Identification of C₁₃‐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 β‐glucosidase hydrolysis were carotenoid degradation products, including megastigma‐6,7‐diene‐3,5,9‐triol, an important precursor of the potent aroma compound β‐damascenone. Additional aglycones comprised the following ionone derivatives: 4‐hydroxy‐β‐ionol, 3‐hydroxy‐β‐ionol, 4‐oxo‐β‐ionol, 3‐hydroxy‐β‐ionone, 3‐oxo‐α‐ionol, 3‐oxo‐ retro ‐α‐ionol (2 isomers), 3‐oxo‐4,5‐dihydro‐α‐ionol, 3‐oxo‐7,8‐dihydro‐α‐ionol (‘Blumenol C’), 3‐hydroxy‐β‐damascone, 3,5‐dihydroxy‐megastigma‐6,7‐diene‐9‐one (‘grasshopper ketone’), 3‐hydroxy‐5,6‐epoxy‐β‐ionone, 3‐hydroxy‐5,6‐epoxy‐β‐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.