Nutrient broth/PEG200/TritonX114/Tween80/Chloroform microemulsion as a reservoir of solubilized sitosterol for biotransformation to androstenedione

Microemulsions (ME) can act as a reservoir of solubilized hydrophobic substrates. The biotransformation of hydrophobic sitosterol to androstenedione (AD) with MEs prepared from nutrient broth and PEG 200 (1:1) as aqueous phase, 40 g/l sitosterol dissolved in chloroform as organic phase, Triton X114 and Tween 80 (1:1) as surfactant phase, was investigated. The phase behavior of this system was studied for ten different ratios(w/w), 10:0, 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9 and 0:10 of the organic phase and surfactant at 30 °C. A pseudoternary phase diagram was constructed to demarcate the region giving stable MEs. The maximum solubility of sitosterol in ME medium was observed to be 8 g/l, which is 3 orders of magnitude higher than the reported sitosterol solubility of 2–4 mg/l in aqueous medium. The ME medium was used for biotransformation studies and a comparative result has been reported. Transmission electron microscopy of cells grown in ME having oil, surfactant and aqueous phase in the ratio of 6:14:80 showed a weakened cell wall structure that permitted production of 465.86 mg/l AD.