Solid-state tyrosinase stability as affected by water activity and glass transition

The effects of water activity (a_W) and the state of the system as dictated by the glass transition temperature (T_g) on tyrosinase storage stability were evaluated in a model food system. Tyrosinase was incorporated into low and high molecular weight polyvinylpyrrolidone (PVP-LMW and PVP-K30, respectively). Samples were equilibrated and stored in desiccators from a_W 0.33 to 0.77 at 20°C. Tyrosinase activity was monitored for a week, and pseudo-first order rate constants for activity loss were calculated. Residual activity after equilibration correlated with T_g rather than a_W. An apparent rate constant increase slightly above T_g was observed and rate constants were the same at constant T_g, suggesting that molecular mobility as dictated by T_g was influencing tyrosinase storage stability.