eatment (Fig. 6D) implies that there could be other contributions by various PPOs to browning in straw mushrooms. High CO2 was applied in short periods prior to storage to investigate browning inhibition. Similarly, a high CO2 content of 40% as a pre-storage treatment reduced browning, resulting in lower browning scores and higher L* values than that of the air pre-treatment (Tables 1and 2). The high CO2 pre-treatment for 6 h was more effective than either the 4 or 12 h pre-treatments. There was no interaction between the CO2 concentrations in the micro- atmosphere and the incubation times of the browning generation. The pH of the mushrooms also decreased in response to highCO2 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