GLASS-TRANSITION AND THERMAL-STABILITY OF LACTASE IN LOW-MOISTURE AMORPHOUS POLYMERIC MATRICES

The thermal stability of a commercial preparation of neutral lactase ( beta-galactosidase) in low-moisture amorphous polymeric matrices of ma ltodextrin (MD dextrose equivalent (D.E.) = 10.9) and polyvinylpyrroli done (PVP; MW 40.000) stored at various temperatures (T) was studied. The main objective was to analyze the usefulness of the glass transiti on temperature (T-g) as a parameter for predicting the thermal stabili ty of lactase in low-moisture glass-forming matrices. Loss;of enzyme a ctivity was observed during storage in glassy conditions (either in PV P or MD matrices), suggesting that, although molecular mobility may be significantly decreased in the glassy state, the protein molecule is still mobile enough to lead to enzyme inactivation. The results indica ted that the change from the glassy to the rubbery state of the PVP ma trix, where the enzyme was embedded, was not reflected in drastic chan ges in the temperature dependence of the thermal inactivation rate, as expected if it could be only predicted on the basis of physical chang es of the matrices. The plasticizing effect of water is not the only f actor to take into account when considering enzymatic stability in hea ted low-moisture amorphous systems.