A SPECTROPHOTOMETRIC METHOD FOR THE QUANTITATIVE MEASUREMENT OF PULLULAN

Pullulan is a biopolymer produced on a large scale for use in several industries, for example as an ingredient in food and drinks, in the plastics industries as nylon-like films and as components in electric materials. However, the quantification of natural pullulan is possible only by gravimetry. This is a time-consuming, non-specific and tedious technique, which involves the precipitation of the broth containing pullulan with solvents and then dried to constant weight at high temperature. In the present work a rapid method for the spectrophotometric determination of pullulan based on complex formation with Congo red is described. The general conditions for optimum assay were pH 4.5-5.0, wavelength 500 nm and 100 muM of NaCl. Several aqueous-alcohol mixtures were assayed to obtain the greatest differences between blanks and samples in the 0.10-10.00 mg range of pullulan. 84% ethanol concentration was selected in the 1.00-10.00 mg range, and 5.3% ethanol concentration for the 0.10-1.20 mg range of pullulan, respectively. In both cases a linear relationship was obtained spectrophotometrically at 500 nm under the assay conditions for determination of the polymer in the ranges used. Interference of diverse substances in the assay conditions was not detected with glycerol and dextran at 50.00 g/l, but dextrin and carboxymethylcellulose at 6.30 g/l and soluble starch at 0.4 g/l cause interference.