PHYSICO-CHEMICAL STUDIES OF ACETONE SUCCESSIVE FRACTIONS FROM ACACIA SENEGAL GUM

Abstract

ABSTRACT There have been substantial developments recently concerning the regulatory aspects of gum arabic and the elucidation of its physico-chemical and functional characteristics. The aim of this research is to fractionate and illustrate the properties of this complex polysaccharide. A total of 8 randomly selected Acacia senegal gum samples (mechanical powder and nodules) have been fractionated by a precipitation method using acetone as a precipitant and successive fractions are produced. The first fractional precipitation resulted in five molecular fractions. Three molecular fractions were isolated after the second and third fractionation process. The main chemical and physical characteristics of the Acacia senegal exudate whole gums and their isolated molecular fractions by precipitation were determined using different methods. All samples were soluble in water. The mean total yield was 89% from the first fractionation, 84% and 92% from the second fractionation of pure and composite samples and 89.5% from the third fractionation. Whole gum samples and their successive fractions have moisture content between 11.15 and 15.10 %. Ash content showed wide variation in the range, 3.30 – 4.65% for the whole gum samples and their fractions, 1.00 – 4.20% for the fractions from the second fractionation and 1.12 – 7.47% for the fractions from the third fractionation. The solutions gave pH values in range (4.48 – 4.64) for the whole gum samples which increased within the successive fractions to (5.36 – 5.87). The specific optical rotations for the whole gum samples ranged from -33.5 O to -31.0 O and increased among the successive fractions from -28.0 O to -11.0 O. The nitrogen content of the whole samples was within the range (0.26 – 0.58 %) and their fractions in the range (0.06 – 1.90%). The higher protein in the second and fourth fractions was (2.71 – 12.54%) for the first fractionation, within the range (2.90 – 13.20%) for the second and (6.34 – 8.32%) for the third fractionations. The solutions of the whole gum samples have intrinsic viscosities in the range 22.30 to 25.16 cm3g-1 and decreased, dramatically, among the successive fractions from16.90 to 1.08 cm3g-1. The osmotic molecular weights in g mol-1 for the whole gum samples were (1.02 – 1.21) x106 and 5.60x105 for the first fraction which dropped to 3x103 for the last one. Determination of the inorganic element contents for the whole gums and their fractions gave, Ca > 0.61%, K > 0.22%, Mg > 0.02% and Na > 0.01%. The Mark-Houwink constants K and ^5; for Acacia senegal were calculated and gave the mean values of 1.32x 10^72;2 and 0.55 respectively. The data support a picture of gum arabic as a very complex, polydisperse glycoprotein, polydisperse in molecular weight and particularly in protein composition.