Stability studies were carried out on the purified papaya (Carica papa
ya L. var. exotica) pectinesterase (EC 3.1.1.11). The enzyme preparati
on was stable at 4 degrees 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 thermo
stability studies showed that enzyme was more heat-stable at pH 7 . 5
than at pH 4. After heating at 60 degrees C for 5 min, 65 . 6 and 82%
activity still remained at pH 4 and pH 7 . 5, respectively. It was com
pletely inactivated by heating for 5 min at 70 and 75 degrees 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 degrees C at pH 4 . 0 were estimated
to be 112 . 14, 23 . 78, 8 . 33 and 1 . 71 min, respectively. Lower in
activation 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 degree
s C. The Z values, which indicate the rise in temperature necessary to
observe a 10-times faster rate of heat-inactivation, were estimated t
o be 7 . 8 degrees C and 8 . 38 degrees C at pH 7 . 5 and pH 4 . 0, re
spectively. The inactivation energies and Q(10) 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 e
nzyme was stable from pH 4-11 after exposure of the enzyme to these pH
s for 24 h at 30 degrees C. More than 85% of the activity was retained
in all of these cases. However, at pH 1 and 12, the enzyme was unstab
le and it completely lost its activity after 24 h of incubation at 30
degrees C.