Being that this is dedicated to the pepper (Capsicum) it would be somewhat of an injustice not to include at least one post covering the source of peppers heat, the Capsaicinoids.  Unique to the Capsicum genus, Capsaicinoids are broadly categorized as colorless, odorless, flavorless, non-nutrient alkaloids and come in many flavors, if you will.  In fact, there are over 20 different Capsaicinoids with capsaicin and dihydrocapsaicin dominating the capsaicinoid profile in most cultivars.  These compounds share a basic chemical structure comprising an aromatic vanillin head derived from the amino acid phenylalanine with a hydrocarbon tail derived from valine.  What makes them different is the number of carbons and/or in the degree of hydrogen saturation of the tail portion.  Figure 1 is a general structure where Table 1 lists some of the Capsaicinoids.

Figure 1.  Generic Capsaicinoid Structure

Table 1: Capsaicinoid Structure Variations²

The total concentration and profile of Capsaicinoids varies among species, within a single cultivar, and even between pods on a single plant.  In general ~90% of the Capsaicinoid concentration comes from capsaicin and dihydrocapsaicin.  Krajewska and Powers¹ were able to show that there is a significant linear relationship between the pungency perceived and concentration of capsaicinoid.  In addition, they found that at low concentrations the intensity of pungency and the perception differed among the Capsaicinoids.  For instance, nordihydrocapsaicin was found to be the least irritating, while capsaicin and dihyrocapsaicin were the most irritating.  The sensation of pungency caused by homodihydrocapsaicin developed more slowly and affected mainly the throat and the back of the tongue.

1. Krajewska and Powers, Sensory Properties of Naturally Occurring Capsaicinoids, J. Food Sci., 53 (3) 1998, 902-905
2. Table derived from Appendino, Giovanni, Modern Alkaloids: Structure, Isolation, Synthesis and Biology, (2008) Chapter 4, pgs 73-109