| Rice dregs, an under-utilized milling co-product of rice syrup industry, contains good-quality extractable proteins. Gluten, as the only major protein of the dregs, comprising at least 85% of the total protein, is hypoallergenic, hypocholesterolemic and rich in essential amino acids. However, it was limited in the application of food industry as its high hydrophobicity and low solubility. To improve the functional properties of the gluten, rice dregs gluten(RDG) and carrageenan(Car) were used to prepare RDG-Car conjugates by dry-heating method. The physicochemical, conformational and functional properties of conjugates were investigated in order to study the mechanism of Maillard reaction and provide a theoretical basis for industrial processing, as a result, to be used to meet the need of industrialization.The gluten extracted from rice residue was purified by a-amylase hydrolysis. The effects of the addition of a-amylase and the hydrolysis time were studied on the protein content and yield of gluten, and the optimum conditions were obtained: a-amylase addition 0.2mg/mL, and hydrolysis time 3h. The protein content of gluten was up to 90.53%, and the yield was 35.27%.The gluten was glycosylated with carrageenan by dry-heated Maillard reaction, The conditions affecting the degree of graft reaction and the functional properties of conjugates were studied, such as the ratio of protein to carrageenan and reaction time. The optimal reaction conditions were as follows:RDG reacted with carrageenan in weight ratios of 1:2, at 60℃,79% relative humidity for 24h, the graft degree of the conjugates was 28.84%. The functional properties of DRG-Car were improved obviously, the solubility, emulsifying property and emulsion stability were increased by 2.04,4.84 and 0.63 times, respectively.Amino acids analysis suggested that the major locus during Maillard reaction contained lysine, arginine and histidine, with the first two contributed more. Analysis by SDS-polyacrylamide gel electrophoresis showed that, with the Maillard reaction continued, the band about 20182D was disappeared gradually, and there was a new high molecular weight band (greater than 170KD) closer to the upper of separating gel. Fluorescence spectra resulted that the DRG-Car conjugates had the typical fluorescent characteristics of Maillard products, the conjugates had the strongest florescence intensity when emission was 340nm and the excitation was 423nm. Also infrared spectra results indicated that carrageenan was grafted to gluten by covalent band. The effect of glycosylation modification on the secondary structure of gluten was studied, results showed that the products were destroyed seriously, of which a-helix andβ-strand content decreased accompany withβ-turns and random coil content increasing. The surface hydrophobicity of gluten determined by ANS probes was reduced rapidly when it was glycosylated with carrageenan. The DSC results suggested that the grafts showed a good thermal stability, with a high thermal denaturation temperature. The study also found that, in comparison with gluten, the solubility, emulsifying property and emulsion stability of conjugates in a wide pH range were improved significantly.The RDG-Car conjugates as a novel emulsifier were applied to the emulsion. Oil-in-water emulsions were prepared by blending 20% soybean oil (v/v) and 80% protein solutions (v/v) together through ultrasonic emulsification. The influence of pH, ionic strength, and storage on emulsion stability was studied, results revealed that at the same protein concentration, emulsions stabilized by conjugates produced smaller droplet size, compared to emulsions stabilized by gluten and mixtures, and the emulsion was stable against long-term storage and ionic strength, but they showed a weak sensitivity to acid pH. |
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