Effect of pH and Aeration Rate on the Production of Destruxins A and B from Metarhizium Anisopliae

The effects of culture medium initial pH ranging from pH 2 to 9 on the production of destruxins A (DA) and B (DB) from the fungus of Metarhizium anisopliae F061 were examined by shaker flask culture, in 5 L stirred tank bioreactor (STR) and 20 L airlift bioreactor (ALR). In shaker flasks, when no control was applied, whatever initial pH used in the medium, the final pH of the broth was always dropped between 3 and 4 at the end of cultivation. During this study, when initial pH was held at below 5, low concentration of DB was obtained. However, the initial pH at 9 is more favorable for the maximal DB production; in contrast, less distinction of DA concentration was observed within the pH range studied. On the other hand, the pH values showed different effects on DA and DB production in 5 L STR cultivation. Maximum yield of DA (71.0 mg/L) and DB (310.6 mg/L) were achieved at pH 9 after 12 and 14 day cultivations, respectively, and these values were three fold higher than those obtained from pH of 2 cultivation. Furthermore, culture medium final pH at 3.7 is more favorable for the high yield of DB production in the above studies. In 20 L ALR with uncontrolled pH, the maximum levels 350 and 550 mg/L of DB were obtained, after 12 and 9 day cultivations by using 6 mesh and without (bubble column mode), net-draft-tube, respectively. On the other hand, the concentration of DB was greatly increased by increasing the aeration rate from 0.5 to 1.5 vvm (volume per volumeper minute). A maximum yield, nearly 700.0 mg/L of DB was obtained with an aeration rate of1.5 vvm and controlled pH at 6 in bubble column mode cultivation.