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bisabolyl cation h e h e h =

ar-Curcumene (20%) a-Zingiberene (22%) p-Zingiberene (14%) p-Bisabolene

Scheme 3.12 Biosynthesis of sesquiterpene hydrocarbons from farnesyl pyrophosphates via a bisabolyl cation (adapted from Rani 1999). P-Zingiberene has been added.

To clarify the generation of geranial in ginger, the alcohol dehydrogenase activity was measured in a crude enzymatic system of ginger. The enzyme solution contained geraniol dehydrogenase (GeDH) specifically acting on geraniol as a substrate with NADP as a coenzyme, which belongs to the pyridine nucleotide dehydrogenase involved in redox reactions (Sekiwa et al., 2001a). Geranial generation and GeDH activity were investigated for different maturity stages and storage periods of ginger. Both were at maximum levels from just after harvesting to initial storage. The GeDH activity subsequently dropped and the generation of geranial also stopped. The substrate specificity is confirmed by adding NADP. These results suggest that the GeDH activity in ginger is related to the generation of geranial.

The higher content of monoterpene and sesquiterpene alcohols in a steam distillate of ginger arises probably from thermal degradation of the nonvolatile glycosides of the corresponding alcohols (Chen and Ho, 1989). Dissacharide glycosides of monoterpenols as aroma precursors from rose flowers have been reported by Oka et al. (1997).

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