Formation of nanoemulsions stabilized by model food-grade emulsifiers using high-pressure homogenization: Factors affecting particle size Cheng Qian, David Julian McClements * Department of Food Science, University of Massachusetts Amherst, Amherst, MA 01003, USA article info Article history: Received 12 April 2010 Accepted 24 September 2010 Keywords: Nanoemulsion Microfluidization Emulsification conditions Viscosity ratio Emulsifier type abstract Nanoemulsions are finding increasing utilization in the food and beverage industries for certain appli- cations because of their unique physicochemical and functional properties: high encapsulation effi- ciency; low turbidity; high bioavailability; high physical stability. In this study, we examined the impact of system composition and homogenization conditions on the formation of nanoemulsions using a high- pressure homogenizer (microfluidizer). The mean particle diameter decreased with increasing homog- enization pressure and number of passes, with a linear logelog relationship between mean particle diameter and homogenization pressure. The minimum droplet diameter that could be produced after 6 passes at 14 kbar depended strongly on emulsifier type and concentration: SDS < Tween 20 < b-lacto- globulin < sodium caseinate. Small-molecule surfactants formed smaller droplets than proteins, which was attributed to their ability to rapidly adsorb to the droplet surfaces during homogenization. The impact of phase viscosity was examined by using different octadecane-to-corn oil ratios in the oil phase and different glycerol-to-water ratios in the aqueous phase. The minimum droplet size achievable decreased as the ratio of disperse phase to continuous phase viscosities (h D /h C ) decreased for SDS- stabilized emulsions, but was relatively independent of h D /h C for b-lactoglobulin-stabilized emulsions. At low viscosity ratios, much smaller mean droplet diameters could be achieved for SDS (d w 60 nm) than for b-lactoglobulin (d w 150 nm). The information reported in this study will facilitate the rational design of food-grade nanoemulsions using high-pressure homogenization methods. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction There is strong interest in the food and other industries in the use of nanoemulsions as delivery systems for non-polar functional components, such as lipophilic bioactive lipids, drugs, flavors, antioxidants, and antimicrobial agents (Hu, Johnston, & Williams, 2004; Kesisoglou, Panmai, & Wu, 2007; McClements, Decker, & Weiss, 2007; Sanguansri & Augustin, 2006; Weiss et al., 2008; Wissing, Kayser, & Muller, 2004). Oil-in-water nanoemulsions contain small oil droplets (d < 100 nm) dispersed within a watery continuous phase, with each oil droplet being surrounded by a protective coating of emulsifier molecules (Acosta, 2009; McClements et al., 2007; Tadros, Izquierdo, Esquena, & Solans, 2004). The stability, physicochemical properties and functional performance of nanoemulsion-based delivery systems can be controlled by altering their composition and preparation conditions so as to produce emulsions with different droplet concentrations, compositions, particle size distributions and/or interfacial proper- ties (Lesmes & McClements, 2009; McClements, 2010). A major potential advantage of nanoemulsions over conven- tional emulsions is that they can be made to be optically trans- parent by preparing droplets with dimensions much smaller than the wavelength of light (d < l) so that scattering is relatively weak (McClements, 2002a, 2010; Wooster, Golding, & Sanguansri, 2008). Consequently, they can be used to incorporate non-polar functional components into transparent aqueous-based food and beverage products. Nevertheless, this type of emulsion may become turbid or even opaque if droplet growth occurs during storage, e.g., due to flocculation, coalescence or Ostwald ripening. Hence, it is impor- tant to prevent these instability mechanisms from occurring in nanoemulsions after they have been formed. Nanoemulsions typically have much better stability to gravitational separation than conventional emulsions because the relatively small particle size means that Brownian motion effects dominate gravitational forces (McClements, 2005; Tadros et al., 2004). They also tend to have better stability against droplet flocculation and coalescence because the range of the attractive forces acting between the droplets decreases with decreasing particle size, while the range of * Corresponding author. E-mail address: mcclements@foodsci.umass.edu (D.J. McClements). Contents lists available at ScienceDirect Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd 0268-005X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2010.09.017 Food Hydrocolloids 25 (2011) 1000e1008