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Corked wine confuses smell receptors

Mixed signals Rather than having an intrinsically bad odour, the chemical that makes wine smell 'corked' is preventing normal smell signals from reaching the brain, finds a Japanese study.

The discovery, made in newts, also fits with smell tests done on human volunteers, according to a report in the Proceedings of the National Academy of Sciences.

The effect of TCA (2,4,6-Tricholoroanisole), the chemical culprit in corking, on smell receptors was being investigated by Dr Hiroko Takeuchi and Professor Takashi Kurahashi and colleagues of Osaka University, Japan.

The researchers had expected to find olfactory (smell) receptor cells in the newt that could detect TCA, but they couldn't find any.

"We applied TCA to more than 100 receptor cells ... but we didn't see any excitation," says Kurahashi.

Meddling with membranes

Instead, to their surprise, they found the chemical inhibited the firing of the newt olfactory receptor cells as they tried to pass messages on to the brain. This means that, regardless of the odour the cell detects, it can't forward the information.

All of the olfactory cells they measured were inhibited by TCA, and at "remarkably low" concentrations. In fact, even the application of an incredibly weak solution of TCA (10 attomolar), still caused a slight suppression of a cell's normal response.

So how is TCA switching off smells? Kurahashi thinks it gets absorbed by the lipid membrane of the olfactory cell where it interferes with the workings of an ion channel. If the ion channel isn't able to function, the cell can't relay its message.

But TCA concentrations are so low that it can't simply be blocking the channels. One TCA molecule appears to be able to knock out many ion channels, most likely by disrupting the lipid membrane that holds them in place.

"It's very similar to the way some anaesthetics work [but] it seems to work at even lower concentrations than local anaesthetics or other drugs," Kurahashi remarks.

TCA could have wide applications as a masking agent, able to switch off unpleasant odours, thinks Kurahashi.

"It could be very useful in the cosmetic industry and in hospitals and retirement homes to mask unpleasant smells," he says.

Musty smell remains a mystery

The discovery reveals that in 'corking', the normal wine smells aren't reaching the brain. But one thing yet to be explained, Kurahashi says, is the characteristic "musty smell" that people report when tasting wine with cork-taint.

In a separate experiment on 20 people, Kurahashi found that the concentrations at which they reported smelling TCA's musty odour were exactly the same as those at which they reported a loss of flavour in the wine.

"This means [smell] suppression may be related to the induction of the musty smell," he says. He thinks the musty smell may result from "some kind of pseudo-olfactory sensation" in the brain.

A smell that affects the system

"It's quite an elegant piece of research", comments Dr Alisha Anderson of the CSIRO's Food Futures Flagship. "It just shows how complex the olfactory system is."

"We've always thought that the receptors were the part interacting with things in the environment, but this shows that you need to think more broadly about that, because things in the environment can also affect the system in a different way."

"The main question still left open for me is - why are we still sensing what's going on as a musty odour?"