Response Surface Optimization of Enzymatic Hydrolysis of Peptides of Chinese Pecan (Carya cathayensis) and Analysis of Their Antioxidant Capacities and Structures

Abstract

Pecan cake, a by-product of pecan processing, has not been fully developed and utilized. It contains proteins with high nutritional value that can be used as a potential source of bioactive peptides. In this study, the compound protease from the four common proteases was selected as the best protease, and the best hydrolysis was determined via response surface optimization. The optimal hydrolysis conditions were as follows: 55 ℃, pH 7.7, 3900 U/g protease concentration, and 2 h. Under these conditions, peptide yield and total antioxidant capacity (TAC) were 34.1 % ± 0.2 % and 1.1 ± 0.1 mmol/g, respectively. The peptides showed desirable 1,1-diphenyl-2-picrylhydrazyl (DPPH) (IC50: 0.2 mg/mL) and 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) (IC50: 0.4 mg/mL) radical scavenging capacities, which were found to be related to their amino acid compositions, molecular weights, and structures. The molecular weight of the peptides obtained from the pecan cake protein hydrolyzed by the compound protease was < 3 KDa, with their contents reaching 94.0 % ± 0.4 %. The yield of protein hydrolysate from pecan cake was about 7.2 %. HPLC analysis revealed that the content of hydrophobic amino acids in the peptides was relatively high. Fourier transform infrared spectroscopy and X-ray diffraction analysis showed that hydrolysis gradually changed the structure of the proteins from ordered to disordered, thereby exposing more antioxidant groups. α-helix and β-turn angles decreased from 29.5 % and 24.9–20.3 % and 15.2 %, respectively. β-sheet, β-antiparallel, and random coil increased from 24.2 %, 7.7 %, and 13.7–33.5 %, 9.3 %, and 21.8 %, respectively.