CO2 treatment (Table 3), which was related to the reduction of browning symptoms. The lower pH in cells could reduce the activity of PPOs, which work optimally at approximately pH 7.0 25, 27, 29. Browning is related to the enzymatic oxidation of phenolic compounds catalysed by PPO 27. The results of the present study agree with earlier findings on the inhibitory effect of high CO2 storage 18, 26 and high CO2 pre-treatment 29 on PPO activity and browning. An extremely high CO2 treatment could induce more rapid deterioration 30, as also obtained in the present study in the 12-h high CO2 pre-treatment.