■ '^--rfst^r^i^^isrs^. ■ 



166 

 of Christiansen et al . (1984) and Taniguchi et al . (1987), it also appears 

 that lobster PPO was more vulnerable to high pressure CO^ treatment than 

 corn germ peroxidase, a-amylase, glucoamylase, ^-galactosidase, glucose 

 oxidase, glucose isomerase, lipase, thermolysin, alcohol dehydrogenase, 

 and catalase. 



High pressure CO^ treatment at 43°C of the three PPO systems for 1 

 min caused a sharp drop in pH from 9.1 to 5.4 for lobster, 6.5 to 4.8 for 

 brown shrimp, and 6.1 to 4.2 for potato (Figures 34, 35, and 36). The pH 

 of the treated lobster, brown shrimp, and potato PPO systems remained 

 constant at 5.3, 4.5, and 4.1, respectively, throughout the experiment. 

 Overall, there was no difference in the profiles of the time-related pH 

 change between the high pressure and atmospheric CO^-treated PPO's (Figures 

 33a, 33b, and 33c). 



Kinetics o f PPO Inartivatinn 



Kinetic parameters for PPO inactivation by CO^ (1 atm) are given in 

 Table 9. The reaction constants for lobster CO^-treated PPO at various 

 temperatures were comparatively higher than those for PPO with no CO 

 treatment. PPO exposed to CO^ showed a higher activation energy (E = 39.7 

 KJ/mole) for inactivation than control (E^ = 26.6 KJ/mole) for the DOPA 

 reaction. The D values of PPO under CO^ treatment are lower than those of 

 temperature controls indicating that at the temperature range used, it is 

 much easier to inactivate PPO by CO, and heat than by heat alone. The 

 smaller z values obtained for CO,-treated samples also implies that the 

 enzyme is more sensitive to elevated temperatures when in a CO, 

 environment. The reaction constants (k) for brown shrimp and potato PPO 



