Inactivation kinetics of horseradish peroxidase in organic solvents of different hydrophobicity at different water contents

The thermal stability of horseradish peroxidase suspensions was studied in three organic solvents of different hydrophobicity (dodecane, octane, and 1-octanol) at three different water contents (14.1, 55.3 and 256.2 mg water g^-1 dry protein). In these conditions, the enzyme is much more stable than in aqueous solutions (inactivation temperatures were in the range of 125-150°C). The enzyme showed a similar stability when in the presence of organic solvents, compared to the enzyme in a solid matrix without organic solvents with the same water content. The inactivation kinetics was well described by assuming the existence of two iso-enzymes, both inactivating according to a first order model. The lowest value for the z-value of both fractions (around 15°C) was obtained at the higher water content studied. The use of solvent and water content variables should be adequate to develop time-temperature integrators to monitor thermal processes at 100-140°C.