Urea degradation kinetics in model wine solutions by acid urease immobilised onto chitosan-derivative beads of different sizes

Abstract

In this work, a purified acid urease preparation was covalently immobilised onto porous chitosan beads of different size. The covalent binding method was found to be more efficient than the adsorption cross-linkage one whatever the glutaraldehyde-to-chitosan bead ratio ( Y GA/CHI) used. At the optimal Y GA/CHI ratio of 0.625 g g −1 , the specific activity ( A Bi) of the biocatalysts decreased from circa 300 to 70 IU g −1 wet support, as the bead average diameter ( d P) increased from 0.14 to 2.2 mm. Generally, A Bi reduced less than 5% after preservation in the wet form at 4 °C for 150–170 days. Only the biocatalyst prepared using the Chitopearl BCW-3001 lost about 40% of its initial activity. The kinetics of urea degradation in a model wine solution using these biocatalysts was of the pseudo-first order with respect to the urea concentration in the liquid bulk, the apparent pseudo-first order kinetic rate constant ( k Ii) ranging from about two thirds to one fifth of that ( k If) pertaining to free acid urease. In the operating conditions tested, the reaction kinetics was estimated as unaffected by the contribution of the external film and intraparticle diffusion mass-transfer resistances. When the model wine solution was enriched with the high-inhibitory tannins extracted from grape seeds, at the maximum level tested (374 ± 2 g GAE m −3) k Ii reduced to no more than (58 ± 9)% of k If, this proving quite a higher protective action against such compounds for the chitosan-based biocatalysts towards free or Eupergit ® C 250 L-immobilised acid urease.