Chili peppers (Capsicums) are different in taste, appearance, aroma, pungency, color, origin, growth process, DNA structure, and toxicity levels. Capsicum consists of 38% pericarp, 2% inner pod, 56% seeds, and 4% stems. The property that separates the Capsicum family from other plant groups and the quintessential chili pepper is an alkaloid called capsaicin (kap-sa-I-sin), an unusually powerful and pungent crystalline substance not found in any other plant. Capsaicin is the source of spiciness and heat in Capsicums.

Capsaicin is a colorless, crystalline, bitter compound present in capsicum [Previously referred to as capsicine]. It has a molecular weight of 305.46. Capsaicin has a melting point of 65_C, a boiling point of 81_C, a vapor pressure of 40, and a vapor density of 1.59 (air=1). The range of capsaicin content in the pericarp of an average pepper is approximately 0.17-0.58% and the inner cap is 6.6-7.7%; the color is mainly concentrated in the pericarp. Chili seeds contain 19% oil with 0.024% capsaicin content. The percentage of capsaicin in the capsicum plant depends on the species, geographic origin, and climatic conditions.

Capsaicin is produced by glands at the junction of the placenta and the sheath wall. Capsaicin is unevenly distributed within the pod and is mainly concentrated in the placental tissue. The seeds are not sources of heat, as is commonly believed. Of a kg of cayenne pepper, for example, approximately 2.13 g. of raw capsaicin can be isolated, which is about 20 times the amount found in paprika.

The potent active ingredient, early considered as a single substance, capsaicin soon proved to be a mixture of two unsaturated and three saturated homologues. This mixture is now called capsaicinoids.

Capsaicin (trans-8-methyl-N-vanillyl-6-nonenamide)

Dihydrocapsaicin (8-methyl-N-vanillyl-nonamide)

Nordihydrocapsaicin (7-methyl-N-vanillyl-octamide)

Homodihydrocapsaicin (9-methyl-N-vanillyl-decamide)

Homocapsaicin (trans-9-methyl-N-vanillyl-7-decenamide)

Capsaicinoids are produced by glands in the placenta of peppers, which is the upper part of the partition just below the stem. The placenta is about 16 times hotter than the meat.

Upon contact with human mucous membranes, capsaicin causes inflammation causing immediate closure of the eyes and difficulty breathing and a burning sensation on the skin. Capsaicin causes irritation of trigeminal cells. These cells are pain receptors located in the mouth, nose, stomach, and mucous membrane. They release Substance P (SP), a chemical messenger that communicates any pain or inflammation from the skin to the brain. SP is a neurotransmitter. Neurotransmitters are intercellular chemical messengers that are secreted by neurons (nerve cells) through specialized structures (synapses) to transmit chemical information to one or more target cells. Capsaicin stimulation of sensory nerves not only results in central transmission of sensory signals, but also releases SP from central and peripheral sensory nerve terminals. Substance P (SP) belongs to the tachykinin family, which represents a group of biologically active peptides with a similar amino acid sequence in the C-terminal region. SP was the first peptide from the tachykinin family to be found in mammals.

Substance P: Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2

Therefore, it is likely that the natural release of SP represents a direct action on postcapillary venules or smooth muscle cells. Both capsaicinoids and SP act as spasmogens on certain viscera that contain smooth muscle, causing contractions. Substance P is one of the key causes of contraction of the esophagus, trachea, airways, and iris muscles of the eyes.