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

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 (YGA/CHI) used. At the optimal YGA/CHI ratio of 0.625 g g−1, the specific activity (ABi) of the biocatalysts decreased from circa 300 to 70 IU g−1 wet support, as the bead average diameter (dP) increased from 0.14 to 2.2 mm. Generally, ABi 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 (kIi) ranging from about two thirds to one fifth of that (kIf) 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) kIi reduced to no more than (58 ± 9)% of kIf, 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.