The English word pepper is ambiguous. Botanically it could be refer to plants that produce black powdered pepper. The angiosperms (flowering plants) who serve as a source of that spice belong to the order Piperales and to the 3600-member family Piperaceae. (Kings play chess only for goodness’ sake is a mnemonic for remembering the order of biological classification: kingdom, a group of related phyla, which in turn, at least in singular form, phylum, is a group of related classes, and so on with orders, families, genera and species. ) Piperine is the compound mainly responsible for black pepper’s pungency, but as expected it does not appear in unrelated plants also dubbed as peppers.
Pepper can also refer to Capsicum chinense, the Red Savino haanero chili, a member of the tomato family, Solanacea. That fruit scores in the hundreds of thousands of units on the Scoville scale of “hotness”. Other hot peppers and milder ones are found among the many varieties of another Capsicum species, known as annum, which includes sweet peppers, jalapeños, and New Mexico chili.
There are four different amides (specifically, capsaicioids) found in hot peppers of the Capsicum genus. A Brazilian study revealed that in these fruits capsaicin makes up anywhere from 24% (in jalapeños) to 95% (pimenta preta) of capsaicioids present. These in turn combine to account anywhere from 0.2 mg to 7 mg per gram of fresh pepper, in other words, less than 1%. The percentage of course soars when the pepper is dried. Except in jalapeños and fatalis, the equally potent compound, dihydrocapsaicin, is the second most abundant amide. The greatest concentration (9.2%) of nordihydrocapsaicin, a substance about half as hot, was found in pimenta de mesa. Finally another related compound, homocapsaicin, the least potent of the quartet, is entirely absent from most spicy peppers, but it makes up 12% of dedo-de-moça pequena’s capsaicioids.
From examining the above structural diagrams notice that in comparison to capsaicin, homocapsaicin’s double bond is slightly further away from the tail end of the molecule. On the other hand, nordihydrocapsaicin’s strength is compromised by being one CH2 shorter than its hotter counterpart, dihydrocapsaicin.
How do these compounds exert such a powerful reaction in our mouths? In all mammals they cause tingling and burning sensations by activating a non-selective cation channel, called VR1, on nerve endings. It’s not a coincidence that the same channel also interacts with compounds released by inflammation from actual intense heat sources or acidic protons. Birds, however, have a variant of VR1, which is still sensitive to heat and acid but which does not interact with capsaicin or its analogues. It’s likely an example of co-evolution between Capsicum plants and animals who can eat their fruits without suffering deterring consequences. They then fly to other destinations to spread ingested seed.
We get more evidence of coevolution thanks to Jordi Altimiras of Linköping University who made me aware of a study revealing that chilli seed germination is decreased in the gastrointestinal tract of mammals but not by the passage through the tract of birds.
Finally why are peppers producing capsaicioids in the first place? Any biosynthesis is catabolic and thus consumes energy. But making capsaicinoids in fruits is a worthwhile investment; it reduces fungal infection and seed mortality. From that narrow perspective we mammals have more in common with fungi than birds.
- Molecular basis for species-specific sensitivity to “hot” chili peppers. Cell. 2002 Feb 8;108(3):421-30.
- Comparative Study of Capsaicinoid Composition in Capsicum Peppers Grown in Brazil
- Directed deterrence by capsaicin in chillies. Nature 412: 403-404 and Tewksbury JJ, Reagan KM, Machnicki NJ, Carlo TA, Haak DC, et al. (2008)
- Evolutionary ecology of pungency in wild chilies. ProcNatAcadSciUSA 105: 11808-11811 .