Thai vs Cayenne

Both are Capsicum annuum and both grew up a long way from where they are famous. The word cayenne entered English in 1756, and the Online Etymology Dictionary traces it to the Tupi word kyynha, meaning capsicum. The link to Cayenne, the capital of French Guiana, is folk etymology that stuck, though UF/IFAS still describes the pepper as named for the city. Either way the trail runs to the Guianas. The Thai pepper had no Thailand to come from until the Portuguese carried chiles east. Bosland and Baral note that Portuguese traders spread chiles along their routes in Africa and Asia soon after Columbus, and that three varieties were recognized in India by 1542. UF/IFAS puts the Thai pepper's origin in Mexico or Central or South America, with its cultivation now centered on Southeast Asia.

The pods could not be more different in build. UF/IFAS describes Thai fruit as small and tapering, about an inch long and under a fifth of an inch wide. A cayenne is three or four inches of thin, curved, wrinkled pod that is usually dried and ground. Iowa State University Extension lists Thai Hot at 50,000 to 100,000 SHU, and UF/IFAS gives cayenne 25,000 to 50,000. On paper the Thai is twice as hot. In the pot a Thai pod is a fraction of the mass, so one pod delivers less total capsaicin than the rating implies, and cooks use several where they would use one cayenne.

Flavor is where the choice gets made. The Thai is sharp and bright with a fast, piercing burn. Green pods taste grassy and red ones a little fruitier. Cooks use them whole, sliced, or pounded into curry pastes and nam pla prik, and they define som tam and larb. Cayenne is comparatively neutral, a clean peppery warmth without much fruit or smoke, which is why it became the default heat in a shaker. It seasons Cajun and Creole cooking, curries, rubs and a long list of hot sauces. Commercial cayenne powder is sometimes a blend of red chiles rather than a single variety.

Substitution runs one way better than the other. A few fresh Thai pods stand in for fresh cayenne with no loss. Cayenne powder in place of fresh Thai gives you the heat and none of the brightness. One more source of confusion: small Thai bird chiles are often sold as Capsicum frutescens, and some are, but this database classes the Thai as C. annuum. The F1 hybrid sold as Thai Dragon is a separate, much longer pepper.

Sources

• Online Etymology Dictionary: cayenne
• UF/IFAS Extension, HS-859: Pepper Production in Miami-Dade County, Florida (Wang, Zhang, Li, Seal, Klassen, Olczyk)
• Iowa State University Extension and Outreach: Growing Peppers in the Home Garden
• Bosland and Baral, HortScience 42(2), 2007: 'Bhut Jolokia', The World's Hottest Known Chile Pepper is a Putative Naturally Occurring Interspecific Hybrid