Effects of Endogenous Polyphenols on the Physicochemical and Functional Properties and Structure of Pine Kernel Proteins

Protein isolates (PI) were obtained from pine kernel meal (PKM) and dephenolized pine kernel meal (DPKM) by alkali dissolution followed by acid precipitation, and globulin (Glo), albumin (Alb) and gluten (Glu) were prepared from PKM and DPKM by Osborne fractionation method. The effects of endogenous polyphenols on the physicochemical and functional properties and structure of these proteins were explored. It was shown that after the removal of phenolics, the contents of phenolic compounds, ash and carbohydrates decreased, while the protein content significantly increased (P < 0.05). The contents of total and free sulfhydryl groups in the proteins from DPKM were significantly higher than those in the proteins from PKM (P < 0.05), whilst the opposite was observed for the content of disulfide bonds and the surface hydrophobicity. Higher solubility, emulsion stability, foaming capacity and foam stability were observed in the proteins from DPKM, while the emulsifying capacity decreased after the removal of phenolic compounds (P < 0.05). The secondary structure of pine kernel proteins was changed by the removal of polyphenols. Dephenolization could alter the secondary structure of pine kernel proteins. The proteins from both PKM and DPKM had typical absorption peaks, and the positions of the amide Ⅰ and amide Ⅱ bands were shifted. In addition, the proportion of β-sheet increased, while the proportions of α-helix, β-turn and random coil decreased in DPKM proteins compared with PKM proteins. Fluorescence spectra showed that polyphenols had a fluorescence quenching effect on PI, Alb and Glu, but not on Glo. The structure and properties of pine kernel proteins can be significantly affected by dephenolization, and this treatment method can be selected according to the application purpose.