Optimization of hexametaphosphate-assisted extraction and functional characterization of palm kernel cake protein

Response surface methodology was applied to study the optimization of palm kernel cake protein (PKCP) hexametaphosphate-assisted extraction. The optimum PKCP yield (28.37%) when extracted using 1.50% sodium hexametaphosphate (SHMP) of pH 10, at 50 °C, and the 1:70 (w/v) ratio of cake-to-solvent was significantly (P < 0.05) higher than the protein yield from an alkaline (pH 10) extraction (8.12 ± 0.24%). Differential scanning calorimetry (DSC) analysis showed a higher denatured temperature (99.78 °C) for PKCP as compared with alkaline extracted one (96.96 °C), suggesting that a less denatured protein population is obtained. Electrophoresis of the PKCP revealed that the protein has 11 bands with MW ranging from 2.11 to 83.19 kDa. Relative to soy protein isolate, PKCP showed higher surface hydrophobicity (165.96 vs. 51.51), better solubility at pH 7 (87.65% vs. 41.21%), oil-binding capacity (7.73 vs. 2.96 g/g) and emulsifying activity (178.50 vs. 32.57 m(2)/g), but lower water-binding capacity (0.36 vs. 11.70 g/g), emulsifying stability (32.24% vs. 43.08%), foaming capacity (20.8% vs. 100.0%) and foam stability (3.80 vs. 19.20 ml). PKCP contained the highest amount of glutamic acid (16.86 g/100 g protein) and followed by arginine (10.78 g/100 g protein). With respect to the 1991 standard of the FAO/WHO for preschool children, PKCP's essential amino acid profile showed deficiencies. Therefore, it can be used as a complementary protein source by supplementing with a tryptophan-rich source, as this was the limiting amino acid.