59D-3 |
Thermal aggregation of rice globulin |
S. K. W. ELLEPOLA and C. Y. Ma. Department of Botany, University of Hong Kong, Kadoorie Biological Sciences Bldg., Pokfulam Rd., Hong Kong, China Studies of structural changes in protein during heat-induced aggregation can provide valuable information for improving functional performance of the protein ingredient. Rice often undergoes diverse heat treatment during processing and its proteins may undergo structural changes and aggregation. Thermal aggregation behavior of rice globulin, the second most abundant protein fraction in rice, has not been studied. The objective of this study was to investigate the thermal aggregation behavior of rice globulin under different environmental conditions and to assess structural changes of the protein during aggregation. Rice globulin was prepared by Osborne fractionation with 0.7M NaCl. Protein dispersions (1% w/v) were aggregated under different temperatures, chaotropic anions and protein-modifying reagents. Structural changes of rice globulin during thermal aggregation were evaluated by circular dichroism (CD) and measurements of sulfhydryl-disulfide contents and surface hydrophobicity. Our results revealed maximum protein aggregation at 100°C with 70% protein precipitation after 60 min heating. Aggregation rate was affected by chaotropic salts, increasing when counterion changed from Cl- to Br-, I- and SCN-. Addition of dithiothreitol led to a marked increase whereas N-ethylmaleimide induced a decrease in aggregation. CD spectroscopy showed a gradual decrease in a-helical and b-sheet contents and increase in random coil structure. Sulfhydryl content was decreased by 38% while disulfide content and surface hydrophobicity was increased by 32% and 60%, respectively upon heating. Results suggest that thermal denaturation of rice globulin was closely followed by aggregation. Chaotropic salts profoundly influenced protein conformation promoting unfolding. The breakage of disulfide bond promoted monomer aggregation, and thiol-disulfide interchange led to the formation of disulfide linkages, stabilizing the aggregates. Denaturation of rice globulin occurred during thermal aggregation and previously buried hydrophobic groups were exposed. These findings provide knowledge to optimize thermal aggregation of rice proteins and may enhance their functionality in rice food systems.
Session 59D, Food Chemistry: Proteins and Physicochemical Properties
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