Direct visualization of capsaicin and vanillylamine in Capsicum fruit with laser fluorometric imaging

Abstract

Capsaicin, the pungent secondary metabolite found in fruit of the genus Capsicum, and vanillylamine, a metabolic precursor to capsaicin, are used in the food science, pharmaceutical, medical, and forensic industries; however, a rapid method to evaluate fruit for the presence of these two compounds is lacking. The present research describes the in situ visualization of capsaicin and its precursor in Capsicum fruit by laser-induced fluorescence imaging and spectrometry. When excited by ultraviolet lasers at 266 nm, capsaicin and vanillylamine have peak fluorescence emissions at 310 nm. The fluorescence spectra of precursors of capsaicin and analogs, i.e., trans-caffeic acid, trans-ferulic acid, trans-para-coumaric acid, vanillyl alcohol, vanillin, vanillic acid, had different peaks than those of capsaicin and vanillylamine. The localization of capsaicin and its immediate precursor, vanillylamine, was imaged with an ultraviolet-sensitive camera after excitation with a laser at 266 nm. Fluorescence images detected at 310 nm showed the localization of capsaicinoid and/or vanillylamine on the surface of placenta and septa of Capsicum fruits. No fluorescence specific to capsaicin and vanillylamine was observed in seeds or pericarp. Both bell pepper and sweet pepper also showed 310 nm fluorescence on the placental surface, suggesting the accumulation of vanillylamine in the placenta. Laser-induced imaging shows considerable promise as a suitable technique for rapidly screening Capsicum fruit for their capsaicin and vanillylamine contents.