Stability studies of papaya pectinesterase

Stability studies were carried out on the purified papaya (Carica papaya L. var. exotica) pectinesterase (EC 3.1.1.11). The enzyme preparation was stable at 4°C in 0·02m sodium phosphate buffer (pH 7·5) solution containing 0·2m NaCl and 0·02% sodium azide and the loss in activity was less than 5% after storage for 1 year. The thermostability studies showed that enzyme was more heat-stable at pH 7·5 than at pH 4. After heating at 60°C for 5 min, 65·6 and 82% activity still remained at pH 4 and pH 7·5, respectively. It was completely inactivated by heating for 5 min at 70 and 75°C at pH 4 and pH 7·5, respectively. The D values (time to inactivate 90% of the enzyme) at 55, 60, 65 and 70°C at pH 4·0 were estimated to be 112·14, 23·78, 8·33 and 1·71 min, respectively. Lower inactivation rates were observed for pH 7·5, with the D values ranging from 143·27 to 1·67 min for temperatures between 60 and 75°C. The Z values, which indicate the rise in temperature necessary to observe a 10-times faster rate of heat-inactivation, were estimated to be 7·8°C and 8·38°C at pH 7·5 and pH 4·0, respectively. The inactivation energies and Q10 values were calculated as 256·9 kJ/mol and 15·59 at pH 4·0 and 284·76 kJ/mol and 19·21 at pH 7·5, respectively. pH stability studies showed that the enzyme was stable from pH 4–11 after exposure of the enzyme to these pHs for 24 h at 30°C. More than 85% of the activity was retained in all of these cases. However, at pH 1 and 12, the enzyme was unstable and it completely lost its activity after 24 h of incubation at 30°C.