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Composition and structure of gum Arabic in solution and at oil-water interfaces

Atgié, Marina. Composition and structure of gum Arabic in solution and at oil-water interfaces. PhD, Génie des Procédés et de l'Environnement, Institut National Polytechnique de Toulouse, 2018

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Abstract

Gum arabic, a tree exudate, is essentially used for its binding and emulsifying properties. This natural product is a complex mixture of covalently linked proteins and charged polysaccharides moieties, differing in their molecular mass and hydrophobicity. A large body of literature now exists on the structure and interfacial properties of gum Arabic but a comprehensive description of the relationship between interfacial composition, interfacial structuration and emulsion metastability remains elusive. In the literature, gum Arabic is described as a mixture of three fractions: an arabinogalactan rich polysaccharide fraction, a polysaccharide-protein conjugates fraction and a fraction of glycoproteins. The conjugate fraction is thought to be responsible for the emulsifying and stabilizing properties of the gum, with the protein part adsorbing at oil droplets surface and the carbohydrate moieties providing steric repulsion between droplets. In this work, we have investigated the microscopic behavior of the gum species in solution and at oil/water interfaces. The first step was to characterize the structure of gum Arabic species in solution. A twodimensional separation of the gum molecules was performed using size exclusion chromatography followed with by hydrophobic interaction separation, which confirmed the highly heterogeneous composition of the gum. Small angle X-ray and neutron scattering measurements on the gum and its fractions led us to propose a structural representation of the gum conjugated moieties. Then the composition of adsorbed gum Arabic films as compared to gum Arabic solutions has been investigated. An increase in the protein rate of the interfacial film showed the crucial role of the polypeptide moieties on the adsorption. The composition of the adsorbed film was shown to differ from the bulk but remained heterogeneous in size and hydrophobicity. A mass balance revealed a strong dependence between the emulsion formulation (gum concentration and physico-chemical parameters) and the surface concentration, while the composition of the interface was only slightly changed. These results suggest that gum Arabic adsorbing species must adopt conformational changes depending on emulsification conditions. In a third stage, we have addressed the stabilization mechanisms resulting from the adsorption of gum Arabic amphiphilic species. For that purpose, we have developed a method to recover the adsorbed species within an oil-in-water emulsion. This method allowed us to unveil a structuration of the adsorbed film. Species recovered from the interface displayed aggregation, the magnitude of which directly depended on the coverage density and protein rate of the adsorbed film. The metastability of emulsions, stabilized with gum Arabic, increased upon promoting interfacial structuration, i.e. when the aggregation rate of adsorbed species was enhanced. Such behavior has not been reported so far in the literature and we believe that it is a key mechanism of gum Arabic-based on emulsions. Finally, small angle neutron scattering experiments (contrast match between the continuous and dispersed phases) disclosed differences of structuration between two regimes of interface coverage. These observations were discussed in the light of the comparison with the scattering spectra of gum Arabic solutions. To conclude, this thesis revolves around the composition/structuration relationship in gum Arabic-stabilized emulsion stabilized and demonstrates that an original mechanism is at play in this complex system.

Item Type:PhD Thesis
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Institution:Université de Toulouse > Institut National Polytechnique de Toulouse - Toulouse INP (FRANCE)
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Research Director:
Masbernat, Olivier and Roger, Kevin
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Deposited On:01 Oct 2018 12:16

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