EFFECT OF LYOPROTECTANTS ON ASCORBATE OXIDASE ACTIVITY AFTER FREEZE-DRYING AND STORAGE

The activity of ascorbate oxidase (AO) purified from green zucchini sq uash (Cucurbita pepo medullosa var. Milano) decreased on freeze-drying . In rehydrated samples, enzyme activity was strictly correlated with the addition of lyoprotectants, the freeze temperature, the enzyme con centration and the storage time. The addition of lyoprotectants such a s sucrose, mannose, glucose or polyvinylpyrrolidone (PVP) improved enz yme stabilization. A notable loss of enzymic activity was observed whe n freezing was performed by immersion in liquid nitrogen in comparison to samples frozen at -20 or - 75 degrees C, Moreover, sugar-based lyo protectants provided better stabilization when the freeze temperature was -75 degrees C. PVP species were most effective in stabilizing AO w hen samples were frozen in liquid nitrogen. The decrease of enzyme act ivity on freeze-drying was inversely related to the enzyme concentrati on . A decrease in enzyme activity was observed in all samples after r ehydration at longer storage times, the decrease being less evident in samples containing the lyoprotectants mentioned earlier. Freeze-thawi ng experiments showed a similar trend considering the presence of lyop rotectant and the enzyme concentration. Samples without additives show ed the highest residual activities after the first freeze-thaw cycle b ut samples were much more prone to a gradual decrease of residual acti vity with subsequent freeze-thawing cycles.