Green notes are present in a wide variety of fresh leaves, vegetables and fruits. These compounds are responsible for a so-called green odor, green aroma, fresh note or green note. Related compounds present in many plants include trans-2-hexenol, cis-2-hexenol, trans-3- hexenol, 1-hexanol, 1-hexanal and cis-2-hexenal; organoleptically, each of these compounds possesses a specific type of green character, which is why they are known collectively as the green notes. Such compounds can be used to sharpen and enhance flavored products, such as those products having fruit flavors. Green notes have been isolated from plants or chemically synthesized: they are present in plant essential oils and have been obtained by steam distillation of plant material. However, the supply of flavor materials for the food industry is mainly based on the biosynthetic capabilities of plants. Cultivation problems, environmental defects and other reasons have led to a decreasing availability of some plant resources. These developments are accompanied by an increasing industrial requirement and various legal restrictions concerning the use of natural and chemosynthetic compounds. This is especially evident in the US market. Natural flavors are perceived as being better than the synthetic varieties and are therefore able to command a higher price. In 1995, the worldwide market for natural green notes was estimated at US$20-40 million per annum. The increasing demand for these natural compounds now exceeds their supply from traditional sources; this has motivated research efforts toward finding alternative natural ways of obtaining these materials.
Moreover, with the recent surge of interest in biotechnology it is easy to understand how flavor-producing microorganisms have turned from being laboratory curiosities into potential new sources of natural flavor compounds. Green notes also have been biosynthetically produced as reported by several authors.
Biosynthetic Pathways of Green Notes
The biosynthetic pathway to the green notes is present in many plant tissues such as leaves, fruits, and vegetables and involves the action of three enzymes (a lipoxygenase, a hydroperoxide lyase and an oxidoreductase).The lipoxygenasecatalysed oxidation of fatty acids containing a 1,4-pentadiene sequence into positionally specific acyl hydroperoxides is a well-documented enzymatic reaction. The hydroperoxide lyase cleaves the hydroperoxide to produce a C-6 unsaturated aldehyde. Then aldehyde isomerase, when present in the environment and under certain conditions, catalyses the formation of an aldehyde which is transformed into an alcohol by the action of alcohol dehydrogenase.