Monday, May 11, 2009

Hot Peppa

Have you ever wondered why chili peppers are spicy? I was asking myself that question several months back and here's what I uncovered.

What makes chilies spicy?


The chemical compound in chili peppers that make them spicy is called capsaicin. It is an irritant for mammals and causes a burning sensation (it actually triggers the same nervous system response as fire or other intense heat) whenever it comes into contact with our mucous membranes (e.g. tongue, lips, nostrils, eyes, genitals...). Interestingly, other animals such as birds are not affected by capsaicin in this way.

Why are chilies spicy?

This is a slightly different and more complicated question. What sort of evolutionary advantage might capsaicin represent for the chili plant?

In general, a lot of plants bear fruit in order to attract animals who, ideally, eat the fruit while the seeds pass through their digestive system intact. When they later excrete the remnants of the meal they will probably have transported the seeds some distance from the mother plant and left them in a pile of fertilizer to boot.

Leaving aside the issue of humans with indoor plumbing, not all of the animals who may be attracted to the fruit are desirable from the plant's perspective. One example are small mammals, such as rodents, who chew up the seeds. Joshua Tewksbury, the University of Washington biologist who seems to be at the forefront of studying chili peppers, originally hypothesized that chili plants evolved this way in order to deter small mammals from eating their fruit. Birds, on the other hand, which would be better for transporting seeds, are unfazed by the capsaicin.

This hypothesis fell short for a couple of reasons. The biggest reason is the timeline: chilies evolved well before birds. Also, in other plants which have developed similar defenses, making them toxic or unpleasant tasting, these effects are found in other parts of the plant as well (e.g. leaves) whereas with chilies only the fruit contains capsaicin.

Tewksbury then came to suspect that capsaicin's purpose might be to protect chilies from another type of predator. Fungal rot is a big problem for wild chilies growing in the tropical forests of South America and capsaicin has strong antimicrobial qualities (protecting against fungus as well as other microbes such as bacteria). In the wild, the amount of capsaicin found in the fruit of different chili plants varies greatly even between members of the same species. Tewksbury observed that fungus was present in the majority of chilies he found in the wild (it seems to be aided by the holes left behind by insects); there also seemed to be a correlation between how spicy the chilies were and how much fungus was present (with the spiciest chilies seeming to be the most resistant to fungus).

Tewksbury is continuing his research to determine whether, in the wild, chilies growing in moister areas where fungal rot is a bigger problem might tend to be spicier on average than those in drier area, and to gather more evidence which might show that chilies with more capsaicin are heartier.

Another interesting fact is that fungus can sometimes be found on even the spiciest chilies, suggesting that different strains of fungus have evolved so that they are able to withstand more capsaicin. Tewksbury says that this shows that there is an evolutionary "arms race" going on, where chilies get spicier so as to ward off fungus, and fungus grows tougher so that it can feed on spicier chilies.

Why do people eat chilies?

Humans are pretty much the only mammals who voluntarily eat spicy chili peppers. Archaeologists have found evidence of people in the New World using wild chilies in their cooking over 8,000 years ago, and they began cultivating the plant around 6,000 years ago.


After Columbus (who called chilies "peppers," forever confusing them with a different group of spice plants that are native to India), the Spanish and Portuguese brought chilies back to the Old World where they would soon become an important ingredient in African, Indian, and Southeast Asian cuisines.

Some scientists believe that one of the reasons chilies became such a popular ingredient, particularly in tropical cuisine, is because of capsaicin's antimicrobial qualities. Because of this chili pepper (along with other spices such as cinnamon, clove, mustard) helped preserve food's freshness longer in the days before refrigeration.

Other reasons people like to eat chili peppers include taste, obviously, as well as the exhilarating effects capsaicin can have on the body like making you sweat, making your heart beat faster, getting your adrenalin pumping, and triggering the release of endorphins. In antiquity, chili peppers may have also been prized for their medicinal benefits as capsaicin can be used as a local anesthetic or to reduce/inhibit infection.

For more information on chili peppers and Tewksbury's research check out his interview on NPR and this article from April's Smithsonian magazine (also check out the Q&A with the guy who wrote the article about what it was like tagging along on one of his expeditions in the jungles of Bolivia, fueled by caffeine, coca leaves and chilies).


Images: Photo of woman in Rajasthan chili field by Giacomo Rossi found on timesonline, photo of chilies in French market by Per Karlsson found on allposters.com

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