Microwaves bedevil a B vitamin.

Diets containing ample vitamin [B.sub.12], also known as cobalamin, not only help prevent pernicious anemia, they may also lower heart disease risk by reducing concentrations of the amino acid homocysteine in the blood. A new study now suggests that to preserve the integrity of this vitamin, chefs should avoid overcooking meats and dairy foods--a major source of cobalamin--especially in a microwave oven.

Fumio Watanabe of Kochi (Japan) Women's University and his colleagues tested 100-gram samples of milk or liquid emulsions made by mixing 10 grams of raw beef or pork with 50 milliliters of water. Heating each sample for 6 minutes in a microwave oven inactivated 30 to 40 percent of the [B.sub.12], that had been present, the scientists report in the January Journal of Agricultural and Food Chemistry.

The milk reached a boiling temperature after about 1 minute and steadily lost [B.sub.12], thereafter. Conventional boiling can also inactivate milk's [B.sub.12] though more slowly. It took about 25 minutes of regular boiling to inactivate as much of a milk sample's [B.sub.12] as microwaving achieved in 6 minutes. Acknowledging that the 6-minute microwave cycle would be considered "lengthy" for the reheating of refrigerated foods, Watanabe notes that such a period wouldn't be unusual for stewing meats or vegetables, steaming chicken, or preparing curries.

While his findings suggest that microwaves may be especially effective at destroying vitamin [B.sub.12], Watanabe says that how they do so remains a mystery.