Study on the kinetic characteristics of the asymmetric production of R-(−)-mandelic acid with immobilized Saccharomyces cerevisiae FD11b

The kinetic characteristics of the reduction of phenylglyoxylic acid (PGA) to R-(−)-mandelic acid (R-MA) by the immobilized yeast Saccharomyces cerevisiae FD11b were studied. Compared with the free cells, the immobilized cells showed a higher enantioselectivity for the production of R-MA. However the specific production rate (qp) of R-MA with the immobilized cells was lower than that with free cells due to the effects of the external and internal diffusion. When the agitation rate was above 200 rpm, the effect of the external diffusion could be eliminated. The effects of the granule diameter and the cell density immobilized in the granules on the internal diffusion rate of the substrate and the specific production rate of R-MA (qp) were investigated. A mathematical model considering the internal diffusion and reduction kinetics was developed to analyze the effect of the internal mass transfer. The experimental data and the simulative results showed that the effect of the internal diffusion on qp could be eliminated when the granule diameter was less than 2.2 mm and the cell density immobilized was lower than 66.7 gdw/L-granules. After excluding the effect of the external and internal diffusion, qp reached 0.355 mmol gdw−1 h−1, close to that with free cells (0.360 mmol gdw−1 h−1) in the same reaction system. The optimal pH for the reduction of PGA with immobilized FD11b was 7.5, and the optimal temperature was 32 °C.