45C-26 |
Residual soybean sulfur metabolism in isolated soy proteins: Sulfate to cysteine |
W. L. BOATRIGHT, Department of Animal Sciences, University of Kentucky, 412 W.P. Garrigus Building, Lexington, KY 40546-0215 A major impediment to the expanded use of soy protein products in human foods is their characteristic flavor. Methanethiol is among the most potent odorants found in soy protein isolates, concentrates and soymilk. This investigation examined components of the chemical mechanism that produces methanethiol in aqueous mixtures of isolated soy protein. Potential substrates and enzyme inhibitors were added into aqueous slurries of isolated soy proteins. Resulting methanethiol was measured using a direct headspace sampling technique and gas chromatography/mass spectrometry with selective ion monitoring. The mean methanethiol headspace concentration above 0.6 percent aqueous slurries of isolated soy protein (ISP) was 0.008 mg per cubic meter. The addition of L-methionine to aqueous slurries of ISP had no effect on methanethiol headspace levels. The addition of L-cysteine produced a 36-fold increase in methanethiol over the control and approximately ten times more hydrogen sulfide than methanethiol. N-acetyl-cysteine had no effect on either methanethiol or hydrogen sulfide. There was no acetaldehyde produced with added L-cysteine indicating that the observed production of hydrogen sulfide and methanethiol was not from the Strecker reaction. L-cystine had the opposite effect of L-cysteine and reduced methanethiol levels. Dithiothreitol, sodium sulfite and ammonium sulfate increased headspace methanethiol above aqueous ISP by 44-, 9- and 4-fold, respectively, over the control but did not produce hydrogen sulfide. In ISP/cysteine mixtures, aminooxy acetic acid (AOA), which inhibits pyridoxial phosphate (PLP) dependent enzymes, slowed both hydrogen sulfide and methanethiol synthesis for 30 min followed by a large increase in hydrogen sulfide by 90 min. When AOA was added to slurries of ISP with DTT, methanethiol production was slowed and large amounts of hydrogen sulfide appeared. These observations indicate that methanethiol from isolated soy proteins is formed by way of a sulfate-to-cysteine pathway involving enzymes such as thioredoxin reductase (or glutaredoxin reductase), ferrodoxin:sulfite reductase and cysteine synthase (a PLP dependent enzyme).
Session 45C, Food Chemistry: Flavor and aroma chemistry
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