EFFECT OF UREA ON MOLECULAR AND COLLOIDAL AGGREGATION OF PROANTHOCYANIDIN POLYMERS FROM PINUS-RADIATA

The effect of urea on the aggregation behavior of proanthocyanidin pol ymer from Pinus radiata (P.r.) extract has been studied by rheological measurements and photon correlation spectroscopy (PCS). The urea indu ced structural changes in the concentrated extracts that were accompan ied by significant reductions of apparent viscosity. The size reductio n of proanthocyanidin polymers upon addition of increasing amount of u rea could also be observed by PCS. In a concentrated alkali-sequential extract, addition of urea leads to a decrease in both the steady shea r viscosity and the dynamic elastic modulus. Since the urea-induced st ructural changes of proanthocyanidin polymers are based mainly on the intermolecular colloidal association, the overall variation of reduced viscosity in the low concentration range of proanthocyanidin in the p resence of various amount of urea was negligible. Thus it is suggested that addition of urea leads to the control of the noncovalent colloid al interactions among proanthocyanidin polymers from Pinus radiata. In the absence of urea, concentration of the P.r. extract to 40% tannin produced stable near-Newtonian solutions of low apparent viscosity. (C ) 1996 John Wiley & Sons, Inc.