THE BIOGENERATION OF GREEN ODOR BY GREEN LEAVES

The 'green odour' of green leaves arises from eight volatile compounds, C6-aldehydes and C6-alcohols which include leaf aldehyde, (2E)-hexenal and leaf alcohol, (3Z)-hexenol. These volatile compounds are synthesized in green leaves from alpha-linolenic and linoleic acids via their respective hydroperoxides. Four enzymes catalyse this biosynthetic pathway and among these four, lipoxygenase and hydroperoxide lyase are particularly interesting enzymes. The enzyme system involved is found bound to the thylakoid membrane of chloroplasts of green leaves. Lipoxygenase adds oxygen stereoselectively to unsaturated fatty acids having a (1Z, 4Z)-pentadiene moiety, e.g. alpha-linolenic and linoleic acids, to produce 13-(S)-hydroperoxides. Hydroperoxide lyase cleaves the bond between C-12 and C-13 of these hydroperoxides to form C6-aldehydes. The activities of the enzymes change throughout the growing season and appear to be closely related to the ambient temperature and solar radiation received. In this review of the green odour of green leaves, attention is focused on the biosynthetic pathway by which C6-aldehydes are formed from alpha-linolenic and linoleic acids, on the enzyme system catalysing the biogeneration of green odour, and on the reaction mechanism and the relationship between enzyme activities and the environment. In addition, structural relationships between green odour and organoleptic properties of n-hexenals and n-hexenols are discussed. The chemistry of the formation of 'Lipton black tea aroma' from leaf alcohol will also be mentioned.