© All Rights Reserved *Corresponding author. Email: veselachalova@gmail.com Tel: 0395 32 603 855 International Food Research Journal 20(6): 2995-3000 (2013) Journal homepage: http://www.ifrj.upm.edu.my Ivanova, P., * Chalova, V., Koleva, L. and Pishtiyski, I. University of Food Technologies, Department of Biochemistry and Molecular Biology, Plovdiv-4002, Bulgaria Amino acid composition and solubility of proteins isolated from sunfower meal produced in Bulgaria Abstract Two protein isolates (PI1 and PI2) were prepared from sunfower 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 frst 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 defcient in these specifc amino acids. Introduction Sunfower meal is a by-product obtained after oil extraction from sunfower seeds. It contains relatively high amount of protein (30-50%) which may reach 66% depending on the effciency of seed dehulling and defatting processes (Bau et al., 1983; Dorrell and Vick, 1997). Sunfower proteins are characterized with high nutritive value. They do not contain anti- nutritional compounds and exhibit well-balanced amino acid composition with the except for lysine which is the frst limiting amino acid (Gassmann, 1983; González-Pérez and Vereijken, 2007). Sunfower proteins are rich in sulfur-containing amino acids which, in general, are defcient in most proteins with plant origin (Gassman, 1983; Ribarova et al., 1987; Canibe et al., 1999). Sunfower seeds contain 2 major protein groups, namely globulins and albumins. Sunfower albumins have low molecular weight and high solubility, which do not depend on pH and ionic strength of the solutions. The globulins, known as helianthinin, consist of oligomers with molecular weight from 300 to 350 kDa (11S globulins) which may dissociate to either oligomers with lower molecular weight (7S) or monomers (2S-3S). The dissociation and association of the helianthinin depends on pH, temperature and ionic strength of the solution (Sripad and Narasinga Rao, 1987; González-Pérez et al., 2004). At low ionic strength, water solubility of the globulins is at minimum at their isoelectric points ranging from pH 4.0 to pH 6.0. At high ionic strength of the solution, the helianthinin is almost insoluble in acidic medium (Canella et al., 1985; Rossi et al., 1985; Vermeesch et al., 1987; González-Pérez et al., 2004, 2008). The amino acid composition and solubility of the proteins in the sunfower meal, remaining after oil extraction, may vary and depend on the pre-treatment of the sunfower seeds and the procedure used for the oil production. Elevated temperature, which is frequently used to increase oil yield, favors protein denaturation and the formation of insoluble protein fraction (Lusas, 1985). In addition, it facilitates the protein interaction with other plant components leading to dark colored products which worsen the functional properties of the proteins (Parrado et al., 1993; Moure et al., 2006; González-Pérez and Vereijken, 2008; Salgado et al., 2011). Most of the studies related to sunfower protein isolates have been performed after defattining of the seeds under mild laboratory conditions where no change of the native structure and functionality of the proteins occurred. However, the industrial procedure of oil extraction from sunfower seeds may affect the amino acid composition, solubility and functional properties of the respective proteins. The protein isolates from industrially obtained sunfower meal, however, are of a higher practical interest since it is an alternative approach for a better and more complete use of this by-product for food application. For the Keywords Sunfower protein isolates Amino acids Solubility Article history Received: 11 June 2013 Received in revised form: 6 July 2013 Accepted: 12 July 2013