Capsaicinoids
Capsaicinoids are a family of about 20 related alkaloids synthesized exclusively in the Capsicum genus. Each is a vanillyl amine joined via an amide bond to a different branched-chain fatty acid. The five major members account for essentially all pungency in commercial pepper varieties.
The Five Major Capsaicinoids
Capsaicin and dihydrocapsaicin together make up ~90% of total capsaicinoid content in most peppers. Nordihydrocapsaicin (C7 saturated chain) is about 70% as potent. Homocapsaicin and homodihydrocapsaicin (C9 chains) are roughly equi-potent to capsaicin but less abundant.
Chain length and saturation are the primary structural variables. Shorter chains reduce TRPV1 binding affinity and potency. Longer chains increase duration at the receptor but reduce peak intensity. This diversity is an engineering lever — changing which acyl-CoA precursors the biosynthesis pathway receives shifts the capsaicinoid profile.
Heat Measurement
The Scoville Heat Unit (SHU) measures total capsaicinoid concentration by HPLC and converts it via the factor of 15 (mg/kg × 15 = SHU). It treats all capsaicinoids as equivalent to capsaicin, which is an approximation. The IFI-H axis improves on this by measuring actual TRPV1 activation rather than raw concentration.