First, peel the skin off an orange, a clementine, or a lime (not so easy to do). I haven’t tried grapefruit or lemons. Yet. Fold the peel so that it is shiny side out. Hold a match flame near where the peel folds. Pinch the fold, and a flame will shoot out of the peel, much like one sees when holding a match in front of an aerosol can of deodorant. You can do it several times before you exhaust the supply. There is some volatile substance in there. But throwing peels into the campfire doesn’t usually result in such a dramatic display. Usually, nothing happens at all. What IS this material? And why doesn’t it ignite in a campfire? I’ve showed this phenomenon to my students, and although I can’t explain it, it certainly is surprising and fascinating. I would like to know more about it.
(I’m not too worried about showing my students about it. I don’t think it’s dangerous. I don’t imagine: “Look out! He’s got a tangerine!” “OK, tell me where the drugs are or I’ll light this lime.” But don’t do it near your eyes, I guess.) Dopers - any help here? xo CC
Citrus fpeels contain citrus oils, which are flamable.
I’ve noticed that folding a citrus peel causes the oils to jet out.
So, oil, dispersed in tiny droplets would be readily flamable. You probably don’t notice it when you toss the whole thing in the fire because the droplets aren’t being dispersed. Additionally, since the rinds have a fair amount of moisture, that would seem to lessen the effect.
According to my Merck Index, these flammable oils (in an orange) are d-limonene (~90%), citral, decyl aldehyde, methyl anthranilate, linalool and terpineol.
Lemons and other citrus are of similar composition.
Limonene is very flammable, a good non-polar solvent, a hazardous irritant and smells rool purty.
[Python]Now, what do you do when someone attacks you with a passionfruit?[/python]
A mixture of oil and water won’t burn until all the water has boiled away. (Boiling water keeps the temperature low.) If the oil is volitile, it all may have evaporated along the steam cloud, so none is left by the time the water is gone.
Furthermore, if you look closely at the citrus peel you can see oil producing glands or sacs (not sure of correct terminology). They are paler ‘bubbles’ interspersed within the rind. Mandarins are good for this.
The oil IS burning in the campfire, however it is subsumed within the flames of the existing combustion.
The oil is also dispersed more evenly in the heat of the fire, rather than spraying out into the cool air from the ‘glands’ in droplets, before igniting in the match flame. As bbeaty said, the water in the rind reduces the rate of oil evaporation, so there is also a temporal dispersion of the fuel. Thus you cannot see the oil burn in the environment of the camp-fire.
Ha ha! Linalool. That’s a good one. Linalool. Ha ha.