is the bubble-free aeration of cells with gases or mixtures of gases, whereby membranes with open pores or diffusion membranes are used. When membranes with open pores are used, the culture is in direct contact with the gas from the membrane and the gas-liquid interface is controlled by pressure. In diffusion membranes, the gas first diffuses in the membrane (often silicone) and then migrates into the culture broth. With diffusion, membranes gas transfer rates in the region of those achievable in direct sparging are reached. However, a high gas pressure as well as a large membrane exchange surface is needed. In addition, the membrane aeration is limited to small reactor systems as a large surface of 2 to 3 m of silicone tubing per liter of culture broth is necessary 4. Besides the membrane aeration, surface aeration and the aeration with spargers is possible.