Unpicking the changes in content, structural properties and in vitro digestibility of pigeon pea seed proteins as influenced by imbibition and heating in sodium bicarbonate

Abstract

Background

Pigeon pea proteins are underutilised mainly because of the long cooking time of the legume seeds. Traditionally, this is mitigated by cooking pigeon pea seeds in sodium bicarbonate (NaHCO₃). This study investigated whether, and to what degree, directly cooking pigeon pea seeds in water (CS0W) or NaHCO₃ (CS0N), soaking in water or NaHCO₃ for 4 h (CS4W and CS4N, respectively) or for 24 h (CS24W and CS24N, respectively) before cooking in water or NaHCO₃ influenced the protein content, structural property and in vitro digestibility.

Results

In vitro protein digestibility of pigeon pea seed flour was lowest in the raw seeds and increased with CS0W<CS4W<S24CW and CS0N<CS4N<S24CN. The α-helix content of the samples also decreased in the same order. SDS-PAGE showed ∼90% decrease in the 7S vicilin subunits (65 and 47 kDa) in CS24N and a corresponding 4-fold increase in protein bands at 10–15 kDa compared to raw seeds. NaHCO₃ treatments decreased the protein isolate yields due to protein losses (up to 7.5-folds) in the cooking fluids.

Conclusion

The findings showed that NaHCO₃ improved protein digestibility by degrading the higher molecular weight proteins, but also increased protein diffusion into the cooking fluid. Therefore, the traditional method of cooking pigeon pea seeds needs optimization to maximize the protein accessibility.