Enzymes typically have a critical instability, which dominates both formulation and process development. In this paper, the ability to preserve the enzyme activity during freeze-drying was investigated for both water-binding and non-water-binding substrates. For this purpose, acid phosphatase was used as model protein. In addition, a procedure for the fast development of a freeze-drying cycle is shown. For the secondary drying part, the effect of processing temperature and time on enzyme activity was investigated. The enzyme activity decreased continuously during secondary drying, with a dramatic drop associated with processing temperatures higher than 293 K. Besides product temperature, the residual moisture level and water mobility are also important. As the residual moisture level and water mobility depend on the product formulation, the stabilizing effect against the enzyme deactivation was studied for a number of formulations which contain different additives, that is, sucrose, lactose, mannitol, and poly-vinylpyrrolidone, with a dramatic activity loss associated with crystallizing excipients. This study also confirmed that not all water-binding substrates can successfully protect the enzyme against deactivation. In fact, water-binding substrates containing reducing sugars (lactose) showed the highest loss of activity.
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