In search of an optimal system for extractive fermentation of an organic acid, fermentations of lactic, citric, and acetic acids were studied in a broth saturated with an organic solvent and in a broth-organic solvent two-liquid phase system. Among the 13 solvents tested, on ly long-chain hydrocarbons, pefluorodecalin, and methyl oleate were found to be nontoxic to the cells of L. delbrueckii in two-liquid phase systems. However, a distinction was made between toxicity due to dissolved solvent molecules and that due to a separate solvent phase. Thus, cells of L. delbrueckii, which were not inhibited in n-dodecanol-saturated broth, but inhibited in a two-phase system, were successfilly protected by immobilization in carrageenan beads. The same solvent however, partially inhibited the growth of A. niger and A. aceti when present at saturated levels in the broth. The effect of tridodecylamine on lactic acid fermentation in a two-liquid phase system was investigated in view of enhancing the acid extraction. Since extraction of an acid is favored by low pH, an extractive fermentation of an acid is practical for microorganisms that can grow at pH < pK,, such as A. aceti and A. niger. The amine was found to be toxic to L. delbrueckii above a concentration of 222 mM, and at a level of 5O%v, it was deleterious to all cell microorganisms tested, either free or gel entrapped.

An intensive and systematic investigation of the reductive transformation of androstenedione (AD) to testosterone by Saccharomyces cerevisiae was undertaken in the presence of natural and chemically modified cyclodextrins (CD). The bioconversion was significantly larger in the presence of/~-and y-CD and hydroxypropyl-fl-CD but only slight in the presence of a-CD and dimethyl-and trimethyl-fl-CD. The performance of the various cyclodextrin media was interpreted in the light of the measured phase solubility diagrams of AD. Further investigation focused on biotransformation of the/%CD-androstenedione complex, the formation of which was studied by differential scanning calorimetry and X-ray powder diffractometry and stoichiometry determined by 1H-nuclear magnetic resonance. A mechanism whereby CDs reduce the effective inhibitory concentrations of substrate and product as well as facilitate transport of the complexed substrate through the yeast cell wall has been suggested for the CD-promoted biotransformation.

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