Molecular associations in acacia gums

The tendency of polysaccharides to associate in aqueous solution has long been recognised. Molecular associations can profoundly affect their performance in a given application due to its influence on the molecular weight, shape and size. This will ultimately determine how the molecules will interact with each other, with other molecules and with water. There are several factors, such as hydrogen bonding, hydrophobic association, ion mediated association, electrostatic interaction, concentration dependence and the presence of proteinaceous components, which affect this behaviour. Our objective is to highlight the role of the proteinaceous component, present in acacia gum, to promote associations when the gum is subjected to various processing treatments such as maturation, spray drying and irradiation. The results demonstrate the ability of the proteinaceous component to promote hydrophobic associations which influence the size and proportion of the arabinogalactan high molecular weight component (AGP). Heat treatment in solid state (maturation) increases the hydrophobic character of the gum and hence its emulsification performance. Spray drying also involves aggregation through hydrophobic association but changes the surface properties of peptide moieties to become more hydrophilic compared to the association promoted by the maturation treatment in the solid state. Irradiation induced cross-linking, in the presence of unsaturated gas, was used to introduce C-C bonds into the carbohydrate moiety and thus confirms the hydrophobic association prompted by the heat used in the maturation and spray drying. This association can be reversed by treatments such as filtration or high pressure homogenisation. The results reported here reconcile the contradiction about structure of gum arabic proposed by the wattle blossom and twisted hairy rope models and shows that the AGP fraction is basically an aggregated fraction made up of AG units stabilised by low molecular weight highly proteinaceous components.