As discussed above, it is generally not the aim of shake flask studies to reach the high cell densities which are commonly reached in bioreactors. In the experiments described here cell concentrations were rather moderate which is due to the lower oxygen transfer rate in shake flasks compared to bioreactors. It was the intention in our experiments to keep the oxygen level high as it is a necessary cofactor in the C-P4H reaction, although for other proteins good production may be achieved also by running the culture into oxygen limitation which has been recently shown for the production of an scFv antibody fragment [23] . Furthermore, the moderate cell densities in our experiments are also due to the use of diluted methanol feed solutions. To avoid long time intervals without any feed and to ensure a feed which is metabolically seen as continuous methanol was fed as a 5% solution, which correspondingly dilutes the medium and results in a lower final cell yield. The use of low-power micropumps instead of microvalves is currently tested, and may provide an improved continuous supply of small liquid amounts.