Why do jalapenos burn skin?

Answer 1

The burning chemical in peppers of the genus Capiscum is capsaicin. It's a strong mucous membrane and skin irritant, and also a neural agent. Molecules of capsaicin bind with the neurosensors that detect abrasion and heat, hence the association between peppers and burning sensations. The exact mechanism of irritation seems to be stimulation of the aforementioned neurochemoreceptor, VR1. This compound is non-water soluable, and as such is difficult to remove. One of the best ways seems to be exposure to the protein casein, common in milk, to which capsaicin seems to readily bond (so you can drink milk to overcome the burning sensation -- somewhat). In ecology, the capsaicin is an irritant only to mammals. With birds, it produces an analgesic effect and in fact birds are the ones that spread Capiscum seeds. Seeds passing through mammal GI tracts fail to germinate, so this clearly is an evolutionary feature that deters mammals from eating these fruit. Capsaicin is also the active ingredient in Pepper Spray, the non-lethal weapon. It's severe enough that the use of pepper spray is banned in military operations. Capsaicin is also now used in a weird variety of pharmaceutical applications ranging from pain control (!) to oncology.