Sesame protein isolate: Fractionation, secondary structure and functional properties

Abstract

This study investigated the selective fractionation of sesame proteins using sodium chloride and ammonium sulfate. Maximum protein extraction occurred at pH 7–10 and in the presence of 0.6–1M NaCl. Sesame protein isolate (Ses-PI) yield increased from 12.5% in the absence of NaCl to 54.6% at 1M NaCl. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) indicated differences in the electrophoretic pattern of water-extracted Ses-PI compared to the NaCl extracts. However, changing the NaCl concentration did not affect the principal components in the profile of the sesame protein extracts. With ammonium sulfate (AS) precipitation, the 11S fraction was preferentially extracted at 30% saturation (AS30%). More 2S and 7S polypeptide bands were found in the AS50% fraction, whereas the AS70% sample had a higher concentration of the 7S fraction. FTIR analysis revealed that heat treatment as well as use of NaCl and AS at lower concentrations induced changes in protein conformation, whereas at higher concentrations the protein conformation was more stable. CD analysis revealed a predominance of α-helices in the water extract which decreased with increasing NaCl content toward more β-strand structures. Changes in secondary structure were also observed as the ammonium sulfate concentration was increased from 30 to 70%. The functional properties of (Ses-PI-1M NaCl) were compared to that of soy protein isolate (Soy-PI). Ses-PI-1M NaCl had significantly (P<0.05) lower protein solubility, water holding and fat binding capacities, but much greater emulsifying and foaming properties compared to Soy-PI.