PHYSICOCHEMICAL STUDY ON THE GUM FROM Acacia sieberana var. sieberana OF SUDANESE ORIGIN

Abstract

Physicochemical characterization, rheological properties and emulsification properties of 24 samples of Acacia sieberana var. sieberana of Sudanese origin from two locations namely Eltwal (South Kordfan state) and Elna'eem (White Nile state) were studied using standard methods. Standard characterization methods were used to obtain the average values of moisture, ash, pH, specific rotation, refractive index, number average molecular weight, intrinsic viscosity, nitrogen content, protein content, total uronic acid and acid equivalent weight of Acacia Sieberana var. sieberana samples from Eltwal and Elna'eem. For Eltwal samples the values were found to be: 8.56 %, 1.61%, 4.34, +104°, 1.337, 2.13× 10 Da, 8.56 ml /g, 0.37%, 2.45%, 9.2% and1933 respectively, while those of Elna'eem samples were found to be: 9.12 %, 1.65%, 4.38, 103, 1.337, 2.29× 106 Da, 9.39 ml /g, 0.37 %, 2.43%, 9.4 % and 1884 respectively. The cationic composition study using atomic absorption technique shows that calcium has the highest value followed by, potassium magnesium and sodium incase of Eltwal samples. In case of Elna'eem samples the most predominant element is calcium followed by potassium, sodium and magnesium. As for the rest of the elements namely Fe, Ni, Co, Mn, Cu and Pb only trace values were obtained for the gum from both locations. The sugar composition study using HPLC technique shows the percentage of the average values of galactose, arabinose, rhamanoe, glucuronic acid and 4-O-methylglcuronic acid for the gum samples from the two locations. For Eltwal samples the values were found to be: 9.8, 57, 4.0, 2.7 and 6.5 respectively, while those of Elna'eem samples were found to be: 9.9, 56, 3.1, 2.4 and 6.6 respectively. The molecular weight distributions of two composite samples of the gum from the two locations were determined by GPC-MALLS. The results indicate the presence of three main fractions namely arabinogalactan protein (AGP), arabinogalactan (AG) and glycoprotein (GP). The weight average molecular weight of Eltwal was found to be 1.8 ×106 Da with Mw/Mn equal (1.2), and that of Elna'eem was found to be 1.6 × 106 Da with Mw/Mn equal (1.7). The GPC- UV elution profiles demonstrated clearly the distribution of protein among the three fractions with appreciable amounts for each fraction and for both locations. The rheological study of the gum reveals a non-Newtonian behavior. At low concentrations below (10% w/v), the behaviour is slightly shear- thickening while at concentrations (10% to 50% w/v) the behaviour is a clear shear- thinning. The study also reveals a viscoelastic behaviour of the gum with G' dominating over G" after a frequency of less than 10 Hz. At (40% w/v) G' dominating over G" the elastic behaviour of the gum from each location dominates the viscous behaviour for all the frequencies tested. I.e. the structure shows certain rigidity, and as a consequence the gum solution may be considered as having a gel structure, and therefore certain form of stability. The emulsifying capacity study of the gum shows clearly the good quality of the gum as an emulsifying agent. The study also reflects that A. sieberana var. sieberana gum is a better surfactant than both Acacia senegal and Acacia seyal.