PHYSICOCHEMICAL STUDIES ON A COMMERCIAL FOOD GRADE XANTHAN .1. CHARACTERIZATION BY SEDIMENTATION-VELOCITY, SEDIMENTATION EQUILIBRIUM AND VISCOMETRY

A popular commercially available xanthan polysaccharide 'Keltrol RD' has been characterised by sedimentation velocity (Schlieren optics), sedimentation equilibrium (Rayleigh interference optics), and viscometry with regard to: (i) homogeneity; (ii) molecular weight; (iii) gross conformation; and (iv) critical overlap concentration, c*. Studies were conducted in phosphate-chloride buffer (pH = 6.5, I = 0.30). Preparations were shown to be homogeneous with a characteristic hypersharp Schlieren peak from sedimentation velocity. Despite the low c* value (similar to 0.5 mg/ml) it was still possible to obtain meaningful physical measurements in the dilute solution regime with a weight average molecular weight M(w) = (5.9 +/- 1.1) x 10(6) g/mol, the second thermodynamic ('osmotic pressure') virial coefficient of (3.4 +/- 0.3) x 10(-4) ml mol g(-2), a sedimentation coefficient s(20,w)(0) =(12.97 +/- 0.26)S, a sedimentation concentration regression parameter k(s) of (2127 +/- 85) ml/g and an intrinsic viscosity [eta] = (7534 +/- 2717) ml/ g. From these data it is also possible to estimate the two 'hydration independent' shape parameters II and the Wales-van Holde ratio k(s)/[eta]; the values obtained (similar to 0.53 +/- 0.20 and similar to 0.28 +/- 0.10, respectively) are consistent only with a stiff, elongated molecule of aspect ratio similar to 70:1.