Rheological characteristics and physicochemical stability of dressing-type emulsions made of oil bodies–egg yolk blends Constantinos V. Nikiforidis a , Costas G. Biliaderis b , Vassilios Kiosseoglou a,⇑ a Laboratory of Food Chemistry and Technology, Department of Chemistry, Aristotle University, Thessaloniki GR-54124, Greece b Laboratory of Food Chemistry and Biochemistry, Faculty of Agriculture, Aristotle University, P.O. Box 235, Thessaloniki GR-54124, Greece article info Article history: Received 7 April 2011 Received in revised form 4 January 2012 Accepted 9 February 2012 Available online 18 February 2012 Keywords: Salad dressings Emulsions Oil bodies Maize germ Rheology Creaming Coalescence abstract Two oil body creams, differing in oil volume fraction and surface protein composition, were obtained by applying alkaline aqueous extraction to comminuted maize germ and then recovering the oil bodies from the extract by centrifugation, either in the presence of 0.5 M sucrose (OB-W) or following isoelectric precipitation at pH 5.0 (OB-A). Oil bodies in the former cream are stabilised by natural oil body surface proteins while in the OB-A cream, exogenous proteins, in addition to natural oil body proteins, are also present. The creams were blended with appropriate amounts of water, NaCl and liquid yolk, and the pH was adjusted to 3.8 to obtain 20% or 45% (w/w) in oil OB-W and 20% (w/w) in oil OB-A model dress- ing-type emulsions. The physical stability of the emulsions, against creaming and coalescence, was mon- itored upon storage, while the development of emulsion structure during ageing was probed by applying steady shear and small deformation oscillatory rheometry. The adsorbed to oil bodies’ surface proteins were analysed by applying SDS–PAGE. Since no yolk protein constituents were detected at the oil body surface layer of the emulsions, it is hypothesised that the presence of unadsorbed yolk particles in the emulsion continuous phase results in the intensification of interdroplet interaction effects, due to deple- tion events, and may therefore have an indirect but nevertheless strong influence on emulsion structure and physical stability. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Oil bodies are small-sized organelles where lipids are found in many oil-rich plant raw materials, including maize germ (Tzen & Huang, 1992). One common feature of all oil bodies is the presence, at their surface, of a mixed phospholipids-protein membrane that confers physical and chemical protection to the oil core against environmental stresses, such as moisture and temperature fluctu- ations (Shimada & Hara-Nishimura, 2010; Shimada, Shimada, Takahashi, Fukao, & Hara-Nishimura, 2008). The oil core is mainly composed of TAG and a small amount (<0.1%) of free fatty acids (Tzen, Cao, Laurent, Ratnayake, & Huang, 1993). The fatty acid pro- file of maize germ oil bodies depends on the variety, with linoleic acid (49.7–62.7%) being the predominant fatty acid component, followed by oleic (23.5–34.9%) and palmitic (9.5–11.5%) acids (Saoussem, Sadok, Habib, & Mayer, 2009). The dominant proteins of the oil body surface are called oleosins (Chen, Lin, Huang, & Tzen, 1997; Jolivet et al., 2004; Simkin et al., 2006; Tzen & Huang, 1992). These are very hydrophobic proteins of a relatively low molecular mass (15–20 kDa). Other minor groups of proteins pres- ent at the oil body surface are the caleosins (27 kDa) and steroleo- sins (40 kDa) which also possess a hydrophobic central domain, but with fewer residues (Lin, Liao, Yang, & Tzen, 2005; Purkrtova, Jolivet, Miquel, & Chardot, 2008). Traditionally, maize germ oil is used for food preparation after extraction with an organic solvent and refinement. This process is characterised by high extraction efficiency and low cost. However, the use of volatile organic solvents very often results in environmental pollution, while safety problems, because of organic solvent inflammability, also constitute a matter of concern. Moreover, the organic solvent extraction processes may have a detrimental impact on protein functionality (Moure, Sineiro, Dominguez, & Parajo, 2006; Rosenthal, Pyle, & Niranjan, 1998). It could, therefore, be more advantageous to apply aqueous extrac- tion to oil seeds in order to recover the oil, as an aqueous disper- sion of oil bodies, as suggested by a number of researchers (Iwanaga et al., 2007; Kapchie, Wei, Hauck, & Murphy, 2008; Nik- iforidis & Kiosseoglou, 2009; Tzen et al., 1993; White et al., 2008). Application of aqueous extraction to oil seeds results in a natural emulsion of oil bodies dispersed in water and, as such, it may be exploited in this form as a pre-emulsified oil food ingredient. For example, these oil bodies could be used in the preparation of liquid or semi-liquid food products, appearing in the form of oil-in-water emulsions, such as mayonnaise, salad dressings, creams, imitation milk and cream liqueurs. The use of the very small-sized (<1.0 lm) 0308-8146/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2012.02.058 ⇑ Corresponding author. Tel.: +30 231 0 997834; fax: +30 231 0 997779. E-mail address: kiosse@chem.auth.gr (V. Kiosseoglou). Food Chemistry 134 (2012) 64–73 Contents lists available at SciVerse ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem