Abstract:

Mycobacterium sp.BD 696-6, a mutant strain of Mycobacterium sp.BD 696-6 that can effectively convert sterols into androst-4-ene-3,17-dione (AD), was immobilized by entrapment in calcium alginate gel capsules. AD was produced from rapeseed sterols by the fermentation of immobilized Mycobacterium sp.BD 696-6. The effects of initial pH of fermentation medium, temperature, substrate concentration and fermentation time on AD yield were investigated by sing-factor method. In addition, the storage stability and repeated usability of immobilized Mycobacterium sp.BD 696-6 were evaluated and gel capsules embedding Mycobacterium sp.BD 696-6 cells after fermentation were observed under scanning electron microscopy. An AD yield of more than 78% was obtained by the fermentation under substrate concentration 0.5%, initial pH 8.5 and temperature 32 ℃ for 144 h which was shortened by 24 h when compared with the fermentation time of free cell. Immobilized Mycobacterium sp. BD696-6 showed excellent storage stability and after used repeatedly 9 times its mechanical strength and AD conversion capacity had no marked decrease. SEM images revealed that the nutrients for calcium alginate-immobilized Mycobacterium sp. BD 696-6 could be freely transported.

Key words: immobilized cells, androst-4-ene-3,17-dione, sterols, calcium alginate, Mycobacterium sp.