Thursday, 17 November 2016

Why do chilis bother making themselves spicy anyway?

Why are chilis spicy? - Spicy food holds a special place in many hearts - some people love it and some hate it. We know that chili peppers are used to make spicy food, but why are chilis spicy in the first place? What's in it for the chili plant? Shouldn't those plants be directing all their energy into surviving or reproducing or something?

Mammals threaten chili pepper seeds - Imagine you're a chili plant: you've just spent all year packaging up your children in stored energy and nutrients (seeds). You've even wrapped your kids up in a tasty, shiny red treat (the meaty part of the pepper), to try and entice a bird to eat your pepper, carry it off far away, and poop your children out into a great new environment to start their lives (I know - plant parents have weird hopes for their children). Despite all this, some mammal suddenly comes by and gobbles up your children, using its molar teeth to grind them into a lifeless pulp, and poops them out less than a mile away to rot. What a nightmare!

Spicy capsaicin protects chili seeds from mammals - Fortunately, as a member of the Capsicum plant family, you have a secret weapon against the murderous mammals - you are a master of organic chemistry! You find (after millions of years of experimentation) that by combining a simple fatty acid and a compound similar to vanilla you can create a remarkable molecule that humans call capsaicin (Fig. 1). By coating your children in this compound you successfully make them extremely unpallatable to marauding mammals. Birds, who are unaffected by capsaicin, are then free to eat your peppers [1]. Since birds don't have the same grinding teeth mammals do, your children pass through the bird intact, and are dispersed widely over the globe to colonize new areas.

Figure 1: Capsicum plants, seeds, and capsaicin. Species in the pepper plant family (example in top left), product fruit with seeds (top right), that contain spicy capsaicin (bottom) to protect the seeds against mammals.

Humans (bizarre mammals) cultivate the chili's capsaicin spice - To humans, capsaicin has a spicy taste that many enjoy. Capsicum species were originally domesticated in Central America over 6000 years ago. After european explorers ventured to the Americas, these plants and their peppers were brought all over the world as part of the Columbian Exchange. Now, cultures all over the globe use spicy peppers in their cooking. Since then, humans have been breeding pepper plants that make incredibly hot peppers, and have developed the Scoville heat unit as a scale to describe how hot different peppers are.

Medicinal properties of capsaicin - Some claim to experience euphoria after eating capsaicin [4], supposedly related to a release of endorphins [5], though I suspect that this is folklore and varies greatly between different people. More widely experienced, and more recent, is the use of capsaicin as a pain-reliever. It is currently approved for topical application in the treatment of backache, arthritis, and sprains.

Chilis are used as a deterrent in agriculture - In large doses, capsaicin creates a burning sensation wherever it touches mammals; it is the primary active ingredient in pepper spray and bear mace. In some parts of Africa and Asia, farmers' crops are in danger of being eaten by elephants, which can step over high fences that keep out other herbivores. Elephants have large and sensitive noses, so farmers can keep them away by planting chili pepper plants in a defensive ring around other crops, and the chilis the barrier plants produce can be sold at market.

Chilis can be used as deterrents in home gardens - Chili seeds can also be added to bird seed to deter squirrels and rodents [2], and applied around flowers and other ornamentals to deter deer and rabbits [3]. Some gardeners report no deterring effects when using certain varieties of paprika (100% ground chilis) or chili powder (ground chilis + cumin and other spices). This is probably because paprika is often made with pimento peppers - chilis that are 4x less spicy than jalapenos, and barely register on the Scoville heat scale. A gardener looking to deter pests should search out a paprika with a punch!

Capsaicin is one of many valuable chemicals that plants have developed over millions of years- Capsicum species and the unique chemical called capsaicin they produce have applications in cooking, medicine, agriculture, and home gardening; all of which make the pepper plant a valuable economic commodity. Indeed, over 10 million acres of chili peppers are cultivated world wide. The plant kingdom is full of species, both known and undiscovered, that synthesize chemical compounds of enormous importance to human life. So, it is important that plant scientists work towards discovering and understanding these natural chemicals to improve both our quality of life and our agricultural practices.

[1] Tewksbury, J. J.; Nabhan, G. P. (2001). "Seed dispersal. Directed deterrence by capsaicin in chilies". Nature. 412 (6845): 403–404.
[2] Jensen, P. G.; Curtis, P. D.; Dunn, J. A.; Austic, R. E.; Richmond, M. E. (2003). "Field evaluation of capsaicin as a rodent aversion agent for poultry feed". Pest Management Science. 59 (9): 1007–1015.
[3] "R.E.D. Facts for Capsaicin" (PDF). United States Environmental Protection Agency.
[4] Gorman J (20 September 2010). "A Perk of Our Evolution: Pleasure in Pain of Chilies". New York Times.
[5] Rollyson WD, et al. (2014). "Bioavailability of capsaicin and its implications for drug delivery". J Control Release. 196: 96–105.
[6] Fattori, V; Hohmann, M. S.; Rossaneis, A. C.; Pinho-Ribeiro, F. A.; Verri, W. A. (2016). "Capsaicin: Current Understanding of Its Mechanisms and Therapy of Pain and Other Pre-Clinical and Clinical Uses". Molecules. 21 (7): 844.