Sodium benzoate occurs naturally in some fruits but is used in far higher concentrations to stop fizzy drinks from going moldy. In an Independent on Sunday exclusive last May, Professor Peter Piper, a chemistry expert at Sheffield University, warned that it could switch off cellular power in yeast and might do similar damage in humans. He called for the Food Standards Agency to fund research into the subject.

Sodium benzoate was also one of seven E-numbers found to worsen hyperactivity in a study by Southampton University. If combined with vitamin A, sodium benzoate can form a potentially carcinogenic substance, benzene. Coca-Cola said it had removed sodium benzoate from Diet Coke production in January.

The company said, "We are looking to phase out the use of sodium benzoate where technically possible." It said that it had not carried out any research into the preservative.

In response to the story, the Food Standards Agency referred Piper's concerns to the Committee on Mutagenicity. In a newly published opinion, the committee dismissed calls for further research into Piper's findings. It pointed to studies on rodents that showed no harm from sodium benzoate and said that human cells were stronger than yeast cells. Piper rebuffed the statement, saying the rodent tests had been done decades before more sophisticated DNA testing was developed, and suggested that the committee lacked expertise in the area. "I regard that statement as a whitewash," he said.

Last month, the FSA demanded a ban on the six food colours in the Southampton University study, but not on sodium benzoate. It added that sodium benzoate was widely used in the soft drinks industry. "Obviously the soft drinks industry is very powerful and they don't want to upset anyone there," said Piper.

Coca-Cola contacted him to discuss his research after the Independent on Sunday published its story last year.

From the May 27, 2008, Prepared Foods e-Flash