Amino acid composition and solubility of proteins isolated from sunflower meal produced in Bulgaria

Two protein isolates (PI1 and PI2) were prepared from sunflower meal produced in Bulgaria. They were obtained by using isoelectric or ethanol precipitation following the extraction of the proteins with 10% NaCl. Their amino acid composition and water solubility as a function of pH and presence of NaCl were investigated. Lysine was established as the first limiting amino acid in the protein isolates with amino acid scores of 35.81% (PI1) and 43.09% (PI2). Both protein isolates contained relatively high amounts of sulfur-containing amino acids with amino acid scores of 99.14%. Arginine reached 8.45 and 9.75 g/100 g protein in PI2 and PI1 respectively. Being obtained by isoelectric precipitation, PI1 was highly soluble in either acidic (pH ≤ 3.5) or alkaline (pH ≥ 7) medium. In contrast, PI2 was insoluble for each pH value below 5.5. The addition of 0.03 M NaCl did not change the solubility pattern of either PI1 or PI2. However, the presence of 0.25 M NaCl diminished the solubility of the protein isolates for the entire pH area from 2 to 8.5. PI1 and PI2 may serve as a valuable source of sulfur-containing amino acids and arginine to complement human diets deficient in these specific amino acids.