THE FRESH GREEN ODOR EMITTED BY PLANTS

The fresh ''green odor'' of green leaves arises from eight volatile co mpounds, C-6-aldehydes and C-6-alcohols which include leaf aldehyde, ( 2E)-hexenal; and leaf alcohol, (3Z)-hexenol. These volatile compounds are biosynthesized in green leaves from alpha-linolenic and linoleic a cids via their respective hydroperoxides. Four enzymes catalyze this b iosynthetic pathway and among these four, lipoxygenase and hydroperoxi de lyase (an enzyme system) are particularly interesting. The enzyme s ystem involved is found bound to the chloroplast membranes of green le aves. Lipoxygenase adds oxygen stereoselectively to unsaturated fatty acids having a (1Z,4Z)-pentadiene moiety, for example, alpha-linolenic and linoleic acids, to produce 13-(S)-hydroperoxides. Hydroperoxide l yase cleaves the bond between C-12 and C-13 of these hydroperoxides to form C-6-aldehydes. The activities of the enzymes change throughout t he growing season and seed development, and appear to be closely relat ed to the ambient temperature, solar radiation, dark-light conditions, etc. Synthesis of green odor is described in this review, and in addi tion, structural relationships between green odor and the sensory prop erties of n-hexenals and n-hexenols are discussed. The chemistry of th e formation of ''Lipton black tea aroma'' from leaf alcohol is also ad dressed. Attention is focused on the biosynthetic pathway by which C-6 -aldehydes are formed from alpha-linolenic and linoleic acids, on the enzyme system catalyzing the biogeneration of green odor, and on the r eaction mechanism and the relationships among various stimuli, the env ironment, and the enzyme activities for producing C-6-aldehydes.