CHARACTERISATION AND TOXICOLOGICAL STUDY OF Acacia nilotica var. nilotica GUM FROM SUDAN

Abstract

Sixty six samples of authentic Acacia nilotica var. nilotica gum were collected from six different states of Sudan during the seasons 2008, 2009 and 2010. A representative composite sample was made from all samples collected during the three seasons. Physicochemical methods were undertaken to characterize the gum and to evaluate its toxicity. The parameters such as moisture content, ash content, pH value, specific optical rotation, intrinsic viscosities, nitrogen and protein content, acid equivalent weight and total uronic acid and tannin content were determined. Results obtained for all parameters show insignificant differences within samples collected from different locations and different seasons. The mean values obtained for the properties studied using the representative composite sample are as follows: moisture content 10.84%, ash content 1.9%, pH value 5.08, specific optical rotation +100, intrinsic viscosity 10.13 cm3g-1, nitrogen content 0.025%, protein content 0.163%, acid equivalent weight 1899.87, total uronic acid 10.21% and tannin content 0.11%. Acid hydrolysis of Acacia nilotica var. nilotica gum followed by HPLC measurements revealed that sugar content was arabinose 41.20%, galactose 17.43% and rhamnose 10.68%. Cationic composition studied using Atomic Absorption technique showed that potassium has the highest value among the cations studied, followed by calcium, magnesium, sodium, lead and iron in the same order. Molecular weight distribution was determined by fractionation of the gum using gel permeation chromatography. The chromatogram showed that three main components designated arabinogalactan protein (AGP), arabinogalactan (AG) and glycoprotein (GP) can be resolved. The molecular weight of the samples was estimated from light scattering measurement using GPC-MALLS technique. The values of Mw and Mn were found to be 3.51x106 Da and 1.57 x 106 Da respectively. The radius of gyration was found to have an average of 33.1 nm. The gum has been fractionated by hydrophobic interaction chromatography (HIC). Two fractions were obtained, hydrophilic fraction (F1) which contained arabinogalactan (AG) as a major component similar to the starting material and hydrophobic fraction (F2) was essentially the GP fraction. The overall distribution of the amino acids composition showed that hydroxyproline and serine were the iv principal amino acids for the whole gum and the dominating amino acids in fraction 1, whereas aspartic acid and leucine were the principal amino acids in fraction 2. An enzymatic degradation study of the whole gum was undertaken and the molecular mass of the degraded products was monitored by GPC. Stiffness values were estimated via the plot of the intrinsic viscosity of A. nilotica var. nilotica gum versus the inverse square root of ionic strength of NaCl. The results showed that the samples studied were either of stiff or semi-stiff backbone chain. Emulsification studies of Acacia nilotica var. nilotica gum showed that the gum has an excellent emulsifying stability compared to A. senegal although it possessed larger droplet size than A. senegal emulsions. Rheological study of Acacia nilotica var. nilotica gum showed that the flow behaviour is shear thinning under low shear rate and behave as a Newtonian fluid at high shear rate and high concentrations. The results of oscillatory test of gum solutions revealed a typical liquid-like behaviour. Toxicological study using in vitro cytotoxic methods involving Acacia nilotica var. nilotica gum on different types of normal and cancer human cell lines was undertaken to assess the safety of using the gum as food additive. The results showed that Acacia nilotica var. nilotica gum has a similar toxicity profile to that of gum arabic.