That red wine is not to be paired with seafood is nearly a religious dogma among connoisseurs. Their reason is that the combination usually results in a strong and unpleasant fishy aftertaste. The traditional explanation for the bad pairing is based on the presence of tannins—the chemicals that make red wines taste dry and cause the mouth to pucker. Yet, every now and again, a tannin-rich red wine that does go well with seafood turns up. Which wines can manage this pairing, and why, has remained a mystery that even the best-trained sommeliers do not understand. A series of experiments just published in the Journal of Agricultural and Food Chemistry has, however, provided the answer.

Takayuki Tamura and his colleagues work at the Product Development Research Laboratory of Mercian Corporation in Kanagawa, Japan. They started their exploration of what was behind the strange aftertaste by asking seven experienced wine tasters to sample red wines and white wines while eating scallops. The panellists were instructed to rate the presence of any fishy aftertaste on a scale of zero to four, with zero indicating no such aftertaste and four indicating an extremely strong one. Over the course of four sessions, they were presented with a grand total of 38 red wines, 26 white ones, 2 sherries, a dessert wine, a port and a Madeira. The drinks were offered in random order, in coded glasses.

What Mr Tamura and his colleagues found was that the wines rated with the strongest fishy aftertastes were those with high levels of iron. To check that iron really was the culprit, they ran a second experiment, in which they tampered with the wines and retested them. Those red wines that contained a lot of iron were treated with a chemical called a chelating agent, which bound up the dissolved iron and made it chemically inert. Wines without natural iron, by contrast, had it added. They also (separately) had zinc, manganese or copper added, to see if the effect was specific to iron or was caused more generally by metals. The team found that the fishy aftertaste did, indeed, vanish when the natural iron was chelated and, vice versa, that the addition of iron caused the fishy aftertaste to get stronger. And the effect turned out to be specific to iron. The other metals had little or no effect.

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