Effects Of Aging On Structures And Properties Of Proteins In Rice During Storage

Aging deterioration of rice is common during storage,which causes a decline in edible and nutritional quality and great economic loss.In order to delay the aging deterioration of rice due to storage,elucidating the aging mechanism is a key way to put forward the methods to delay or prevent the aging.During rice aging,the change of protein is closely related to the deterioration of rice quality.It is necessary to elucidate the mechanism of rice aging through studying the changes of protein structure and properties during storage.At present,the research on protein changes in rice aging has focused on the molecule weight and property changes in surface hydrophobicity of the protein and the content of sulfhydryl and disulfide bonds.However,there are few studies on advanced protein structure and oxidation of other groups,and the majority of the raw materials used are the total rice protein.The outer layer of rice grains is rich in protein,and it is exposed to air during storage,so the outer layer of rice grains is a good test material for studying the changes in rice protein.In this project,the outer layer of rice grains obtained by abrasive milling(grinding degree 15%)was used as the test material.The Osborne method was used to extract proteins from fresh rice and aged rice one by one according to respective solubility.The proteins extracted from fresh rice were stored separately under simulated aging conditions.The structures of fresh rice proteins,aged rice proteins and aged fresh rice proteins were characterized to find out the changes caused by rice aging,and to provide a basis for elucidate the mechanism of rice aging.The main results are as follows:(1)The effect of rice aging on protein structureRaman and infrared spectroscopy were used to study the structural changes of fresh rice glutelin,aged rice glutelin and fresh rice glutelin-aged.Raman spectroscopy showed that the normalized Raman intensities of aged rice glutelin at 1665 and 1218 cm-1 were significantly lower than fresh rice glutelin,indicating a decreased ?-helix in glutelin after rice aging.The disulfide bonds(the peak intensities at 516 and 527 cm-1),sulfoxides(the peak intensity at 1035 cm-1)and sulfones(the peak intensities at 1124,1152,1159,1316 and 1334 cm-1)of the aged rice glutelin were significantly higher than those of the fresh rice glutelin,indicating the obvious oxidation of sulfur-containing amino acid residues.The intensity ratio of Fermi resonance at 857/830 cm-1 of tyrosine in aged rice glutelin was obviously larger than fresh rice glutelin,indicating more exposed tyrosine residues in glutelin after rice aging.The Raman intensity of the tryptophan indole ring near 751 cm-1 of aged rice glutelin was significantly higher than fresh rice glutelin,indicating more buried tryptophan residues after rice aging.The O-H stretching strength of the aged rice glutelin at 3423 cm-1 was significantly higher than that of the fresh rice glutelin,indicates that the degree of intermolecular bonding was increased,and the association between glutelin and starch strengthened.Except the peak intensities of tyrosine Fermi resonance and sulfone at 1333 and 1152 cm-1 were not higher,the Raman intensities of fresh rice glutelin-aged at other peaks were higher than those of aged rice glutelin,which indicates that the oxidation degree of fresh rice glutelin-aged is higher.Infrared spectroscopy showed that the absorption peaks of sulfur oxides at 1153,1078 and 1026 cm-1 in aged rice glutelin and fresh rice glutelin-aged increased,which further supported the oxidation of glutelin after rice aging.(2)The effect of rice aging on protein morphologyFluorescence microscope and scanning electron microscope were used to characterize the morphology of protein and protein body before and after rice aging.Fluorescence microscopy showed that the fluorescence intensity of aged rice protein and fresh rice protein-aged was higher than that of fresh rice protein,indicating that the protein was oxidized seriously after rice aging,so it was more concentrated in a local area.Scanning electron microscopy showed that the fresh rice protein is relatively dispersed,low aggregation degree,high particle integrity,and the edges are close to spherical.The aged rice protein was mostly aggregated with a high degree of aggregation.(3)The effect of rice aging on the adsorption and binding capacity between protein and starchThe adsorption and binding constants of albumin,globulin,glutelin and prolamin extracted from fresh rice and aged rice,respectively,and amylose,amylopectin and deproteinized crude starch were determined.The results showed that the adsorption and binding constants of the four protein isolates basically showed a decreasing trend,indicating that the protein-starch adsorption and binding capacity was weakened after rice aging;the amount of albumin and glutelin per unit starch bound decreased after rice aging,while per unit amylopectin the binding globulin,per unit amylopectin and starch residue binding prolamin have increased.(4)The effect of rice aging on functional properties of glutelin and their improvement of enzymatic hydrolysisCompared with the functional properties of fresh rice glutelin,the solubility,water holding capacity,emulsifying properties and emulsifying stability of aged rice glutelin were significantly reduced,while oil holding capacity increased,which supported the obvious oxidation of aged rice glutelin.The solubility(except for pH 9),water holding capacity and emulsifying properties of fresh rice glutelin-aged were lower than those of aged rice glutelin,and its oil holding capacity were higher,which indicated that glutelin had a higher degree of oxidation when it was extracted from fresh rice and aged alone.After the aged rice glutelin and fresh rice glutelin-aged are treated with amylase and protease,except for the foaming stability,other functional properties are significantly improved.Therefore,amylase and protease can effectively improve the functional properties of rice glutelin after rice aging,suggesting that the binding of protein and starch after rice aging is enhanced.In summary,the protein structure and properties in the outer layer of rice grains changed significant,which resulted in the changes of functional properties of protein.The changes in functional properties supported the results of structural changes,which indicated that obvious oxidation of proteins occurred during rice aging,it might be an important reason for protein affecting starch gelatinization after rice aging and deterioration of rice quality.The results provided a basis for revealing the mechanism of protein affecting starch gelatinization during rice aging,and laid a foundation for controlling rice aging to reducing postpartum grain loss.