Does Your Wine Smell Like a Wet Dog? Don't Blame the Dog

If your wine smells like wet dog, soggy cardboard, or grandma’s basement, don’t blame the dog. A strange phenomenon called "cork taint" arises from groundwater pollution followed by microbial metabolism. 

Wanna know what's going on? Grab a goblet

The microbial production of a small amount of an otherwise-unremarkable organic chemical called 2,4,6-trichloroanisole (TCA) causes the scent.  But in reality, TCA is anything but unremarkable; it does some really strange things to your sense of smell, making your wine smell like all kinds of icky stuff, like a wet dog, damp basements, or wet cardboard. Wine enthusiasts may be familiar with the common term "corked wine," which is so-named because it can result from TCA leaching from the cork into the wine. The way TCA gets into the cork is quite interesting.

Boozing it up with chemistry 

Two chemical reactions must (sequentially) take place to form TCA, but they are very different. One is the result of an environmental pollutant making its way from groundwater into wood, primarily the cork oak (Quercus suber). The other is carried out by microbes. Pour yourself a drink. Here's the chemistry.

Step one: Blame the environment

1. During the bleaching of wood pulp, small amounts of 2,4,6-trichlorophenol (TCP) [1] are formed by the reaction of sodium hypochlorite (bleach) with lignin, a component of wood that gives it strength. (Figure 1)

Figure 1. Chlorination/oxidation of lignin gives trace quantities of trichlorophenol (TCP)

Step two: Blame the bugs.

2. TCP itself doesn't make your wine smell good, bad, or otherwise. It takes nature to finish the job. Certain microbes, such as yeast and bacteria perform a methylation reaction (addition of a methyl group and loss of a hydrogen atom), which converts TCP into  2,4,6-trichloroanisole (TCA) (Figure 2).

Figure 2. TCP is converted to TCA by S-adenosyl-L-methionine-dependent methylating enzymes found in certain microbes. S-adenosylmethionine is "nature's methylating agent." It performs this critical function in almost all living organisms.

What's so special about TCA?

This is where things get strange. To an organic chemist's eye, 2,4,6-trichloroanisole is just another molecule. It has a trichlorobenzene fragment; there are plenty of molecules containing this fragment in the universe of organic chemistry, and a methoxy (OCH3) group - ubiquitous in natural products. To a chemist, TCA also looks like a big snoozer. Except it isn't. It does something really strange to your sense of smell, which is where the wet dogs and damp basements– rarely associated with wine – come in.

The explanation can be found in an article entitled "2,4,6-Trichloroanisole is a potent suppressor of olfactory signal transduction." This sounds a bit daunting, so let's call it "2,4,6-Trichloroanisole Screws Up Your Nose."

There are a number of naturally occurring chemicals, called off-flavor substances, that can alter the smell and taste of food and drink, for example, musty-smelling chickens, medicinal-tasting coffee, and muddy-tasting sake. These off-flavor substances are assumed to work by binding to certain olfactory (scent) receptors and triggering unnatural excitation, causing them to misfire, thus the strange odors. 

This ability to dull the sensitivity of the olfactory system is a phenomenon known as olfactory suppression in which certain chemicals directly inhibit the signal transduction in olfactory receptor neurons, probably through interactions with membrane proteins or ion channels. So, it's not TCA itself that is stinking up your vino. Rather it's the effect of the chemical on your nose that makes you perceive one scent as another. 

Of course, if you drink too much wine, you could end up sharing a wet cardboard box in a damp basement with a wet dog. Then you have a different sort of problem. 

NOTE: 

[1] A cofactor is a non-protein chemical compound that is required for an enzyme's activity. It helps the enzyme perform its function—often by stabilizing its structure or participating directly in the chemical reaction.