By Janet Raloff

Web edition: April 11, 2006

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The safest way to prepare this rice? Pressure-cook it, a new study finds.

Credit: PhotoDisc

Rice—white, fluffy, pure, nutritious—can nevertheless carry fungal poisons called aflatoxins. Two years ago, Je Won Park and his colleagues reported that "rice is the major contributor to the dietary intake of aflatoxin B₁ in Korea." Aflatoxin B₁ is the most poisonous of these contaminants, and Park's group had found it in 6 percent of uncooked rice collected from markets in Seoul.

Park and his colleagues now report that pressure-cooking appears to largely eliminate the poison from rice. The new finding suggests one way that East Asians, renowned for their rice-based diets, can limit exposure to aflatoxins, which are known human carcinogens.

The researchers found aflatoxins in 5 out of 88 rice samples. That would yield a "probable daily intake" of 1.19 to 5.79 nanograms of the toxin per kilogram (ng/kg) of body weight among the Seoul residents regularly eating rice contaminated with Aspergillus fungus, which produces aflatoxins. Such a dose exceeded the maximum tolerable intake, the researchers noted in the January 2004 Food Additives and Contaminants, and thus raised serious health concerns.

For comparison, the U.S. National Toxicology Program (NTP) has estimated that the median intake of all aflatoxins—including B₁—by U.S. residents in the Southeast, where aflatoxin contamination of foods tends to be highest in this country, is 2.7 ng/kg body weight.

The new pressure cooker data indicate a means for substantially lowering exposure to these liver carcinogens in populations that depend on rice as a dietary staple. Indeed, Park told Science News Online, aflatoxin intake by Koreans who prepare rice with pressure cookers should fall to "safe" levels.

Not really a surprise

Last year, Park's team showed that washing rice and then cooking it in a steamer—the usual Korean method—could cut the grain's aflatoxin B₁ concentration by one-third. The finding appeared in the July 2005 Journal of Food Protection.

In their current study, the researchers found that preparing rice in a pressure cooker reduced aflatoxins to concentrations of just 12 to 22 percent of those that had been present in the uncooked rice. Their report appears in the March 22 Journal of Agricultural and Food Chemistry.

Park notes that researchers nearly 3 decades ago suggested that pressure-cooking could substantially cut aflatoxin contamination of rice. At that time, however, scientists didn't have the tools to reliably quantify the reduction in aflatoxins. Because of new measuring techniques, Park's team was able to confirm that the reductions in the fungal poison are sufficient to bring even tainted rice into a healthful range.

Park, a food scientist, notes that Korean rice is probably not any more contaminated with aflatoxin than is rice elsewhere, including the United States. However, he notes, "the amount of rice [eaten by] Koreans is much higher than in Western countries," suggesting that aflatoxins and preparation methods that reduce them are much more important issues for people eating typical Asian cuisines.

His team is planning to evaluate how processing techniques affect aflatoxins in corn products, such as breakfast cereals. His group is also investigating whether crops grown organically are any more or less likely to bear significant aflatoxin contamination than typically grown crops are.

Substantial epidemiological data and research on animals have demonstrated that aflatoxins are a potent trigger of liver cancer and may play a role in a host of other cancers as well. For instance, decades ago, residents of southeastern U.S. states had high intakes of aflatoxins from local foods, and the population there had a 10 percent higher rate of liver cancer than did people elsewhere in the nation.

Concentrations of the toxins in U.S. foods have tended to fall in recent years. Today, U.S. foods most likely to be tainted with Aspergillus fungi or their aflatoxins tend to be peanuts, corn, and milk. Total aflatoxins in corn today typically range from less than 0.1 picogram/kilogram to 80 pg/kg, the National Toxicology Program reported last year. A picogram is just one-thousandth of a nanogram.

Park says his team doesn't know which aspect of pressure-cooking—the pressure versus the high temperature that builds up—does the most to rid rice of aflatoxin B₁.