Identification of C-13-Norisoprenoid Flavour Precursors in Starfruit (Averrhoa carambola L.)

HRGC and HRGC-MS identifications of glycosidically bound constituents from starfruit (Averrhoa carambola L.) were achieved in extracts isolated by Amberlite XAD-2 adsorption followed by methanol elution. The majority of aglycones released by almond beta-glucosidase hydrolysis were carotenoid degradation products, including megastigma-6,7-diene-3,5,9-triol, an important precursor of the potent aroma compound beta-damascenone. Additional aglycones comprised the following ionone derivatives: 4-hydroxy-beta-ionol, 3-hydroxy-beta-ionol, 4-oxo-beta-ionol, 3-hydroxy-beta-onone, 3-oxo-alpha-ionol, 3-oxo-refro-alpha-ionol (2 isomers), 3-oxo-4,5-dihydro-alpha-ionol, 3-oxo-7,8-dihydro-alpha-ionol ('Blumenol C'), 3-hydroxy-beta-damascone, 3,5-dihydroxy-megastigma-6,7-diene-9-one ('grasshopper ketone'), 3-hydroxy-5,6-epoxy-beta-ionone, 3-hydroxy-5,6-epoxy-beta-ionol, 3,4-dihydro-3-hydroxyactinidol, vomifoliol ('Blumenol A'), 4,5-dihydrovomifoliol, and 7,8-dihydrovomifoliol ('Blumenol B'). Several of these new starfruit constituents are easily degraded upon heat-treatment at natural pH condition of the fruit pulp, thus rationalizing the formation of a number of C-13-aroma compounds, which have recently been reported as starfruit volatiles.