You should scale it by using different pH, For producing maximum toxin in fermentor by using chemical defined media should be slight;y acidic may be ranging from 5.8-6.7. In highly acidic medium low concentration of toxin are secreted. These are secondary metabolite and are secreted during idiophase that displays utilization of primary metabolites and release of toxins, these are further utilized by bacterial cells.

Thanks for your reply. I am already tried in acidic PH, where i am getting less toxin. Is Iron play i vital role in toxin producation? If yes, at what concentartion and when it wili be secreated in medium?

Thanks & Regards

Dipak Das 

Thanks to all of you for valuable suggestion. I have achieved 350-400 Lf/mL in diphtheria production ( in 75 liter fermentor and 1200 liter fermentor) in semi synthetic medium. The important point is that Kf is less than 5 minutes.