Hydrocolloids with emulsifying capacity. Part 3 – Adsorption and structural properties at the air–water surface

Abstract

The spreading capacity and surface layer characteristics (morphology and mechanical properties) of different hydrocolloids emulsifiers were compared at air–water interfaces. Acacia senegal, Acacia seyal (AcSey), sugar beet pectin (SBP) in conventional and matured states and two conventional samples of gum Ghatti (GG) were studied in aqueous solutions at pH 4.5 and 3.1. Samples were analysed by their Langmuir compression isotherms. At pH 4.5, the conventional A. senegal (GAc) and A. seyal (AcSeyc) samples exhibited low spreading capacity compared with the other hydrocolloids. The capacity to spread on the surface was improved by the maturation process applied to A. senegal gums and to SBP, but had little effect on the A. seyal samples. The elasticity at zero deformation rate was homogenous for the hydrocolloids. Only A. seyal samples showed high resistance to layer compression, probably due to their low specific volume because of their more compact structure. The matured A. senegal gum was hydrophobically fractionated to separate the main components which were studied individually. The arabinogalactan protein (AGP) component dominates the surface behaviour of this hydrocolloid, and it was confirmed that the arabinogalactan (AG) component did not exhibit surface activity. Acidification of the sub-phase to pH 3.1 improved the spreading and mechanical properties of the layers for all conventional and matured samples, which is in agreement with previous results at n-hexadecane–water interface. However, the GG samples were not affected by pH changes and exhibited similar Langmuir isotherms at both pH values.

A. senegal gums were further studied by atomic force microscopy (AFM) and ellipsometry. In aqueous solutions circular particles of about 11 nm in diameter were detected at pH 4.5 for GAc, whereas matured A. senegal gum (designated as EM2) showed circular objects of about 25 nm diameter. Aggregated particles were observed in both samples, but their proportion in EM2 was higher than for GAc. By AFM analysis, the film layer formed by EM2 samples at the air–water interface (pH 4.5) at a surface pressure of 28 mN/m was thick (about 3 times the height of the particles). The roughness and larger particle sizes were more evident for EM2 samples. AFM and ellipsometry at pH 3.1 showed that the surface layer of both samples were thicker and made up of a higher proportion of aggregates.

Introduction

In the previous papers of this Series (Parts 1 and 2) we have described the influence of aggregated structures on the improvement of the emulsifying and interfacial properties of Acacia senegal gum exudates and other new emulsifying hydrocolloids. The aggregated structures were shown to increase the elasticity of the interfacial film and enhance both the emulsifying capability and the speed to lower the interfacial tension. Surface charge was found to be an important property in controlling their interfacial behaviour. Changes in the pH from 4.5 to 3.1 enhanced their adsorption properties and film elasticity. Although oil–water hydrocolloids-interfacial-properties have been widely studied, the morphology adopted by the components at the interface is still unknown. Structural aspects for A. senegal components in solution or hydrated conditions have been reported (Cowman et al., 2006, Dror et al., 2006, Wang et al., 2008), but no information has been presented about the morphology and the structure of the interfacial films formed by these molecules. Neither has the behaviour at the air–water interface for hydrocolloids such as the Acacia gums and sugar beet pectin been quantified. Fauconnier et al. (2000) showed that A. senegal gums exhibited better spreading properties and produced films which were more elastic than Acacia seyal gums.

Emulsification agents are generally characterised by their behaviour at the air–water interface, since their reactivity at the oil–water interface is not significantly different (Williams & Prins, 1996). Already we have shown that the film forming properties at the air–water interface of A. senegal in its conventional and matured forms parallel their emulsification effectiveness (Part 1 of this Series).

Natural hydrocolloids are usually highly polydispersed. For A. senegal and A. seyal three main components have been described: arabinogalactans (AG), arabinogalactan proteins (AGP), and glycoproteins (GP). Between them, it was well established that AGP is the main component which is active at the interface in dispersed systems.

Atomic force microscopy (AFM) has been applied successfully to visualise hydrocolloids and others soft material components at the molecular and supramolecular levels (Bottier et al., 2008, Gunning et al., 2004, Ikeda et al., 2005, Sanchez et al., 2008, Wang and Somasundaran, 2007). This technique has also been used to characterise the morphology of air–water interfacial films formed with egg yolk lipoproteins or β-casein proteins (Dauphas et al., 2007, Mackie et al., 1999, Martinet et al., 2003). We propose also to use this technique in the present investigation.

To extend our previous studies (Part 2 of this Series), which describe the interfacial properties at the n-hexadecane–water interface, we added Ghatti gum (GG), A. seyal (AcSey), and sugar beet pectin (SBP) to our investigation to get information about their behaviour at air–water interfaces. Our objective is to characterise the spreading capacity of these emulsifying hydrocolloids, and learn about the morphology of their films at air–water interfaces. To achieve this we have used ellipsometry, measured Langmuir trough pressure–area isotherms, and applied AFM to Langmuir–Blodgett films.