Sodium erythrobate (2.3 mM) and dithiothreitol (6.5 mM) caused headspace hexanal from aqueous slurries of commercial isolated soy proteins (ISP) to increase by as much as 13-fold compared with headspace hexanal in the controls. When linoleic acid was also added, headspace hexanal levels from commercial and laboratory-prepared ISP were further increased by as much as 18-and 136-fold, respectively, compared with the levels in the controls. When 13 Clabeled linoleic acid was added to aqueous mixtures of laboratory-prepared ISP along with reducing agents, 96% of the resulting hexanal contained the 13 C isotope. Commercial ISP samples had about 40% of the isotopically labeled carbon incorporated into hexanal. These large increases in hexanal were almost completely prevented by heating the aqueous ISP to 100°C or above prior to adding the reducing agent and linoleic acid. Adding cysteine or sodium sulfite reduced headspace hexanal levels from aqueous ISP slurries by more than 88%. Thiol blockers, iodoacetic acid and N-ethylmaleimide, did not affect hexanal levels.
Commercial isolated soy proteins (ISP) contain 22 and 31 ppm sulfite as measured by the optimized MonierWilliams method (Sulfites in Foods, Official Methods of Analysis, 16th edn., AOAC, Washington, DC, 1995, Official Method 990.28). A method was developed to cryogenically trap and quantify the sulfur dioxide produced by this method using GC-MS. The same commercial ISP samples were found to contain 17 and 26 ppm sulfite, respectively, with GC-MS. ISP prepared in the laboratory contained 33 ppm sulfur dioxide, and defatted soybean flakes contained only a trace. Adding dithiothreitol after beginning the boiling step of the Monier-Williams assay had no significant effect on the sulfite content of a commercial ISP, whereas adding dithiothreitol prior to bringing the sample to a boil reduced the sulfite content from 17 to about 1 ppm.
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