Fractionation, physicochemical properties, nutritional value, antioxidant activity and ACE inhibition of palm kernel expeller protein

Five protein fractions (albumin, globulin, prolamines, glutelin-1 and glutelin-2) from palm kernel expeller were fractionated and characterized. Their nutritional value, physicochemical properties, antioxidant activity and angiotensin I-converting enzyme (ACE) inhibition were also investigated. Results showed that the globulin and glutelin-2 were the predominant fractions. The albumin and globulin were rich in arginine, and the glutelin-2 had a high content of sulfur amino acids and showed good nutritional quality. In both the native and reduced states, albumin and glutelin-1 consisted of three polypeptides (18.8, 22.1 and 51.5 kDa) and five peptides (20.7 to 51.5 kDa), respectively. The globulin and glutelin-2 were composed of polypeptides linked via one or more disulfide bonds. Albumin, prolamines and the total palm kernel expeller protein (TPKP) had endothermic peaks at 53.8, 126.5 and 102.3 °C respectively. All the fractions except globulin exhibited a good emulsifying capacity (137.87–269.54 m2 g−1) and emulsion stability (75.13–89.71%) but poor foaming capacity (≤47.56 g/100 g). Among the fractions, prolamines showed the highest inhibiting ability of lipid autoxidation, ACE inhibition (13.63 ± 2.18%) and free radical-scavenging activity on DPPH (79.45 ± 6.80%), hydroxyl (93.40 ± 1.46%) and ABTS (79.35 ± 1.99%). Overall, these palm kernel expeller protein fractions could be used in the food or other industries.