Summary 229 Review 1．Introduction Gum Arabic is certainly the most commonly used among the different natural hydrocolloid emulsifiers. It is employed by the industry in the formulation of many products 1) , and especially as emulsifier of citrus flavours in soft drinks. The gum is a complex polysaccharide consisting mainly of D- galactopyranose, L-arabino-pyranose and furanose, with some proportions of L-rhamnopyranose, D-glucuropyranosyl uronic acid, and 4-O-methyl-D-glucuropyranosyl uronic acid. Besides, it contains a small amount of protein (～2％) as an integral part of the structure. By hydrophobic interaction chromatography, Randall et al. 2) , have shown that gum arabic contains three main components, two of low-molecular-weight namely arabinogalactan (AG) and glycoprotein (GP) and one of high- molecular-weight namely arabinogalactan protein (AGP). The origin of gum arabic functionality in emulsions is not due to its thickening properties because this gum forms aqueous solutions of low viscosity even at high concentrations as 50 w/w％ 3) . Instead, its capacity to form interfacial films at oil-water and air-water interfaces has been well established 4, 5) . Moreover, the emulsifying and interfacial properties were mainly ascribed to the high-molecular-mass-protein fraction (AGP) 4～6) . This fraction usually contains a high proportion of the gum protein (about 50％) even if it represents only between 10 and 20 w/w％ of the gum 4, 6, 7) . Surface activity of a set of Acacia gums, presenting nitrogen contents between 0.1 and 8.5％, was studied by Dickinson and co-workers 8) . They suggested some correlation between protein content and long-time interfacial tension, grouping samples in three categories. In spite of that, nitrogen content and emulsifying effectiveness were not correlated, and they proposed that the distribution between low- and high-molecular- weight of proteinaceous material must influence the emulsifying behaviour. The complexity of gum arabic molecular composition is at the origin of this variability in interfacial and emulsifying characteristics 6, 8) . Besides, changes in molecular weight, nitrogen content, and proportion of each fraction in the gum were detected depending on the region where the tree was developed or depending on its age 9) . Particularly, the molecular weight of the gum was increased with the age of the trees. These negatively characteristics generate some difficulties to the complete industrial acceptance of gum arabic, even if it remains the emulsifier of choice for citrus oil flavored Four gum samples (Acacia senegal (control: GAc, and matured: EM2) and Gum Ghatti (Anogeissus Latifolia: GGsd and GGrd)) with different characteristics were evaluated as surface and interfacial active natural hydrocolloids. Adsorption kinetic and equilibrium interfacial tension were measured by axi-symmetric drop shape analysis at n-hexadecane-water interfaces. Diluted aqueous solutions of gums (pH 4.5) showed three stages during interfacial film formation: induction, development, and equilibrium states. Final interfacial tension provided by EM2, GGsd and GGrd was lower than that of GAc gum (about 30 mN/m versus 40 mN/m). As measured by interfacial rheology, films were basically elastic, with no frequency dependence of their dilatational parameters. Elasticity was developed in three hours after which its evolution was minor. EM2, GGsd and GGrd formed more elastic films (|E| ~ = 50-60 mN/m) than GAc (|E| ~ = 20 mN/m). Air-water spread films of gums were evaluated by compression isotherms, and the results correlated with those of n-hexadecane-water interface. GAc showed uniform surface pressure increment during compression. GG samples and EM2 presented the highest pressure values during compression and structural transitions at 8 mN/m and 25 mN/m. Collapse pressure was similar for all samples. Interfacial characteristics of previously high pressure (5 MPa) treated gum solutions were not changed. Conversely, all gum solutions at pH 3 (citric acid 20 mM) showed improved interfacial activity (kinetic and final interfacial tension), and more elastic interfacial films (higher elastic modulus). The correlation of results with both pH induced modifications and the proportion of arabinogalactan protein (AGP) in samples is discussed. Surface and Interfacial Properties of Natural Hydrocolloid Emulsifiers Oscar F. Castellani a) Saphwan Al-Assaf b) Monique Axelos a) Glyn O. Phillips b,c) Marc Anton a) a) UR1268, Biopolymers Interactions Assemblies, INRA F-44300 Nantes, France b) Glyn O. Phillips Hydrocolloid Research Centre, North East Wales Institute Plas Coch, Mold Road, Wrexham LL11 2AW, UK c) Phillips Hydrocolloids Research Ltd. 45 Old Bond Street, London W1S 4QT, UK 特集：天然ハイドロコロイド乳化剤（1） Feature Articles : Natural Hydrocolloid Emulsifiers (1)