Structural characterization and performance analysis of flaxseed isolate protein

This study aims to realize the full processing and utilization of isolated protein from flaxseed meal. Flaxseed isolate protein (FIP), and degummed and defatted flaxseed isolate protein (DD-FIP) were prepared from the cold-pressed flaxseed meal (FM) using ultrasound-assisted aqueous extraction. A comparison was also made on their physicochemical (purity, extraction rate, molecular weight, and amino acids), structural (Fourier Transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), X-ray diffraction (XRD)) and functional properties (amphiphilicity, solubility, foaming ability, foam stability, emulsification activity and emulsion stability). The results showed that the protein mass fractions in the FM, and degummed and defatted flaxseed meal (DD-FM) were 37.52% ± 0.04% and 37.47% ± 0.02%, respectively. The purity values of FIPand DD-FIP after ultrasound-assisted aqueous extraction were 74.88% ± 0.02% and 89.85% ± 0.16%, respectively, with the extraction rates of 50.20% ± 0.12% and 41.68% ± 0.08%, respectively. There were the relatively high purity and extraction rate of the two proteins. The reason was probably because the FM was better retained a large amount of protein. The purity of FIP was significantly lower than that of DD-FIP, probably because the residual oil in FM and flaxseed gum were interfered with the protein extraction. There was also a residual oil amount of (9.53±0.39) g/100g in FM, and the extracted FIP contained (1.28±0.21) g/100g of oil. There was the irremovable flaxseed gum, leading to reduce the sedimentation of the protein separation. But there was a large effect on the purity of the isolated protein. The degumming and degreasing treatment of FM increased the protein purity by about 15% before protein extraction. The molecular weights of FIP and DD-FIP were determined to be 10-55 kDa, with the water-soluble low molecular weight albumin (10, 14, 15 and 17 kDa), and salt-soluble high molecular weight globulin (30, 33, 35, 40, 45 and 55 kDa), indicating the most outstanding protein bands. 17 amino acids were then identified in each of them, including the abundant essential and non-essential amino acids. Both FIP and DD-FIP were contained all the essential amino acids (EAA) (30.35%, 32.60%) and non-essential amino acids (NEAA) (69.70%, 67.40%), similar to the soybean isolate protein (SIP) (37.13%, 67.55%). FIP and DD-FIP were rich in the sulfur-containing amino acids, such as methionine and cystine, and branched-chain amino acids, such as isoleucine, in addition to abundant acidic amino acids (aspartic acid and glutamic acid) and basic amino acids (lysine, arginine and histidine). The composition and content of EAA in both FIP and DD-FIP were higher than those recommended by FAO/WHO for children and adults. The secondary structures of FIP and DD-FIP were similar to the looser structure and average stability after FTIR spectroscopy; SEM observed that the FIP share the lower microscopic porosity than DD-FIP. The XRD diffraction intensities and peak shapes showed that there was less crystallinity or orderly arrangement in the structures of both extracted FIP and DD-FIP, where the crystallinity of DD-FIP was lower than that of FIP. The amphiphilicity, hydrophilicity and the lipophilicity of two proteins were better than those of soybean protein isolate. Both FIP and DD-FIP presented the excellent alkali solubility at different pH (2~11) and salt ion concentration (0~1.25 mol/L). The foaming ability, emulsification activity and emulsion stability of DD-FIP were better than those of FIP, while the foam stability was the opposite. Therefore, the two isolated proteins can be expected to apply into the field of health food. The finding can provide the strong references and data support to the application in food industry.