Small-deformation rheology of mesquite gum stabilized oil in water emulsions M.A. Valdez a,d, * , J.I. Acedo-Carrillo a , A. Rosas-Durazo a , J. Lizardi b , M. Rinaudo c , F.M. Goycoolea b a Departamento de Investigacio ´n en Polı ´meros y Materiales, Universidad de Sonora, Blvd. Transversal y Rosales, 83000 Hermosillo, Sonora, Me ´xico b Laboratory of Biopolymers, Centro de Investigacio ´n en Alimentacio ´n y Desarrollo, A.C. (C.I.A.D., A.C.), P.O. Box 1735, Hermosillo, Sonora 83000, Mexico c Centre de Recherches sur les Macromole ´cules Ve ´ge ´tales, C.N.R.S. Affiliated with University Joseph Fourier, B.P. 53, 38041 Grenoble, Cedex 9, France d Departamento de Fı ´sica, Universidad de Sonora, Blvd. Transversal y Rosales, 83000 Hermosillo, Sonora, Me ´xico Received 20 June 2005; received in revised form 5 November 2005; accepted 11 November 2005 Available online 10 January 2006 Abstract The influence of the nature of the oil phase on the viscoelastic properties of emulsions stabilized with mesquite gum has been investigated. Mesquite gum-stabilized emulsions of D-limonene, n-decane, n-dodecane, n-tetradecane n-hexadecane and orange oil were analyzed, the observations showed that the nature of the oil used was determinant for the viscoelastic and gel formation ability of the emulsions. Orange oil-in- water emulsions stabilized with mesquite gum developed a gel-like structure with time, in contrast to the emulsions obtained with alkane oils and with D-limonene. This is rationalized in terms of a weak gel network of protein-polysaccharides molecules interconnecting spherical oil droplets. The gelling behavior of these systems has been described with a model derived from percolation theory. q 2006 Elsevier Ltd. All rights reserved. Keywords: Mesquite gum; Oil in water emulsions; Rheology; Gel 1. Introduction The rheological properties of emulsions have been investigated from both the theoretical and the practical standpoint. In very dilute emulsions stabilized by nonionic surfactants or proteins as long as the droplets are completely dispersed, the viscosity of dilute nonaggregated emulsion systems has been described by the well-known Einstein equation (Buffo & Reineccius, 2002; Dickinson and McCle- ments, 1996). As emulsions become more concentrated, their viscoelastic properties become more apparent. In fact, the mechanical properties of highly concentrated emulsions (with volume fractionsR0.9) have much more in common with foams. The rheology of such systems is largely determined by the thin liquid film between the droplets (Dickinson and McClements, 1996). In flocculated protein-stabilized systems, it has been observed that the main contribution to the shear modulus comes from micro-gel formation involving adsorbed macromolecules on neighboring emulsions droplets. It has also been known for decades that gum arabic forms cohesive and elastic films at the air–water and especially so at air–oil interfaces and the time-dependent thickening behavior has been studied using small-amplitude oscillatory rheological tests (Sanchez, Renard, Robert, Schmitt, & Lefebvre, 2002). In a recent study, a conventional rheometer has been modified so as to study the interfacial shear viscosity and interfacial viscoelastic properties of mesquite mixed with oppositely charged chitosan, an aminoglycan, at the oil–water interface using mineral oil. The role of the nature of the oil phase on the thermodynamic stability of emulsions stabilized by MG or gum arabic and on their electrical properties (zeta potential) has been addressed in an accompanying paper (Acedo-Carrillo, Rosas-Durazo, Herrera-Urbina, Rinaudo, Goycoolea, & Valdez, accepted 2006). It was firmly demonstrated that the droplet size of MG- stabilized emulsions of orange deterpenated essential oil remain unchanged for periods up to 120 h, while emulsions of D-limonene and n-hexadecane are unstable and their average diameter increases monotonically with time. The mechanism of emulsion destabilization with time in these systems was attributed to Ostwald ripening. Indeed, Ostwald ripening is the Carbohydrate Polymers 64 (2006) 205–211 www.elsevier.com/locate/carbpol 0144-8617/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbpol.2005.11.028 * Corresponding author. Address: Departamento de Fı ´sica, Universidad de Sonora, Blvd. Transversal y Rosales, 83000 Hermosillo, Sonora, Me ´xico. Tel.: C52 2259 2108; fax: C52 2259 2109. E-mail address: valdez@fisica.uson.mx (M.A. Valdez).