Hydration characteristics of the gum exudate from Acacia senegal

Abstract

Using Differential Scanning Calorimetry the interaction of water with the gum exudate from Acacia senegal can be investigated. The first water molecules bind strongly to the hydrophilic groups of the core carbohydrate structure and are non-freezing. As the water content is increased several metastable freezing-bound states can be identified. One transition at ~255 K does not change with increasing water content. This metastable freezing-bound water state is closely associated with the polysaccharide structure. The transition of the main freezing-bound state, however, moves towards that of free water with increasing water content. This broad transition embraces at least two other metastable states, which can be recognised by variations in thermal history. There is one freezing-bound state at ~268 K and an accompanying metastable state which readily converts to free water on heating at 323 K, but when left to stand at room temperature this metastable state re-forms. Overall, A. Senegal can be seen to have a structure which effectively binds water. The freezing-bound states do not overlap with free water until ~20% solution has formed. From the ΔH value of the transitions it is evident that the binding of water is a necessary driving force not only in the conventional confectionary applications, but also in emulsification when the protein core performs the contrasting hydrophobic directive force.