Identification of the odour-active aldehyde trans-4,5-epoxy-(E,Z)-2,7-decadienal in yuzu (Citrus junos Sieb. ex Tanaka)

Yuzu (Citrus junos Sieb. ex Tanaka) is a variety of sour citrus fruit that is popular in Japan. In this study, we investigated its aroma, which many people, especially in Japan, find appealing. Aroma extract dilution analysis was used to screen the odour-active compounds in the yuzu volatile oil. An unknown odorant with an albedo-like odour and a high flavour-dilution factor was detected. To identify this odorant, it was separated via flash column chromatography on silica gel and then fractionated by preparative high-performance liquid chromatography on a reverse-phase column. The volatile fraction, which contained a significant amount of the target unknown compound, was analysed by gas chromatography-mass spectrometry and gas chromatography-olfactometry to determine its mass spectrum, retention index, and odour quality. By comparing the resultant data with that of a synthesised authentic reference compound, it was evident that the unknown odorant was trans-4,5-epoxy-(E,Z)-2,7-decadienal (3); this is the first time that it has been identified as an odour-active unsaturated aldehyde in yuzu volatile oil. From the results of a model formation experiment, we propose that trans-4,5-epoxy-(E,Z)-2,7-decadienal originates from alpha-linolenic acid via 13(S)-hydroperoxy-(Z,E,Z)-9,11,15-octadecatrienoic acid (13(S)-HPOT), which is a lipoxygenase-induced oxidation product of alpha-linolenic acid.