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

Flavor is an essential quality characteristic of soymilk. (E)-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 (E)-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 Fe²⁺ and chelators. In a dry matter base, soybean hypocotyls generated a much higher amount of (E)-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)-sn-glycerol-3-phosphatidylethanol-amine (12-PEOOH). The decomposition of 12-PEOOH in the presence of ferrous ions to form (E)-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 (E)-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 Fe²⁺ concentration, while the molar conversion to (E)-2-heptenal was 74% and independent of the Fe²⁺ concentration. The formation of radicals LOO^• and R^• showed similar pH and Fe²⁺ concentration dependence with those of (E)-2-heptenal. (E)-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.