For ages, diners have been told that drinking red wine while eating seafood can produce an unpleasant fishy aftertaste. The rule of thumb has been red wine with meat, white wine with fish. But the rule is not hard and fast. Seafood can taste fine with some reds, whereas some whites can ruin the meal. What's the common factor?

Researchers at Mercian Corp. in Fujisawa, Japan, a division of which produces wine and spirits, decided to find out. They conducted an experiment with seven experienced wine tasters who were offered 38 varieties of red and 26 types of white. Over four sessions, the volunteers tasted the samples, along with pieces of scallops, the seafood most likely to produce the fishy effect. Then the researchers chemically analyzed the wines for a possible link to the aftertaste.

The culprit appears to be iron, the team reports in a recent issue of the Journal of Agricultural and Food Chemistry. When the element's content rose above 2 milligrams per liter or so, the seafood-dining experience turned sour. The team double-checked their results by soaking pieces of dried scallops in samples of wine. Scallops dunked in vino with low iron content smelled normal, but pieces soaked in samples with high iron content reeked of fish.

The researchers report that they haven't yet isolated the compound in the scallops that reacts with the wine, but they suspect it's an unsaturated fatty acid, which could be breaking down rapidly and releasing the decaying fish smell when exposed to iron. How much iron a wine contains depends on the amount in the soil where the grapes were grown, as well as other factors such as how the grapes are harvested and processed. Red wine tends to have a higher iron content, hence the admonition against mixing it with seafood.

"We were surprised in our finding," says research chemist and lead author Takayuki Tamura, "because we thought that polyphenols or sulfur dioxide [produced] the unpleasant sensation." These components represent a larger percentage of wine content than does iron. He explains that because iron does not "induce color change, accelerated oxidation, or cloudiness," vintners tend to ignore its potential role as a meal-spoiler. But the new findings, he says, offer winemakers the opportunity to reconsider the downside of iron contamination.

The paper's science is sound, says enologist Gordon Burns of ETS Laboratories in St. Helena, California. Still, he says, there are better reasons to avoid red wine with fish: Any robust red wine, regardless of iron content, would likely overwhelm the delicate, subtle flavor of many seafood dishes. Red wine, he says, often pairs better "with a big stew or a hearty chunk of meat."