Chemistry of odor-impact volatiles from soy protein concentrates

The extensive application of soy protein products, a source of food proteins with unique nutritional value, excellent functional properties and potential health benefits in human foods, is limited by their characteristic beany odor. Compared with other soy products such as soy protein isolates, soy oil, soy flour, and textured soy protein, soy protein concentrates have received minimal research on their odor-contributing volatile compounds. The major objectives of the current study were: (1) to identify the primary odor-impact volatile components for soy protein concentrate aqueous slurries; (2) to investigate the factors influencing the occurrence of the key odorants; (3) to elucidate the potential precursors for the formation of the powerful odorants.; Based on gas chromatography olfactometry/mass spectrometry (GCO/MS) analysis, acetaldehyde, methanethiol, hexanal, dimethyl trisulfide, and 2-pentyl furan were identified as the most potent odorants from both static and concentrated headspace of soy protein concentrate aqueous slurries. Besides hexanal and dimethyl trisulfide, 2-heptanone, octanal, 2-octanone, 1-octen-3-one, 3-octen-2-one, 2-decanone, benzaldehyde, 2-pentyl pridine, trans,trans-2,4-nonadienal were also revealed to be possible odor contributors by aroma extract dilution analysis from the concentrated SPC aqueous slurry volatiles recovered with chloroform.; Since both methanethiol and dimethyl trisulfide were documented for the first time as important odor-impact volatiles from soy protein concentrates, further investigations were performed to determine the factors influencing their occurrence and the potential precursors for their formation in soy protein concentrates. With an established methanethiol quantification method using ethanethiol as an internal standard, it was observed that the concentrations of methanethiol in soy protein concentrate aqueous slurries were significantly increased by high temperature (65°C), basic pH condition (pH 9.0), existence of transition metal ions, lipoxygenase, and EDTA. In contrast, low temperature (5.5°C) and an acidic condition (pH 4.6) reduced the level of methanethiol. Further quantitative data and the mass spectra obtained with incorporation of stable isotope labeled potential precursors indicated that L-methionine were possible precursors for methanethiol formation in soy protein concentrates with methional and methionine sulfoxide being likely intermediates.; With deuterium labeled DMTS as an internal standard, dimethyl trisulfide, another important odorant, was quantified from soy protein concentrates under various treatments. It was observed that both high temperature treatment and addition of ascorbic acid into SPC slurries resulted in a significant increase in dimethyl trisulfide. However, in acidic (pH 4.6) or basic conditions (pH 9.0) and with sodium benzoate, a decreased dimethyl trisulfide level was observed in soy protein concentrates. The mass spectra for DMTS obtained from soy protein concentrate samples treated with carbon-13 or deuterium-labeled compounds together with quantitative data indicated that methionine and cysteine were the precursors with methanethiol and H₂S being possible intermediates for dimethyl trisulfide formation in soy protein concentrates.