Influence of some operational parameters on the bioconversion of sitosterol with immobilized whole cells in organic medium

Mycobacterium sp. NRRL B-3805 cells immobilized on Celite were used to selectively cleave beta-sitosterol to 4-androstene-3,17-dione (AD), with bis(2-ethylhexyl)phthalate as the bioconversion medium. The amount of water retained in the immobilization matrix proved to be a key parameter for biological activity retention, 0.5 to 0.8 g water/g dry support giving the best results. Mechanically stirred batch reactors were effectively used for sterol biotransformation, no increase in bioconversion rates being obtained for stirring speeds above 100 rpm. Kinetic studies were performed with beta-sitosterol concentrations up to 30 mM, with different support particle sizes. Kinetic control of the biotransformation process becomes apparent for support particle sizes below 0.12-0.2 mm. (C) 1998 Elsevier Science B.V. All rights reserved.