Some chilli tastes bitter because of the peppers that are used in the recipe for many types of chilli. The peppers taste bitter to us because they have chemicals that trigger our "bitter taste receptors" in our mouths. These taste receptors are basically proteins embedded in the membranes of cells in our mouths. The receptors are meant to bind to certain chemicals, called "ligands."

Chillies contain poisons that are meant to discourage the wrong animals from eating them. These poisons are also responsible for the heat. However, in small doses, we humans can deal with the poisons.