Improved emulsifying properties of soy proteins by acylation with saturated fatty acids Athanasia O. Matemu a , Hisataka Kayahara b , Hisashi Murasawa b , Shigeru Katayama a , Soichiro Nakamura a, * a Department of Bioscience and Biotechnology, Shinshu University, 8304 Minamiminowamura, Ina, Nagano 399-4598, Japan b Food Research Laboratory, Asahimatsu Foods Co. Ltd., Japan article info Article history: Received 2 November 2009 Received in revised form 6 May 2010 Accepted 22 June 2010 Keywords: Soy proteins Saturated fatty acids Acylation Emulsifying properties Oil binding capacity Surface hydrophobicity abstract Effects of acylation on emulsifying properties of soy proteins were investigated using a variety of satu- rated fatty acids. Beta conglycinin (7S), glycinin (11S), and acid-precipitated protein (APP) were acylated with activated fatty acid esters (6C–18C) to form covalent linkage between the carboxyl group of the fatty acid and the free amino groups of the protein. Reduction in the free amino groups of acylated 7S, 11S and APP resulted into the dissociation of the protein, indicating a structural change, as evidenced by the fluo- rescence spectra and the degree of modification. It was shown that the emulsifying activity (EAI) and emulsion stability (ES) of 7S and 11S were significantly improved (p < 0.05) upon acylation with all sat- urated fatty acids, whereas no change in EAI and ES for the acylated APP was observed upon attachment of short and long chain fatty acids. The fluorescence intensity was also remarkably affected by acylation showing significant changes in protein structure. Covalent attachment of fatty acids resulted into 1.4–2.2 and 1.1–1.8-fold increase in the oil binding capacity (OBC) of 7S and 11S respectively, however no changes in acylated APP. Acylated 7S showed 3.0–9.4-fold increase in the water binding capacity (WBC), with no change in acylated 11S, while acylated APP with longest chains showed low WBC. The surface hydrophobicity of 7S was significantly improved (p < 0.05) by acylation; no changes were observed in the acylated 11S. Furthermore, acylation decreased the surface hydrophobicity of APP. Thus, it was demonstrated that saturated fatty acids with adequate chain length are suitable candidates for the preparation of functional lipoproteins from soy proteins. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Soybean protein has been used as a source of nutrients in Asian countries for many decades. Soybean protein is well known for its nutritional value, different functional properties and also as a po- tential source of bioactive peptides. However, soy globulins (7S, 11S and APP) have poor surface functionality, which makes it dif- ficult to be directly utilised in functional food processing. Proteins exist in a compact folded structure with buried hydrophobic groups and exposed hydrophilic groups. The hydrophobic groups are necessary for the surface properties of protein. The large vari- ety of hydrophobic groups that might be incorporated into the protein molecules opens a very wide range of possibilities for obtaining improved surface activity (Magdassi & Toledano, 1996). For the past few decades, different modification techniques have been used to alter the protein structure, changing its physical and chemical properties, hence influencing its structure–functional properties. Acylation has been used among other chemical meth- ods for protein modification. Acylation of the free amino groups on a polypeptide chain involves the utilisation of reagents that re- act covalently at the amino sites of the proteins. The mechanism, depending on the nature of the modifying agent, can affect the charge balance of the molecule in three different ways, namely, by preserving the positive charge on the amino groups, by abolish- ing the charge and bringing it to neutrality, or by imposing a neg- ative charge as a substitute for the original positive charge (Lakkis & Villota, 1992). The need for multiple functional food products or ingredients has increased the pressure to food industries and researchers to develop different modification techniques to enhance and diversify the protein functionalities, such as emulsifying, water and oil binding. Attachment of hydrocarbon chains may modify the charge and structural properties of soy proteins, increasing their hydro- phobicity, thus improving their surface functionality. The acylation of the e-amino groups of the lysine residues, particularly with suc- cinyl groups, markedly enhances several properties of proteins, including their emulsifying properties (Aoki, Taneyama, Orimo, & Kitagawa, 1981; Malabat, Sanchez-Vioque, Rabiller, & Gueguen, 2001; Mirmoghtadaie, Kadivar, & Shahedi, 2009; Wong, Nakamura, & Kitts, 2006); water and oil binding (Lawal & Adebowale, 2004; Mirmoghtadaie et al., 2009) and surface hydrophobicity 0308-8146/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2010.06.081 * Corresponding author. Tel./fax: +81 265 77 1609. E-mail address: snakamu@shinshu-u.ac.jp (S. Nakamura). Food Chemistry 124 (2011) 596–602 Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem