Xylanase thermoinactivation followed first-order kinetics and the activation energy for the decay was 311 kJ mol(-1). Xylanase showed at 60 degreesC a half-life of 2.7 min in absence of polyols but the addition of 2 M erythritol, xylitol or sorbitol increased it above 8, 32 and 112 times, respectively. Moreover, xylanase showed 80% residual activity after 19 It at 60 degreesC with 3 M sorbitol. The thermodynamic parameters for the inactivation process in absence of additives were calculated and the increase in DeltaG after the addition of polyhydroxylic cosolvents showed parabolic dependence on both the number of hydroxyls per molecule and the polyols concentration. Xylanase protection by ethylene glycol (EG) and glycerol was the more effective the higher the thermoinactivation temperature. Erythritol and sorbitol did not inhibit xylanase activity so that neither of them acted as ligand to protect xylanase. Other polyols tested showed competitive-lineal inhibition in the beechwood xylan kinetics with K-i = 2.13 M for glycerol and K-i = 1.14 M for xylitol whereas ethylene glycol showed competitive-parabolic inhibition. Thus, they can protect the enzyme not only as polyols, through their interaction with the medium, but also as ligands. (C) 2003 Elsevier Inc. All rights reserved.