30C-27 |
Thermal aggregation of Dolichos lablab globulin |
K. -. M. CHING and C. -. Y. Ma. Department of Botany, The University of Hong Kong, Pokfulam Road, Hong Kong Dolichos lablab is an underutilized legume indigenous to China and has been shown to be a good source of dietary protein. In order to enhance the utilization of this legume protein as a food ingredient, the study of its thermal behavior is important since it may affect the functional properties (e.g., gelation) and nutritional quality of the product. The objective of this study was to investigate the thermal aggregation behavior of Dolichos lablab globulin (DLG) under different buffer conditions and to evaluate structural changes of the protein during aggregation. DLG was prepared by Osborne fractionation with 0.5 M NaCl. Protein dispersions (1% w/v) were aggregated under different temperatures and time, chaotropic anions and protein-modifying reagents. Physicochemical changes of DLG during thermal aggregation were evaluated by differential scanning calorimetry (DSC), measurements of sulfhydryl-disulfide contents and surface hydrophobicity. Our results showed that maximum aggregation occurred at 0.2 M NaCl. Heating at the temperature above 80¢J did not increase aggregation significantly. Aggregation rate did not change when the counterion changed from Cl- to Br-, I- and SCN-. Addition of dithiothreitol enhanced aggregation whereas SDS inhibited it. DSC data showed that the protein was undergoing denaturation when subjected to preheat treatments (at temperature below the denaturation temperature of DLG, 99¢J). The sulfhydryl and disulfide contents did not change markedly during aggregation while surface hydrophobicity increased two folds. Results suggest that thermal aggregation of DLG is preceded by denaturation. Chaotropic salts influence protein conformation and promote aggregation. The breakup of disulfide bonds by dithiothreitol enhances thermal aggregation of DLG, whereas ionic repulsion by SDS interferes with the process. Denaturation of DLG occurring during thermal aggregation leads to exposure of previously buried hydrophobic groups. The present findings provide information for optimizing thermal aggregation of DLG and may enhance their utilization in food systems.
Session 30C, Food Chemistry: Proteins
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