• soy protein;
  • methanethiol;
  • hydrogen sulfide;
  • L-cysteine

ABSTRACT: The addition of L-cysteine to aqueous slurries of commercial isolated soy proteins (ISP) increased methanethiol headspace levels by 17- to 36-fold over the control. Corresponding levels of hydrogen sulfide were about 10 to 19 times greater than methanethiol. Neither methanethiol nor hydrogen sulfide were detected when L-cysteine was added to aqueous slurries of hexane-defatted soy flour. The production of hydrogen sulfide and methanethiol in aqueous slurries of commercial ISP was inhibited by the addition of a component(s) recovered from the pH 4.6 supernatant obtained during laboratory preparation of ISP by isoelectric precipitation. The inhibitory component had a molecular weight (MW) of ≥ 100000 and an isoelectric point of about 5.9. This component was not serine acetyl transferase. Its inhibitory properties were inactivated at 70°C and diminished with elevated levels of methionine. Adding the ≥ 100000-MW soluble-proteins (from the isoelectric precipitation step) back to a nonheat-treated laboratory ISP during processing reduced the methanethiol level by 88%.