Researchers from the University of Illinois at Chicago said yesterday that one of five raisin compounds tested, oleanolic acid, was particularly effective in killing bacteria.
The findings counter the public perception that raisins promote cavities.
"Raisins are perceived as sweet and sticky, and any food that contains sugar and is sticky is assumed to cause cavities," said lead author of the study Christine Wu, professor and associate dean for research at UIC.
"But our study suggests the contrary. Phytochemicals in raisins may benefit oral health by fighting bacteria that cause cavities and gum disease."
The data, presented at the annual meeting of the American Society for Microbiology running in Atlanta this week, come as food manufacturers and oral care companies show increasing interest in natural compounds that can fight gum and tooth disease.
Dental floss and other products containing cranberry extracts have recently been launched in the US on the back of research showing that the fruit contains compounds that prevent adhesion of the bacteria Streptococcus mutans, an agent for dental caries, to teeth.
The new study shows that a raisin compound may have the same action.
Wu and her colleagues performed routine chemical analyses to identify five phytochemicals in Thompson seedless raisins: oleanolic acid, oleanolic aldehyde, betulin, betulinic acid and 5-(hydroxymethyl)-2-furfural.
Oleanolic acid, oleanolic aldehyde, and 5-(hydroxymethyl)-2-furfural inhibited the growth of two species of oral bacteria, including Streptococcus mutans, and Porphyromonas gingivalis, which causes periodontal disease.
The compounds were effective against the bacteria at concentrations ranging from about 200 to 1,000 micrograms per milliliter.
Betulin and betulinic acid were less effective, requiring much higher concentrations for similar antimicrobial activity.
At a concentration of 31 micrograms per milliliter, oleanolic acid also blocked S. mutans adherence to surfaces. Adherence is crucial for the bacteria to form dental plaque, the sticky biofilm that accumulates on teeth. After a sugary meal, these bacteria release acids that erode the tooth enamel.
It is estimated that more than 5 million people visit the dentist with toothache every year in the UK, and the vast majority of these are the result of tooth decay.
Gum disease causes bleeding gums and bad breath and is the number one cause of tooth loss in the UK. It has also been linked to conditions such as heart disease, lung disease, diabetes and heart attacks.
In an earlier unpublished study, Wu's collaborator Shahrbanoo Fadavi, a pediatric dentist at UIC, found that adding raisins alone to bran cereal did not increase the acidity of dental plaque. Raisin bran cereal with added sugar, however, did raise acidity levels.
"Foods that are sticky do not necessarily cause tooth decay. It is mainly the added sugar, the sucrose, that contributes to the problem," Wu said.
Raisins contain mainly fructose and glucose, not sucrose, the main culprit in oral disease, she added.
The study was funded by the California Raisin Marketing Board.