Comparison of structural and functional properties between two protein isolates from flax (Linum usitatissimum L.) seeds

This study aimed to realize the full processing and utilization of isolated protein from flaxseed meal. Flaxseed protein isolate and degummed and defatted flaxseed protein isolate (DD-FPI) were prepared from the cold-pressed flaxseed meal (FM) using ultrasound-assisted aqueous extraction. A comparison was also made on their physicochemical, structural and functional properties. The results showed that the protein mass fractions in the FM, and degummed and defatted flaxseed meal were (37.52 +/- 0.04) % and (37.47 +/- 0.02) %, respectively. The degumming and degreasing treatment of FM increased the protein purity by about 15% before protein extraction. The molecular weights of FPI and DD-FPI were determined to be 10-55 kDa, indicating the most outstanding protein bands. A total of 17 amino acids were identified in each of them, including the abundant essential and non-essential amino acids. FTIR spectroscopy showed that the secondary structures of FPI and DD-FPI were similarly the looser structure, withaverage stability. SEM observed that the FPI exhibited the lower microscopic porosity than DD-FPI. The XRD diffraction intensities and peak shapes showed that there was less crystallinity or orderly arrangement in the structures of both FPI and DD-FPI, where the crystallinity of DD-FPI was lower than that of FPI. The amphiphilicity, hydrophilicity and the lipophilicity of the two proteins were better than those of soybean protein isolate. Both FPI and DD-FPI presented the excellent alkali solubility at different pH and salt ion concentrations. The foaming ability, emulsification activity and emulsion stability of DD-FPI were better than those of FPI, while the foam stability was the opposite. Therefore, the two isolated proteins can be expected to apply into the field of functional food.