A scientific paper that has just been published in journal Proceedings of the National Academy of Sciences (PNAS) examines the way that 2,4,6-trichloroanisole (TCA), the key compound responsible for musty cork taint is perceived. The results are surprising.
They suggest that TCA acts on olfactory receptor cells (the cells containing the proteins that detect smells) by suppressing protein channels called cyclic nucleotide gated (CNG) channels in the cell membranes, rather than actually eliciting odorant responses. In plain English, what this means is that we don’t actually smell TCA. These results indicate that the effect of TCA is to suppress our smell mechanism.
You can read the original paper here (it’s free to access).
This result explains why extremely low levels of TCA suppress the character of a wine, even when no musty defect is present. This is something familiar to most of us who’ve tasted lots of wines: some bottles just don’t seem right, and opening a second bottle reveals a very different wine. It’s bad news for the cork industry.
But they are also puzzling. We know very well the extremely musty smell of badly corked bottles. This paper suggests that TCA doesn’t have a smell, but that it is having its effect through suppression. The concentrations at which TCA gives a musty smell appear to be too low for activating olfactory receptors.
The authors propose two explanations. First, that the mustiness of TCA is a pseudoolfactory sensation. Second, that suppression of the olfactory receptor cell output by TCA causes an olfactory sensation.
NOTE ADDED LATER:
I have been thinking about these results. I suspect that humans actually do possess olfactory receptors that can detect TCA as a smell; these were newt cells. There is a selective advantage for humans to be able to smell TCA as it is an indicator of fungal activity. If we detect it at low concentrations and it is aversive, then this would stop us consuming food that has fungal contamination. Remember: fungi create some nasty secondary metabolites that can cause us a lot of damage. While these results explain how low levels of TCA can suppress other wine components (an observation that has always puzzled me: how would such low concentrations of one molecule affect others without such a mechanism as that explained here?), they don’t answer the question of why higher levels of TCA smell nastily musty. But even this doesn’t seem consistent: if they suppressed fruity smells, why wouldn’t they supress musty smells? It is all very intriguing, and still quite confusing.