Reduced-lipid soy protein isolate (SPI), prepared from soy flour treated so that most of the polar lipids have been removed, exhibited an increase in protein solubility of 50% over that of the control SP1 prepared from hexane-defatted flour. Adding lipids from a commercial SPI during processing of reduced-lipid SPI decreased SPI solubility by 46%. The 19% decreased solubility caused by the lipids (primarily phospholipids) was largely recovered by treating the protein with a reducing agent (2-mercaptoethanol). The balance of protein insolubility, caused by the lipids, was attributed to a smaller lipid fraction (approximately 5% of the total lipids). Adding lipids during SPI processing contributed to both the formation of oxidized protein sulfhydryls, incapable of being reduced by 2-mercaptoethanol, and to oxidative deterioration of protein as determined by protein carbonyl contents.
Using gas chromatography/olfactometry (GCO), major odors from the headspace of aqueous solutions of soy protein isolates were evaluated. Many corresponding odorants were identified by correlating GCO with GC/mass spectrometry (MS) on two separate stationary phases followed by comparing retention times, mass spectra, odor descriptions and odor intensities with authentic standards. Based on aroma extract dilution analyses, the most powerful odorants (strongest and most volatile first) were (1) dimethyl trisulfide, (2) trans,trans-2,4decadienal, (3) an unidentified burnt soy sauce-like odor, (4) 2-pentyl pyridine, (5) trans,trans-2,4-nonadienal, (6) hexanal, (7) an unidentified charred sweaty feet-like odor, (8) acetophenone, and (9) 1-octen-3-one. This is the first reported occurrence of dimethyl trisulfide in soy protein isolates.
The effect of oxygen on the two separate one-electron reactions involved in the oxidation of ascorbic acid was investigated. The rate of ascorbate radical (Asc(-)) formation (and stability) was strongly dependent on the presence of oxygen. A product of ascorbic acid oxidation was measurable levels of hydrogen peroxide, as high as 32.5 μM from 100 μM ascorbic acid. Evidence for a feedback mechanism where hydrogen peroxide generated during the oxidation of ascorbic acid accelerates further oxidation of ascorbic acid is also presented. The second one-electron oxidation reaction of ascorbic acid leading to the disappearance of Asc(-) was also strongly inhibited in samples flushed with argon. In the range of 0.05-1.2 mM ascorbic acid, maximum levels of measurable hydrogen peroxide were achieved with an initial concentration of 0.2 mM ascorbic acid. Hydrogen peroxide generation was greatly diminished at ascorbic acid levels of 0.8 mM or above.
Addition of tert-butylhydroquinone or a mixture of butylated hydroxyanisole and tert-butylhydroquinone (200 ppm on a lipid basis) during SPI processing gave increased protein solibility over that of the con&l (55%. 56% and 34%. resuectivelv). These increased solubilities correspond to 32% and 18% decrease in oxidation of free sulthydryls and 20% and 12% reduction in protein oxidation, as determined by protein carbonyl content. Increased protein solubilities, due to added antioxidants, were accompanied by higher total protein surface hydrophobicity, as determined by the sodium dodecyl sulfate (SDS) binding method, and soluble protein hydrophobic&y, as determined by the fluorescence probe S-anilino-1-naphthalene sulfonate (ANS).
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