The operational stability was assessed for both free and immobilized lipase in phos­phate buffer of pH 7. The immobilized lipase was also stored in 50 mM Tris-HCl buffer (pH 8.0) containing 0.3 M CaCl2 (T/Ca solution). All these solutions were stored at 4°C, and relative activities were determined for a period of two weeks and are shown in Fig. 12.5d. Also, the immobilized lipase was repeatedly used for hydrolysis ofp-NPP in order to evaluate its reusability property. From Fig. 12.5e, it can be seen that the immobilized lipase retained 75.54% of its initial activity upon tenth hydrolysis of p-NPP. Loss of activity can be attributed to enzyme leakage and also deactivation of lipase during the reuse procedures.