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W. L. BOATRIGHT and C. J. Stine. Dept. of Animal and Food Sciences, Univ. of Kentucky, 412 W.P. Garrigus Building, Lexington, KY 40546-0215 Soybeans are the second largest food crop in the U.S. and only about 2% of the available proteins from soybeans are used in human foods, largely because of their characteristic flavor. Methanethiol is one of the most potent odor compounds in isolated soy proteins (ISP). Our objective was to elucidate the post-harvest biochemical reactions responsible for the occurrence of sulfites (a methanethiol precursor) in ISP. Total sulfate, specific organic-sulfate compounds and adenylate compounds were determined by HPLC. Adenosine-5'-triphosphate (ATP) was measured by bioluminescence. Total sulfites were measured with a modified Monier Williams procedure. Methanethiol content was assayed by static headspace/gas chromatography/mass spectrometry. The total sulfate content of hexane-defatted soybean flours was 1,100 to 1,480 ppm. The ATP content, which is required by ATP sulfurylase to convert inorganic sulfate into active forms of sulfate, was 980 pmol per 0.1 g defatted flour. Unlike the ATP increases upon germination of intact codyledons, ATP levels decreased rapidly upon hydration of defatted flour. The common forms of activated sulfate in plants (adenosine-5'-phosphosulfate and 3'-phosphoadenosine-5'-phosphosulfate) were not found in defatted flour or samples obtained at various stages of ISP processing. The addition of magnesium, ammonium or sodium sulfate during ISP processing had no significant effect on the sulfite or methanethiol content of ISP. The sulfite content of hexane-defatted soybean flour was about 1 ppm and the corresponding ISP contained 21 ppm sulfite. No sulfites were formed during ISP processing until immediately after the isolated soy proteins were separated from an unknown component(s) in the pH 4.6 supernatant during protein isoelectric precipitation. Identification of the precise point during ISP processing where sulfites are being formed provides us with important information necessary for developing novel processes to minimize sulfite and methanethiol formation in ISP.
Session 32, Food Chemistry: Component interactions and processing effects
2005 IFT Annual Meeting, July 15-20 - New Orleans, Louisiana |