The thermal denaturation of lysozyme was studied as a function of different additive (sucrose, glucose, fructose, mannitol) concentrations, phosphate buffer concentration and scan rate using differential scanning calorimetry (DSC ) and analyzed by an isoconversional method. The denaturation temperature T_m increased with the increase of scan rate and the additive concentrations, while decreased with the increase of phosphate concentration. The phosphate accelerated the denaturation process of lysozyme and reduced the thermal stability of lysozyme, while the additives enhanced it. One part of the lysozyme denaturation process was irreversible in which aggregation occurred, and the other part was reversible in the presence of all additives. By the isoconversional method, it was showed that the apparent activation energy did not keep invariable at different conversion ratios (α) under all conditions, but decreased with the increase of α, which suggested that a simple reaction mechanism could not be used for the denaturation process. The denaturation was still not a process involving the two standard reversible states, but a multi-step process in the presence and the absence of all additives.

Key words: DSC, lysozyme, additive, isoconversional method, multi-step process