Black raspberries have a chemical that seems to prevent cancer of the esophagus, researchers from Ohio State University said.

Lead author Gary Stoner said his study found that chemicals called anthocyanins in the berries helped stop cancer in rats treated with a carcinogen.

The rats were given an extract rich in anthocyanins, but a news release said it was about the same amount as would be found in whole berries. Often, research touting the benefits of an extract of fruit use concentrations much higher than could be obtained in a normal diet.

The release said the work confirmed in animals what had been found in test-tube trials.

"Now that we know the anthocyanins in berries are almost as active as whole berries themselves, we hope to be able to prevent cancer in humans using a standardized mixture of anthocyanins," said Stoner.

The study was published in the journal Cancer Prevention Research.