Ever send a bottle of wine back at a restaurant? If you weren't just being a pretentious snob, then it was probably because the wine seemed “corked”—had a musty odor and didn’t taste quite right. Most likely, the wine was contaminated with a molecule called 2,4,6-trichloroanisole (TCA), the main cause of cork taint. But a new study by Japanese researchers concludes that you do not smell TCA directly; rather, TCA blocks up your sense of smell and distorts your ability to detect odors. The findings could help the food and beverage industry improve its products and lead to less embarrassment for both you and your waiter.
Wine tasters are well aware that TCA can ruin a bottle of wine. “More experienced tasters can recognize TCA taint at the first sniff of an affected wine,” says Thomas Matthews, executive editor of Wine Spectator magazine. “However, some of our tasters are much more sensitive than others to the flaw and pick it up at much lower levels.” In some cases, wines that tasters said were OK to drink but that reviewers rated as inferior turned out to have small but detectable concentrations of TCA when subjected to laboratory analysis, Matthews adds.
Researchers had long assumed that contaminants like TCA activate nerve cells involved in smell called olfactory receptor cells (ORCs), although they have not been able to explain how even extremely low concentrations of the molecule produce the corked odor. So Hiroko Takeuchi, a biophysicist at Osaka University in Japan, and colleagues set out to see what's really going on when ORCs are exposed to TCA and similar contaminants. First, the team isolated ORCs from newts, whose ORCs are the largest among vertebrates and about three times as large as those of humans. The cells' size made it easier to measure electrical currents across an ORC’s outer membrane by puncturing it with a thin glass tube called a micropipette. Such electrical currents activate the cell and are carried by calcium ions entering the cell.
To the researchers' surprise, TCA did not stimulate calcium to flow across the cell membrane. Instead, it stopped the flow by blocking passages through the membrane, called cyclic nucleotide-gated (CNG) channels. TCA was up to 1000 times more effective in blocking CNG channels than other odor blockers sometimes used in making perfume, the researchers report today in the Proceedings of the National Academy of Sciences.
Turning to human subjects, the researchers recruited 20 volunteers from the Daiwa Can Co. in Tokyo, which manufactures packaging for foods and beverages. The volunteers were experienced at tasting for “off” flavors and odors but were not experts in wine tasting. The subjects tasted glasses of both red and white wines to which the researchers had added minute amounts of TCA. In double-blind tastings, the subjects recognized cork taint from TCA at concentrations as low as 10 parts per trillion. (They also detected similar levels of two other common wine contaminants.)
TCA and other contaminants don't activate your sense of smell directly but distort it, giving the sensation of a musty odor, the researchers say. They suspect that TCA inhibits only a subset of CNG channels and thus mixes up the signals sent to the brain. TCA is also found in apples, raisins, chicken, shrimps, peanuts, cashews, sake, green tea, beer, and whiskey, as well as in packaging for these products. So the food and beverage industry “has to pay attention to TCA contamination,” Takeuchi says.
The study still doesn't reveal how such small concentrations of TCA create the effect. The molecules cannot stopper the channels directly because the typical ORC has about 100,000 CNG channels but the lowest concentrations of TCA applied to the newt cells would supply only about 600 molecules to a cell. So one TCA molecule must be affecting a number of CNG channels simultaneously. Takeuchi and her colleagues hypothesize that TCA-like contaminants are probably absorbed into the fat layers that make up the cell membrane to cause widespread disruption of the channels.
The findings are “not totally surprising,” says Barry Ache, director of the Center for Smell and Taste at the University of Florida in Gainesville. Many other chemicals are capable of blocking ion channels, he says, and “there is reasonable, if not compelling, evidence that a reduction in excitation is as much a part of the olfactory [sense] as excitation itself.” He agrees that TCA might be able to exert its powerful effect by dissolving into the cell membrane.
So, the next time you send back a bottle of wine, tell the waiter that you have detected unacceptable levels of TCA. That—and a decent tip—might persuade him that you're not just being pretentious.
*Correction, 16 September, 6:18 p.m.: The last sentence of the seventh paragraph initially began, "Takeuchi and his colleagues," when it should have read, "Takeuchi and her colleagues," as Hiroko Takeuchi is a woman. This has now been corrected, and we apologize for the error.