lab mice and rats.