International Journal of Biological Macromolecules 57 (2013) 76–82 Contents lists available at SciVerse ScienceDirect International Journal of Biological Macromolecules jo u rn al hom epa ge: www.elsevier.com/locate/ijbiomac A comparative study on the emulsifying properties of various species of gum tragacanth Mina Farzi a , Zahra Emam-Djomeh a,∗ , Mohammad Amin Mohammadifar b a Transfer Phenomena Laboratory (TPL), Department of Food Science, Engineering & Technology, Faculty of Agricultural Engineering and Technology, University of Tehran, P.O. Box 4111, Karaj 31587-77871, Iran b Department of Food Science and Technology, National Nutrition and Food Technology Research Institute, Faculty of Nutrition Sciences and Food Technology, Shahid Beheshti University of Medical Sciences, Tehran, Iran a r t i c l e i n f o Article history: Received 24 December 2012 Received in revised form 4 February 2013 Accepted 2 March 2013 Available online xxx Keywords: Gum tragacanth Model emulsion Emulsification activity Rheological properties Particle size a b s t r a c t Emulsification activities of three different species of gum tragacanth containing Astragalus gossypinus, A. compactus and A. rahensis were investigated. Emulsion stability indexes, particle size distributions, steady and unsteady rheological properties and some other physicochemical attributes including the surface tensions and uronic acid contents were taken into consideration. It was revealed that A. gossypinus created the most stable emulsions although having lower viscosity than A. compactus. It is believed that higher insoluble fraction and higher uronic content made this species a good steric and electrostatic emulsifier. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Gum tragacanth (GT), exuded from different species of Astra- galus, is a heterogeneous branched anionic biopolymer [1] with a high molecular weight (about 8.4 × 10 5 Da) [2] which belongs to the adsorbing polysaccharide category [3]. GT has been in Gener- ally Recognized as Safe (GRAS) list since 1961 and can be used at 0.2–1.3% level in food stuffs [4]. Due to the unique properties such as high acid resistance, stabilizing, emulsifying and gelling ability, GT has numerous applications in food, pharmaceutical and cosmetic industries [5,6]. It is well known that GT consists of two different fractions: tra- gacanthin (water-soluble) and bassorin (water-swellable) [7]. It is not well understood yet if the two fractions are in physical or chem- ical association, but the fact that they can be separated easily, favors the probability of a physical mixture [8]. Balaghi et al. [4] reported that the ratios of soluble to insoluble parts, physicochemical and rheological properties of various species are completely different. They also suggested that the sugar composition of GT is strongly species-dependent and the functional properties of the gums are greatly influenced by their sugar compositions [9]. Anderson et al. [10] reported that bassorin and tragacanthin of different species have different amount of uronic acid, methoxyl and neutral sugar ∗ Corresponding author. Tel.: +98 26 3224 8804; fax: +98 26 3224 9453. E-mail address: emamj@ut.ac.ir (Z. Emam-Djomeh). contents. So regarding different characteristics of various species, different applications could be expected for each species [9,11]. Emulsions form an important component of many processed food formulations [12]. They consist of two immiscible liquids (usu- ally oil and water), one dispersed in the other as small droplets. Emulsions are thermodynamically unstable but can be kinetically stabilized for a reasonable period of time by adding emulsifiers and/or thickening agents [13]. It should be noted that most low molecular weight emulsifiers do not have health clearance, are limited in their use levels or are restricted to certain foods [14], while biopolymers such as proteins and polysaccharides are com- pletely accepted as food ingredients [15]. So food industry has presented an increasing interest in replacing these biopolymers instead of low molecular weight emulsifiers [16,17]. Most polysaccharides exhibit thickening rather than emulsi- fying properties; means they prevent phase separation through increasing the viscosity of the continuous phase [18]. Although, it is reported that some polysaccharides such as gum Arabic (GA), modified starches, sugar beet pectin, some galactomannans and soy soluble polysaccharide exhibit emulsifying properties [18–23]. These surface active polysaccharides adsorb to the oil/water inter- face and stabilize dispersed phase against coalescence through steric or electrostatic interactions. Most studies about surface active polysaccharides have been carried out on GA and it is reported that the emulsifying activity of this gum is attributed to the small amount of protein which is included in its structure as an arabinogalactan–protein complex (AGP) [23–25]. On the whole, 0141-8130/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijbiomac.2013.03.008