()-2-Heptenal in Soymilk: A Nonenzymatic Formation Route and the Impact on the Flavor Profile.

Flavor is an essential quality characteristic of soymilk. ()-2-Heptenal has a fatty and fruity flavor with the sensory threshold value of 13 μg/L in water. This study demonstrated that the formation of ()-2-heptenal was independent of the lipoxygenase (LOX) and hydroperoxide lyase (HPL) activity as well as oxygen concentration but was related to the presence/absence of Fe and chelators. In a dry matter base, soybean hypocotyls generated a much higher amount of ()-2-heptenal than cotyledons. A phospholipid hydroperoxide was purified from the chloroform/methanol extract of soybean hypocotyls and was identified as 1-palmitoyl-2-(12-hydroperoxyoctadecadienoyl)--glycerol-3-phosphatidylethanol-amine (12-PEOOH). The decomposition of 12-PEOOH in the presence of ferrous ions to form ()-2-heptenal was studied in a model system. The rate of decomposition decreased sharply at pH values higher than 6, but the molar conversion of 12-PEOOH to ()-2-heptenal increased with an increase of pH. At a constant pH of 5.8, the decomposition rate of 12-PEOOH was positively linearly related to the Fe concentration, while the molar conversion to ()-2-heptenal was 74% and independent of the Fe concentration. The formation of radicals LOO and R showed similar pH and Fe concentration dependence with those of ()-2-heptenal. ()-2-Heptenal displayed an enhancement of bean aroma and fruity flavor of soymilk at low concentrations, but a fatty flavor was noticed at high concentrations.