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Preparation, characterization and functionality of acid-modified gluten Wu, Chiu Hui

Abstract

The insolubility of gluten in aqueous solutions is one of the major limitations of its more extensive use in food industry. This property is due to the presence of the high concentration of nonpolar amino acid residues such as proline, leucine and glutamine; and the lack of many ionizable side chains such as lysine, arginine, glutamic acid and aspartic acid. The hydrogen bond interactions between glutamine and asparagine side chains play an important role in promoting association between gliadin and glutenin molecules. The purpose of this study is to search for the best condition for solubllizing gluten by mild acid treatment; this treatment is expected to decrease the amount of side chain amides, thereby weakening the hydrogen bond interactions between the molecules. Gluten solutions containing various concentrations of HCl (0.02 to 0.5 N) and acetic acid (1.75 to 8.75 N) were heated at different temperatures (from 100 to 121 C) for different lengths of time. The following results were observed from this study. (1) Soluble gluten was recovered with yields of 85 to 92% of the original gluten protein by dialysis and yields of 70 to 78% by isoelectric precipitation. (2) The amount of amide nitrogen in the treated samples was dependent mainly on the concentration of the acid used. Gluten can be soiubilized when the amide content was reduced by approximately 10%. (3) Gel filtration through Sepharose 6B columns showed better separation of higher molecular weight components than through Sephadex G-100. The results suggested that with the increasing HCl concentrations the amount of higher molecular weight components in solubilized gluten decreased; acetic acid was considerably milder for gluten modification, (4) The chloranil test proved that a small amount of low molecular weight peptides and free amino acids had been formed during the acid treatment. (5) The molecular weight distribution of the products was further evaluated by Trautman plot for the approach-to-equilibrium method of ultracentrifugation. The results showed that the HCl-treated gluten was polydisperse and that when the concentration of the acid used to treat gluten was low, weight average molecular weight was much greater. (6) The 2,4,6 tri-nitrobenzenesulphonic acid method and the chloranil test were used to determine the amino groups exposed by peptide hydrolysis. As the concentration of acid was increased for acid solubilization of gluten, more amino groups were exposed in solubilized gluten. (7) No significant changes were detected in the content of SS-groups of gluten during acid modification. (8) Amino acid analysis revealed no significant changes in the pattern with the acid treatments. (9) The acid modification of gluten remarkably improved the emulsifying capacity and the emulsion stability; the emulsifying capacity of the gluten modified by dilute HCl and acetic acid was equal to or in some cases better than that of soybean protein isolates. (10) The pH 5.4 soluble fraction prepared from the isoelectrically precipitated gluten modified with 0,02 and 0.05N HCl exhibited excellent ioamability and stability. (11) A warm activated carbon column was effective in elimination of wheaty flavour of the acid-modified gluten solutions. The best conditions for the acid modification of gluten were found to be the heat treatment of 5% gluten suspension in 0.02H HCl at 121 C for 30 min or in 0.05N HCl for 15 min. These conditions were adequate for achieving solubility of gluten while minimizing the hydrolysis of protein molecules.

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