| Akebia trifoliata var. australis seed, the co-product of Akebia trifoliata var. australis fruit used for both food and medicine, contains good-quality extractable proteins and oils. Akebia trifoliata var. australis seed protein isolate and its fractions were prepared using the methods of alkali extraction and acid precipitation and Osborne from the seeds. The physicochemical, structual and functional properties of prepared proteins were investigated in order to study the basis properties and the mechanism of antioxidant properties as influenced by enzymatic hydrolysis and provide a theoretical basis for industrial processing, as a result, to be used to meet the need of industrialization.As acidic amino acids, glutamic acid and aspartic acid were the most abundant amino acids found in the protein isolate (API), consistent with most storage proteins. Very similar amino acid patterns were found in albumin, globulin, glutelin fractions and API. With respect to nutritional parameter, threonine was found as the first limiting amino acids for all proteins prepared from the A. trifoliate var. australis seed. From the analysis of SDS-PAGE, albumin and glutelin displayed8and2distinct subunits, respectively. API, with molecular weights ranging from10.0to55.0kDa, contains the bands of both albumin and glutelin. Accounting for31.2%and36.6%, β-strand and random coil were found to be the major secondary structures in A. trifoliata var. australis seed protein. A similar secondary composition was shown in API and glutelin. API showed the highest contents of total and exposed sulphydryl in the three proteins. The amount of disulphide bonds was the highest in glutelin, which indicated that glutelin would be more stable under thermal treatment in food processing. All the protein samples showed two characteristic X-ray diffraction peaks around8°(small intensity) and20°(high intensity). With respect to the functional properties, albumin was characterised by a higher solubility, surface hydrophobicity index, foaming and emulsifying capacity when compared with API and glutelin.API was limited enzymatic hydrolysis by Alcalase with the conditions as follow:temperature55℃, E/S2%and pH8.0. The physicochemical, functional and antioxidant properties of the resulting hydrolysates as a function of different reaction time were investigated. The total content of hydrophobic amino acids and several amino acids involved in antioxidative activity of hydrolysates increased. The same amino acid compositions were observed in the hydrolysates with different degree of hydrolysis (DH). The thermal transition of API and its hydrolysates was investigated by DSC, the thermal denaturation temperature of major endothermic peak significantly increased from92.4℃to97.6-98.4℃when DH increased from6.8%to8.9%or12.8%, while the enthalpy change significantly decreased. After enzymatic hydrolysis for10min, the solubility, emulsion capacity and foaming capacity of the resulting hydrolysate increased by2.04、4.84and0.63times, respectively. These functional properties decreased as the reaction time increased from10min, however, the level was still higher than those of API.The antioxidant activity increased with increasing the concentration of peptides and the highest antioxidant activity at all tested concentrations were observed in the hydrolysate with DH12.8%in the system of DPPH radical scavenging. At the concentration of5mg/mL, its radical scavenging activity was76.14%, which was close to that of ascorbic acid. The reduce power of all hydrolysates increased first and then decreased slightly as the DH increased at the same concentration. Both API and its hydrolysates suppressed the discoloration of β-carotene at different levels and showed increase in antioxidant activity index (AAI) with increasing protein or peptides concentration. Consistent with the DPPH radical scavenging, the hydrolysate with DH12.8%showed the highest AAI at the same concentration. |
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