LAUSR

Many ketoses or organic acids can be produced by membrane-associated oxidation with Gluconobacter oxydans. In this study, the oxidation of meso-erythritol to L-erythrulose was investigated with the strain G. oxydans 621HΔupp BP.8, a multideletion strain lacking the genes for eight membrane-bound dehydrogenases. First batch biotransformations with growing cells showed re-consumption of L-erythrulose by G. oxydans 621HΔupp BP.8 in contrast to resting cells. The batch biotransformation with 2.8 g L−1 resting cells of G. oxydans 621HΔupp BP.8 in a DO-controlled stirred-tank bioreactor resulted in 242 g L−1 L-erythrulose with a product yield of 99% (w/w) and a space-time yield of 10 g L−1 h−1. Reaction engineering studies showed substrate excess inhibition as well as product inhibition of G. oxydans 621HΔupp BP.8 in batch biotransformations. In order to overcome substrate inhibition, a continuous membrane bioreactor with full cell retention was applied for meso-erythritol oxidation with resting cells of G. oxydans 621HΔupp BP.8. At a mean hydraulic residence time of 2 h, a space-time yield of 27 g L−1 h−1 L-erythrulose was achieved without changing the product yield of 99% (w/w) resulting in a cell-specific product yield of up to 4.4 gP gX−1 in the steady state. The product concentration (54 g L−1 L-erythrulose) was reduced in the continuous biotransformation process compared with the batch process to avoid product inhibition.