| Walnut(Juglans Regia L.)is one of the four major nuts in the world.Xinjiang walnut with thin shell and large fruit and rich nutrients is very popular among consumers.However,the thin and fragile walnut shell made it hard for storage,especially rich in fat and protein,which makes it susceptible to various factors in the storage and processing process,resulting in oxidation and deterioration.Therefore,it is very important to explore the changes of walnut quality during storage.This experiment is based on the storage conditions of walnut in the actual production and processing,select room temperature and cold storage as storage conditions,using three kinds of packaging materials for walnut kernel packaging,and natural storage with shell walnut as a control.The protein quality of four kinds of walnut samples stored at room temperature and cold storage for 0,1,3,6 and 9 months were investigated.The effects of storage conditions on the quality of walnut protein were explored through the determination of physical and chemical indexes and the characterization of structure.Finally,the digestion of walnut protein in different storage periods and the antioxidant activity of digestive products were explored through in vitro simulation digestion.In addition,three different packages of walnut kernel were separately accelerated storage under high temperature and high humidity conditions,in order to quickly obtain the protein oxidation situation and the impact of protein oxidation on amino acid content,protein structure and digestibility,and explore the correlation between each index.The main research results are as follows:(1)During storage,temperature and vacuum degree have significant influence on the oxidation of walnut protein,room temperature > cold storage,PE > CK > PA/PE > PET/AL/PE.With the increase of storage time,the values of carbonyl group,dimertyrosine,turbidity and free amino group increased significantly,while the free sulfhydryl group and solubility decreased,indicating that oxidative aggregation occurred during storage.The values of carbonyl group,dimertyrosine and turbidity changed significantly after 6 months of storage(P < 0.05),indicating that the protein oxidation degree of walnut increased after 6months of storage.(2)With the increase of storage time,the secondary structure of walnut protein becomes disordered and loose,the proportion of α-helix and β-sheet decreases,and the proportion of β-turn and random curling increases.Protein aggregates formed during storage conceal tryptophan residues,resulting in reduced fluorescence intensity,and exposed hydrophobic groups interact with each other,reducing surface hydrophobicity.At the 6th month of storage,high molecular weight protein aggregates appeared on the interface of separation gel and accumulation gel of protein electrophoresis bands of walnut at room temperature and cold storage,indicating that the oxidation degree of walnut protein was significant at the 6th month of storage,and the new high molecular weight peak appeared in the chromatogram at the later stage of storage also indicated this.(3)Simulation of gastrointestinal digestion showed that intestinal digestion could significantly improve the digestibility of walnut protein,but the digestibility decreased with the increase of storage time.The molecular weight distribution of peptides showed that after 2 h of gastric digestion,90% of gastric digestive juices contained small molecular peptides less than 3000 Da,mainly in the range of 100-1000 Da.After intestinal digestion for 3 h,no peptide larger than 3000 Da was detected,and the digestive juices were mainly distributed in 100-500 Da.With the increase of storage time,the large molecular weight peptide gradually increased.The gastrointestinal digestive products of walnut protein showed high antioxidant activity,and the scavenging ability of ABTS·+ was the highest,followed by DPPH and ·OH free radicals.In addition to DPPH free radicals,intestinal digestion significantly increased antioxidant activity.However,during storage,the antioxidant capacity decreased,and the overall cold storage and PET/AL/PE vacuum packaging decreased slowly.(4)The accelerated experiment showed that with the increase of storage time,the contents of amino acids decreased,the contents of carbonyl group and dipoly tyrosine increased significantly,and the secondary structure became disordered and loose.SDS-PAGE showed that after 15 days of storage,the protein oxidation degree deepened to form macromolecular aggregates.Correlation analysis showed that the degradation of amino acids,formation of protein aggregates and structural changes during storage resulted in the covering of enzyme action sites,which reduced the digestibility of walnut protein.Vacuum packaging can slow down the protein oxidation rate,PET/AL/PE vacuum effect is more significant. |
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