PHYSICOCHEMICAL CHARACTERIZATIONS OF ACACIA SIEBERANA VAR. SIEBERANA OF SUDANESE ORIGIN

Kamal M. Saeed. Twenty four authentic samples of Acacia sieberana var. sieberana were collected from South Kordofan (SK) and White Nile (WN) states. The samples were analyzed to determine 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 the gum. For (SK)samples average values, for the above parameters, 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% and 1933 respectively, while those of WN samples were found to be: 9.12 %, 1.65%, 4.38, 103, 1.337, 2.29× 10 Da, 9.39 ml /g, 0.37 %, 2.43%, 9.4 % and 1884 respectively. The cationic composition showed the following sequence: calcium > potassium> magnesium > sodium. Traces of: Fe, Ni, Co, Mn, Cu and Pb were detected. Percent average values of sugar composition show that galactose, arabinose, rhamanoe, glucuronic acid and 4-O-methylglcuronic acid for the gum samples from both locations are similar. For SK samples the values were found to be: 9.8, 57, 4.0, 2.7 and 6.5 respectively, while those of WN 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 indicate the presence of three main fractions. The weight average molecular weight of SK and WN, composite samples, is 1.8 ×10 Da and 1.6 × 10 Da respectively. The emulsifying capacity study of the gum shows that the gum is a grade one emulsifying agent.

The emulsification function is investigated by measuring the droplet size distribution of the emulsion at three different temperatures: as fresh sample and after storage for 3 and 7 days once at ambient temperature and once at 60 o C (using accelerated stability test). The samples were subjected to Mastersizer 2000, a laser diffraction particle size analyzer (Malvern Instruments). Distilled water was used as dispersant and a value of 1.450 was used for the refractive index for oil phase Octanoic/Decanoic acid triglyceride oil (ODO) .

Emulsion preparation:-
Distilled water was added to about 12 g of the gum sample (based on dry weight) in glass bottle to become about 40 g in total with a concentration of 30 % (w/w) gum solution. The sample was agitated on a tube roller mixer overnight until the sample completely dissolved and hydrated. Exact calculated grams for each samples (~27 -28 g) of the prepared gum solution was filtered using 100 µm mesh then mixed with 0.52 ml of 10 % (W/V) sodium benzoate solution as a preservative, and 0.48 ml of 10 % (W/V) citric acid solution to adjust the pH to 4, distilled water was added until the total weight become 32.0 g. then, 8.0 g ODO oil was added to the gum solution to give a total of 40 g and final concentration of 20%. The mixed solution was homogenized for 3 minutes using a polytron (PT-2100) homogenizer at 26000 rpm. Impeller (PTDA21 9 mm tip diameter) was used as dispersing tool. To achieve small particle size < 1 micron, the pre-emulsified mixture was homogenized using a high-pressure Nanomizer (NM2-L100, Yoshida a kikai Co. Ltd.). In order to achieve effective disaggregation of the gum it was passed twice at 50 M Pa. The final emulsion was divided into two aliquots and kept in closed glass universals. One of the aliquots was kept at 60 oC in the Vacuum Oven (Gallenkamp. OVA031.XX1.5), and the other was kept at ambient temperature . . Tables (1 and 2) show the results of the physicochemical characterization of the samples from SK and WN states. Average values for % moisture, % ash, %nitrogen, % protein, % total uronic acid pH, specific rotation, number average molecular weight, intrinsic viscosity, and equivalent weight are almost identical for all samples, however very slight variations are observed.

Results and discussion:-
Low intrinsic viscosity indicates a highly branched globular structure (Barrow, 1979). Percentage of nitrogen and protein for SK and WN samples are three times that of Acacia seyal (Hassan et al., 2005) and are in the same order for those of Acacia senegal (Osman et al., 1995). Eequivalent weights for SK and WN samples are slightly different and are higher than the values reported for Acacia seyal (Hassan et al., 2005). The high equivalent weight for Acacia sieberana var. sieberana may be attributed to its high molecular weight and relatively low uronic acid content.
Tables (3 and 4) show the major cationic compositions of Acacia sieberana var. sieberana gum samples from SK and WN respectively. The results indicate that the major elements are in the order Ca > K > Mg > Na for SK samples and in the order Ca > K > Na > Mg for WN samples. These differences may be attributed to differences in soil element composition.
Tables (5 and 6) show the sugar compositions of Acacia sieberana var. sieberana gum for SK and WN samples respectively. The results indicate two facts: arabinose: galactose ratio is >1 and rahmnose Content is low. These two facts are typical features of gums from the Gummeferae (Anderson and McDougall, 1987).       1 and 2 show the GPC elution profiles for SK and WN respectively. The profiles reveal the presence of three main fractions differing very much in their molecular weight, emphasizing the polydispersity of the gum. These fractions resemble the arabinogalactan-proteins (AGP), arabinogalactan (AG) and glycoprotein (GP) described for Acacia senegal (Randall et al, 1988). However, the distribution patterns of these fractions are slightly different indicating different detailed structure of the three components.    (Fig .3). The high molecular weight peak was processed separately and the remainder of the gum processed as the second peak; M w = weight average molecular weight; M n = number average molecular weight.

Summary and conclusions:-
The physicochemical study of Acacia sieberana var. sieberana gum shows the followings characterizations: positive optical rotation, low viscosity and low rhaminose content. The GPC fractionation of this polysaccharide gum shows the presence of three fractions and the protein is distributed among all these fractions with considerable ratios. From a functional application view, the gum is considered a grade one emulsifier.