Some new research sheds light on why chili plants are spicy:

It has been thought that the chemicals known as capsaicinoids, which surround the seeds and give peppers their characteristic heat, are the chili's way of deterring microbes. But if so, then microbial infestation should bring selective pressure on chilis -- the more bugs, the hotter the peppers should be.

That has never been shown in the wild. Now, however, in a study of wild chili plants, Joshua J. Tewksbury of the University of Washington and colleagues show that the variation in heat reflects the risk that the plants will be attacked by a seed-destroying fungus.

My own little vegetable garden has been hard hit by fungus this year, but the hot peppers are in full bloom. Now I know why - capsaicin is the original fungicide.

Of course, that still doesn't explain why capsaicin tastes "hot," and why spicy food "burns" our tongue. For that, we need to investigate the physiology of taste. It turns out that capsaicin - this plant protectant - binds to a special class of vanilloid receptor inside our mouth called VR1 receptors. After binding capsaicin, the neuron is depolarized, and it signals the presence of spicy stimuli.

But here's the strange part: VR1 receptors weren't designed to detect capsaicin. They bind spicy food by accident. The real purpose of VR1 receptors is the detection of heat. They are supposed to prevent us from consuming food that is too hot, in the thermal sense. (That's why our VR1 receptors are clustered in our tongue, mouth and skin.) So when they are activated by capsaicin the sensation we experience is that of excessive heat. We start to sweat and get the urge to drink lots of water. But that pain is just an illusory side-effect of our cell receptors. There is nothing "hot" about spicy food.

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If you like spicy food - and I love spicy food - then you'll find this report from Harold McGee's blog rather interesting. It concerns the evolution of capsaicin, the pungent chemical that makes chilis so spicy: Levey, Tewksbury and colleagues tested the theory that capsaicin selectively repels…
Food from countries all over the world owes a lot of its flavour to a fungus. The species in question isn't one of the many edible mushrooms used for cooking, the baker's yeast that gives rise to bread, or the moulds that spread through blue cheese. It's a little known species called Fusarium…
Some plants do not want to get eaten. They may grow in places difficult to approach, they may look unappetizing, or they may evolve vile smells. Some have a fuzzy, hairy or sticky surface, others evolve thorns. Animals need to eat those plants to survive and plants need not be eaten by animals…
(First posted on July 21, 2006) Some plants do not want to get eaten. They may grow in places difficult to approach, they may look unappetizing, or they may evolve vile smells. Some have a fuzzy, hairy or sticky surface, others evolve thorns. Animals need to eat those plants to survive and…

I suppose the next question would be 'why does capsaicin bind to the heat receptors?' Some sort of analysis what it does to the fungi would play a part there :)

Interesting stuff!

~Lab Rat

VR1 receptors weren't designed to detect capsaicin

"Designed"? Tsk. (nitpicking, yes, but still)

A funny aside to this topic -- I saw a show during Discovery channel's Shark Week recently (I think it was MythBusters) during which the hosts tested the myth that sharks are repelled by chili peppers (evidently there's an old Indian tradition of carryting a bag of chilis in the water when swimming where sharks are present). The end result was, even when the sharks swallowed a balloon full of chili peppers, there was absolutely no effect. I gather now, after reading this post, that's because sharks lack the receptors to respond. I wonder how many folks learned that the hard way? :)

Strange that the VR1 receptors detect food that is too hot. I would think that for almost all of our evolutionary history, that would never happen. I wonder how long we have actually been cooking our food?

By Chris King (not verified) on 12 Aug 2008 #permalink

A friend grew habanero peppers in her yard as ornamentals, for the strong green and the bright orange. She gave me the ripe peppers because she knew I liked them. They were great.

The next year, she planted again, but in a different spot with different soil. The entire crop was mild, far less spicy than jalapenos or serranos.

Clearly, the 'heat' has something to do with what minerals are available in the soil. In their native land, the plant's ability to defend against fungus would depend upon soil chemistry.

... a special class of vanilloid receptor ...

Is capsaicin chemically similar to vanilla?

It would take a world-class chef to blend those flavors successfully.

By Pierce R. Butler (not verified) on 12 Aug 2008 #permalink

"It would take a world-class chef to blend those flavors successfully."

I wonder if jalapeno jelly bellys have vanilla in them.

So, any pepper fan who cooks has probably had the experience of chopping up those compact packets of gustatory pleasure. Then he or she may be brushing wayward hair away from the face and touch the edge of an eyelid. As Gollum would say of the Sun: it burns!! Other heat receptors outside the mouth are just doing their jobs.

And where does the delicious factor fit in with spicy food? Is that another dopamine link... Stravinsky, Montague's trust game, it never ends. How about an fMRI look at people eating a really good curry vs a bland but tasty dish.

And do the garden pepper plants also ward off insects, rodents, deer and other non-fungal consumers?

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I'm with Chris King above. I can't buy steaming hot food as any kind of evolutionary pressure. Does the author expect us to believe that this capacity evolved in the last couple of thousand years? Pooh. It makes you wonder what possible basis in research this claim could have.

Humans have been cooking with fire for at least 250,000 years, judging by hearths found in caves.

By Pierce R. Butler (not verified) on 14 Aug 2008 #permalink

The ethical implications if this research are troubling. Capsaicin binds to pain receptors. Fungi are susceptible to capsaicin. Therefore, fungi can sense pain, and those who do not eat meat for ethical reasons should consider dropping fungi from their diets as well.

By Mustafa Mond, FCD (not verified) on 15 Aug 2008 #permalink

The VR1 receptors are quite old. If I'm not mistaken, all mammals have them. However birds do not (the research even points out that birds are the main dispersers for peppers seeds and aren't affected by the heat). So now the evolutionary question comes down to "Did early mammals often eat food that was too hot?"