The fruit of most species of Capsicum contains capsaicin (methyl vanillyl nonenamide), a lipophilic chemical that can produce a strong burning sensation (pungency or spiciness) in the mouth of the unaccustomed eater. Most mammals find this unpleasant, whereas birds are unaffected. The secretion of capsaicin protects the fruit from consumption by insects and mammals, while the bright colours attract birds that will disperse the seeds.
Capsaicin is present in large quantities in the placental tissue (which holds the seeds), the internal membranes, and to a lesser extent, the other fleshy parts of the fruits of plants in this genus. The seeds themselves do not produce any capsaicin, although the highest concentration of capsaicin can be found in the white pith around the seeds.
The amount of capsaicin in the fruit is highly variable and dependent on genetics and environment, giving almost all types of Capsicum varied amounts of perceived heat. The most recognizable Capsicum without capsaicin is the bell pepper, a cultivar of Capsicum annuum, which has a zero rating on the Scoville scale. The lack of capsaicin in bell peppers is due to a recessive gene that eliminates capsaicin and, consequently, the "hot" taste usually associated with the rest of the Capsicum family. There are also other peppers without capsaicin, mostly within the Capsicum annuum species, such as the cultivars Giant Marconi, Yummy Sweets,Jimmy Nardello, and Italian Frying peppers.