With the holidays concluding (and stocks of candy canes dwindling) it's natural to ask: what's the difference between spearmint, peppermint, and wintergreen flavorings?
Beyond the differences in their biological origin, various people have (not surprisingly) analyzed the constituents of the essential oils of each.
Peppermint oil (Mentha × piperita L., a cross between watermint, Mentha aquatica, and spearmint, Mentha spicata) has seasonal and regional variation, but it is primarily a mixture of menthol (>30%) and menthone (>15%, sometimes >30%). Also present are a large diversity of additional compounds, including menthyl acetate, eucalyptol, limonene, beta-pinene, beta-caryophyllene, trans-carane, pulegone, (+)-carvone, and neomenthol. Some of these components (pulegone and menthofuran, for instance), are undesirable from flavor or toxicity standpoints, and have been the subject of metabolic engineering. A sampling:
Spearmint comes from the essential oil of Mentha spicata and differs markedly from peppermint oil mostly in the relatively low abundance of menthol (<1%); it's still a complex mixture. The primary flavor component in this case is R-carvone (>50-75%). Limonene is more prominent than in peppermint. Other components include eucalyptol, trans-carveol, dihydrocarveol, caryophyllene, beta-bourbonene, linalool, beta-pinene, and germacrene D.
Wintergreen (essential oil commonly from Gaultheria procumbens) is arguably simpler than either peppermint or spearmint and has a greater variety of sources. The chief flavor component here is methyl salicylate (a whopping ~98%), although one also finds limonene, myrcene, cadinene, carene, and pinene (alpha-pinene, in contrast to the predominantly beta-pinene in peppermint/spearmint).
For a really nice graphic of chemical compounds as flavor components in herbs and spices in general, see this post at Compound Interest.
All in all, terpenes are nice.