Science Questions

Why do chilli peppers change colour when they go ripe?

Sun, 2nd Aug 2009

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Neil Denham asked:

Why do chilli peppers change colour when they go ripe?


Weíre growing some chilli peppers on our window sill.  Actually, my chilli plant got some fungal disease and died though or nearly died.  Iíve managed to resuscitate it but I had to spray something on it.  Itís says, ďDo not use on things you intend to eat,Ē so while I saved the plant, itís now useless because I canít actually eat it.  Anyway, weíre growing some chilli peppers on our window sill. For ages theyíve been green but suddenly theyíre going beautiful red and yellow colours.  I understand the process why most plants are green but why do the peppers turn red and yellow?  Whatís the point in that?

Chilli peppersHelen -   Actually, Iím growing my own chilli peppers too, failing too as well.  So if youíve got any tips on how to do it better, Iíd love to hear from you. But, well if you think about it, itís kind of a two-sided question, really.  First of all, why does any kind of fruit or anything that might be eaten change into a bright red colour and usually thatís if itís ripe and ready to go, itís advertising itself to be eaten by a disperser.  An animal of some sort is going to come along, have a nibble, eat it, take the seeds in its stomach and release them somewhere else in the faeces to help disperse this plant.

Thatís okay if youíre a nice tasty thing like a tomato.  They start out green and they donít want to be eaten before theyíre nice and ready, before the seeds have actually developed enough.  So they will later on turn to be red and there you go.  Thatís why they turn red, but why do chillies turn red?  Do they actually want to be eaten given that theyíre so spicy?  Do animals actually like to eat chillies?  Well, thereís lots of theories about why chillies evolve to have such spiciness and capsaicin is the chemical that actually makes your tongue burn when you have a mouthful of chilli, and I love it as well.  And a guy called Josh Tewksbury, a scientist in the University of Washington who spent a lot of his time looking into this question of how spicy chillies evolved, why they evolved. Whatís the purpose of them?  Heís been out in the Bolivian and Peruvian jungles where we think Ė well, sorry, the rainforest, the dry mountainous areas, actually not rainforest, where we think these chillies first evolved and heís found some really interesting things out. 

So, we think that it could actually be that mammals are no good at eating chillies because they actually crunch them up. Theyíre seed predators.  Thatís not much use to the chilli plants; their seeds get destroyed that way.  But maybe birds are the ones that the chillies are trying to attract because birds usually just swallow down the seeds whole and they donít actually get affected by the chilli and theyíve watched to see.  There are natural variations in the amount of capsaicin you get in plants within one population of chillies. They went out and looked and they saw that mammals donít like to eat the ones where thereís lots of capsaicin in the plants.  But birds donít mind, they would just go for any of them.

Chris -   I think, they actually lack the receptor that the capsaicin locks on to on the nerve fibres.

Helen -   Right, there you go.

Chris -   So they canít detect it.

Helen -   That as well.

Chris -   Because one suggestion we did have for people who keep chickens and are fed up with rats eating the chicken food is that you put loads of curry powder in with the chicken food and the chickens donít notice the curry powder because theyíre insensitive to the effect of capsaicin.

Helen -   There you go.

Chris -   But the rats do and the other benefit of that is that you get a sort of premarinated chicken, so when you come to eat it, itís already nice and currified.

Helen -   Super! Super! Itís also could be that it could deters fungal attack and that in fact having more capsaicin in the chilli plants, in their seeds helps to avoid fungus coming along and destroying the seeds. Maybe itís attracting birds thatís why chillies are red but weíve been doing it for an awfully long time, at least 6000 B.C. we think that chillies were being cultivated.

Chris -   With good reason, theyíre fantastically tasty.

And we had a comment from Judith in Northhampton who said that capsaicin powder can be used to stop things eating your chicken food. She uses the same trick to stop the squirrels in nicking her bird feed.  


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Neil Denham asked the Naked Scientists: We are growing some chilli peppers on our windowsill, for ages they have been green, but suddenly they are turning red and yellow. I understand the process of why most plants are green, but how do the peppers turn red and yellow, and what is the purpose of the colours? What do you think? Neil Denham, Tue, 4th Aug 2009

Thanks for the response guys! Sorry to hear you have not had luck in growing chillies Chris, not sure what our secret was, we didn't do much special and just stuck them in pots on a sunny windowsill! To make you jealous here is a link to a few pictures!

The long ones taste pretty spicy, even when they were green, the fat ones are more like normal peppers with a slightly chilli taste. Neil Denham, Thu, 6th Aug 2009

I have been growing dorset nagas (the hottest chillis in the world) this year and have a grow log peteboy, Sun, 16th Aug 2009

you were asking on chilli growing tips in your article :) - Conrad, Thu, 2nd Sep 2010

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