Changes Of Protein And Its Correlation With Metal Elements In Seeds Of Camellia Oleifera Based On Different Oil Extraction Methods

The camellia seed cake(Camellia oleifera Abel.)as a high-quality plant protein resource with a balanced amino acid ratio has been concerned in the product development of multiple foods and feeds.The existence of heat effects,pressure,and shear forces in the oil extraction methods would change the physicochemical properties of the protein,while the protein was very susceptible to metal ions in the denaturation process,and then affecting the comprehensive development and utilization of oleifera seed meal protein.This study aimed to assess the effects of oil extraction methods on the structural changes,physicochemical properties,emulsifying properties,and antioxidant activities of protein isolates derived from the camellia seed cake,including hydraulic pressing cake(HPC),twin-screw pressing cake(TPC),and organic solvent extraction cake(OEC).Finally,the effects of metal elements on protein are discussed in combination with the migration characteristic of metal elements in camellia seed cakes.This study could help develop the application of protein of camellia seed cake more specific and targeted,provide basic data for the study and,be an important reference for the development of the camellia seed.The main results are as follows:1.Extrusion and heat effects during oil preparation resulted in protein denaturation and aggregation,and extrusion enhanced the antioxidant activity of the protein.The total protein content of HPC,TPC,and OEC was 103.87 ± 1.30,97.65 ±5.36,and 100.80±5.62 mg/g.The protein composition of camellia seed cake was albumin(ALB)and gluten(GLU),accounting for 36～39%and 54～58%of the total protein content.The least protein extraction rate was found in TPC.The constituent amino acids of protein are glutamic acid,arginine,and PROartic acid,and the strong oxidation property of the proteins in the camellia seed cake may be affected by the distribution of antioxidant amino acids in it,and the aino acid composition is also related to the particle size and emulsification stability.It was found by the surface activity results that the increased surface tension would decrease the stabilization of the water/oil interface,thus reducing its emulsification stability.The proteins of camellia seed cake showed excellent antioxidant activity,proving that extrusion can enhance the antioxidant capacity of proteins.2.Combined with the scanning electron microscopy(SEM)results,it was found that the protein of HPC and TPC exhibited broken,and rough surface morphologies,due to the denaturation and aggregation of proteins caused by extrusion and heat effects during the oil extraction methods.The results of Fourier spectroscopy showed that β-sheet and random coils are the two major structures of ALB,globin(GLB),and gliadin(PRO),with high β-sheet content,being the reason for the large particle size of PRO.While the primary structure of GLU is α-helix and β-turn.Extrusion enhanced C-O stretching vibrations of protein and increased the breakage of disulfide bonds.The particle size and dispersity index(PDI)of isolate protein of TPC were lower than those in the other cake,the particle size of PRO was larger than that of other proteins because of its higher β-sheet content reached more than 70%.The surface tension of the protein solution would be affected by the charge in the solution,thus reducing its emulsification stability.The results of SDS-PAGE showed that the molecular weight of isolate protein in camellia seed cake was distributed at 15～25,and 35～55 kDa.The isolate proteins of TPC appeared to form new bands,some bands disappeared and some bands deepened in color.3.The contents of 12 metal elements in camellia seed cake were determined,combined with the changes in protein content and structure.Camellia seed cake was rich in metals,with the highest contents of K,Ca,and Mg elements reaching 9 431,1589,and 1 517 μg/g;The content of heavy metal elements is less,the highest content of heavy metal elements is Ni,at 3.14 μg/g with the content order Ni>Cr>As>Cd.The metallic elements in the camellia oleifera seeds were mainly transferred to the pressed seed cake,and the number of metallic elements in the oil migrated less.The coordination of Cd to the protein lead to a turn-sheet transition in the secondary structure of the subunit,the content of Cu may affect the α-helices and β-sheet.Finally,the correlation between metal elements and protein contents showed that the concentration of Fe was significantly correlated with the content of GLB(p<0.05),the concentration of Cr was significantly correlated with the content of GLU(p<0.05)of the camellia seed cake.The concentration of Cr was significantly correlated with the emulsifying property of GLU(p<0.05),the concentration of Cd was significantly correlated with the emulsifying property of ALB(p<0.05)of the camellia seed cake.This study found that extrusion and heat effects in hydraulic pressing and twinscrew pressing were found to have significant effects on the composition,molecular weight,and structure of the protein in camellia seed cake,while the amino acid composition and secondary structure of the protein are related to the particle size,emulsifying properties,and metal elements such as Cd and Cu.Extrusion enhanced the antioxidant activities of proteins.The significant positive correlation was found between Fe and GLB,and Cr and GLU,which may provide new ideas for improving the content of Fe and reducing health risks of heavy metal in the applications of the plant protein.