Simplified kinetics and thermodynamics of geraniol acetylation by lyophilized cells of Aspergillus oryzae

Kinetics and thermodynamics of geranyl acetate production by direct geraniol acetylation with lyophilized cells of Aspergillus oryzae were studied in n-heptane and compared with those of ethanol acetylation. Batch tests were performed varying the starting substrates equimolar level from 25 to 150 mM, the cell concentration from 5.0 to 30 g l−1, and the temperature from 30 to 95°C. The progressive increase in the starting product formation rate observed with increasing temperature up to 80°C and the successive fall beyond this value confirmed the occurrence of reversible biocatalyst inactivation. The simplified Arrhenius model was used to estimate the apparent activation enthalpies of both the acetylation of geraniol (ΔH# = 35 kJ mol−1) and the reversible inactivation of the biocatalyst (ΔH#i = 150 kJ mol−1). The thermodynamic parameters of the irreversible enzyme denaturation were also estimated by residual activity tests performed on lyophilized cells previously exposed in the solvent at different temperatures for variable times (ΔH#d = 28 kJ mol−1; ΔS#d = −0.28 kJ mol−1 K−1). These results on the whole suggest that the reversible inactivation and the irreversible denaturation of mycelium-bound carboxylesterases are thwarted by increases either in the hydrophobicity or in the molecular weight of the alcoholic substrate.