Profiles of the time-related pH changes among these CO^-treated PPO 

 were similar (Figures 33a, 33b, and 33c). A sharp drop in pH from 8.5 to 

 5.3 occurred after the PPO solutions were bubbled with CO2 for 1 min. The 

 pH remained constant at around 5.4 for the duration of the experiment. 

 The pK value for the equilibrium between dissolved CO2 and H* and 

 HCO3" is 6.1. (Montgomery and Swenson, 1969). According to the Henderson- 

 Hasselbalch equation, the molar concentration of HCO3" to CO2 in solution 

 '/ was reduced from 251 to 0.16 when the pH of the solution dropped from 8.5 



.^fV^ to 5.3. This study showed that exposure to COj, yielded a lower pH 

 environment and resulted in a rapid acidification (Aickin and Thomas, 



^ 1975; Thomas and Ellis, 1976). ^. , . 



>; When the change in enzyme activity was compared to the change in pH 



; ', among the CO^-treated PPO solutions, it was noted that PPO activities 

 ! ;. , decreased with an increase in CO^ treatment time. The treatment of PPO 

 solution with CO2 caused an instant drop in pH and it then remained 

 constant at 5.3 after 1 min. Using the pH control study, it was found 

 that enzymes under an environmental pH of 5.3 still had 66, 60, and 35% of 

 original activity after heating at 33°, 38°, and 43°C, respectively, for 

 30 min. Results from this study thus demonstrate that the loss in 

 activity was not due entirely to pH changes. 



(k,; 



n 



Effect of N . on PPO Activity 



Nitrogen gas did not inactivate PPO activity. In contrast, the 

 relative activity of N2-treated PPO increased with time (Table 8). Such 

 significant (P < 0.05) increase in enzyme activity accompanied by a 

 decrease in volume and thus an increase in protein concentration were 



