Retro‐aldol degradations of unsaturated aldehydes: Role in the formation of c 4‐heptenal from t 2, c 6‐nonadienal in fish, oyster and other flavors

Alterations of cucumber‐, melon‐like notes in aromas and flavors caused by retro‐aldol degradations of t 2, c 6‐nonadienal were confirmed using gas chromatographic measurements of volatile compounds in model systems. The data indicated that 3‐hydroxy‐ c 6‐nonenal was formed first by the addition of water to the alpha/beta double bond of t 2, c 6‐nonadienal, and this was followed by a retro‐aldol condensation of 3‐hydroxy‐ c 6‐nonenal to yield c 4‐heptenal and ethanal. Compared to the reaction rate in aqueous systems at neutral pH, formation of c 4‐heptenal was enhanced substantially at alkaline pH, but was greatly diminished at acidic pH values. Heating (to 90 C) of aqueous model systems of t 2, c 6‐nonadienal held at neutral pH also enhanced the rate of formation of c 4‐heptenal substantially compared to that at ambient temperature (21 C). Rates of formation of c 4‐heptenal in aqueous model systems held under air or nitrogen atmospheres were similar. c 4‐Heptenal was not formed when t 2, c 6‐nonadienal was held at 21 C for 96 hr under air or nitrogen in nonaqueous commercial corn oil.