Oil-in-water emulsions stabilized by chitin nanocrystal particles Maria V. Tzoumaki a , Thomas Moschakis a , Vassilios Kiosseoglou b , Costas G. Biliaderis a, * a Department of Food Science and Technology, School of Agriculture, P.O. Box 235, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece b Laboratory of Food Chemistry and Technology, Department of Chemistry, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece article info Article history: Received 25 November 2010 Accepted 8 February 2011 Keywords: Pickering emulsions Oil-in-water emulsions Chitin nanocrystals Rod-like particles Creaming Rheology Microstructure abstract The aim of the present study was to investigate the oil-in-water emulsion stabilizing ability of chitin nanocrystals (colloidal rod-like particles) and the factors that may influence the properties of such systems. Chitin nanocrystal aqueous dispersions were prepared by acid hydrolysis of crude chitin from crab shells and oil-in-water emulsions were generated by homogenizing appropriate quantities of a chitin nanocrystal stock aqueous dispersion with corn oil, using an ultra-sonic homogenizer. The resulting emulsions were visually evaluated for their creaming behaviour upon storage. Additionally, the samples were studied with static light scattering, small deformation oscillatory rheometry and optical microscopy, under different conditions of nanocrystal concentration, ionic strength, pH and temperature. The chitin nanocrystals were proven quite effective in stabilizing o/w emulsions against coalescence, over a period of one month, as evidenced by static light experiments and microscopy, and this could be attributed to the adsorption of the nanocrystals at the oilewater interface. The rheological data provided evidence for network formation in the emulsions with increasing chitin nanocrystal concentration. Such a gel-like behaviour was attributed to an inter-droplet network structure and the formation of a chitin nanocrystal network in the continuous phase. The stability of the emulsions to creaming increased with an increase in nanocrystal concentration. Finally, by raising the temperature (20e74  C), NaCl concen- tration (up to 200 mM) or pH (from 3.0 to 6.7) there was an enhancement of the emulsion elastic character and creaming stability. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Emulsions are of great practical interest because of their extensive occurrence in food, cosmetics and pharmaceutical industries (Vignati, Piazza, & Lockhart, 2003). Emulsion preparation and stabilization can conventionally be achieved by prolonged mechanical agitation and addition of surfactants or other surface active polymers (Ashby & Binks, 2000; Vignati et al., 2003). Early works have shown that emulsion stability does not necessarily require amphiphilic surfactants to reduce the interfacial tension, but can also be efficiently achieved by dispersed solid particles of colloidal dimensions (Pickering, 1907; Ramsden, 1904). The emul- sions stabilized by such particles are usually referred as Pickering emulsions. The conventional explanation for emulsion stabilization by solid particles is their accumulation at the oilewater interface in the form of a densely packed layer, which may prevent droplet floc- culation and coalescence by a steric mechanism (Binks & Horozov, 2006; Dickinson, 2006, 2010). The extent of steric barrier depends on how difficult it is to remove particles from the interface, and is greater when most of the particles’ surfaces lay on the outer side of the oil droplets (Dickinson, 2010). Therefore, the contact angle q made by the stabilizing colloidal particles at the wateroil contact line, determines the particle location at the interface and the nature of the emulsion. Contact angles less that 90  , imply the hydrophilic nature of the colloidal particles that gives rise to o/w emulsions, whereas contact angles greater than 90  imply the hydrophobic nature of the particles that favours w/o emulsion formation (Lagaly, Reese, & Abend, 1999; Vignati et al., 2003). Moreover, the energy of desorption per particle is related to the contact angle q and as long as q is not close to 0  or 180  , it is predicted to be of the order of several thousand kT (Aveyard, Binks, & Clint, 2003; Dickinson, 2010; Yusoff & Murray, 2011). This denotes that once the particles are at the interface, they are effectively and irreversibly adsorbed. Hence, one of the most striking features of the particle-stabilized emulsions is that, in most of the cases, they are extremely stable to coalescence even when the droplets are quite large (Binks, Dyab, & Fletcher, 2007; Dickinson, 2010). In addition to the particle layer formation around the droplets, some other ordering mechanisms responsible for the prevention of * Corresponding author. Tel./fax: þ30 2310 991797. E-mail address: biliader@agro.auth.gr (C.G. Biliaderis). Contents lists available at ScienceDirect Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd 0268-005X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2011.02.008 Food Hydrocolloids 25 (2011) 1521e1529