Influence of microbial cultivation during bio-reduction with in-situ product crystallization (ISPC)

An in-situ product crystallization process (ISPC) was developed for a crystalline product formed during bioreduction coupled with biocatalyst cultivation. The model reaction was the asymmetric reduction of 4-oxoisophorone (OIP) by baker's yeast (Saccharomyces cerevisiae). Yeast cells were cultivated fed-batch in the reactor with an initial cell concentration (C_XI) of 1 gdw.L^-1 to reach a maximum concentration of 30 gdw.L^-1. The desired product, 6R-dihydro-oxoisophorone (DOIP), is also degraded by baker's yeast mainly to an unwanted by-product (4S,6R-actinol); thus, it was removed immediately from the fermentor via an external crystallization unit in the integrated process. The OIP reduction rate was five times higher (≅ 0.33 mmol.gdw^-1.h^-1) with growing cells as compared to the reduction rate with resting cells. During the integrated process, OIP reduction was started when the optimum cell concentration was already reached in the reactor as the substrate (OIP) was found to inhibit cell growth at a constantly high concentration (C_OIP ≥ 55 mM) in the reactor. Final DOIP yield and selectivity were 85% and 99%, respectively. The product crystals were readily recovered, rod-like in shape, and tend to form aggregates. Typical DOIP crystals have an average diameter of 12 μm and length of 20 μm.