The fed -batch technique is the most frequently applied operation mode for an effi cient biotechnological production. It is often characterized by the achievement of higher product yields compared to the batch mode. For the case when the fedbatch process is operated under substrate -limiting conditions, substrate uptake rates lower than the maximum uptake capacities of cells occur. Thus, the accumulation of branch -point intermediate compounds of the cell ' s metabolism is more or less avoided. The synthesis of unwanted side -products is not at all or only supported barely with low amounts of substrate. Usually, under substrate -limiting conditions, carbon is provided for the most important products in order to keep the cellular system functional and viable.Such substrate -limiting conditions are easily achieved at the laboratory scale and pilot scale. Most often, the feed solution is introduced through a pipe at the top of the fermenter. A mixing time of several seconds is achieved in stirred tank reactor s ( STR s), which is so fast that the even distribution of the substrate in the reactor is not disturbed. Hence, even up to high cell densities, no accumulation of unwanted byproducts under fed -batch conditions is observed, except that viscosity is very high and oxygen availability becomes insuffi cient in late process stages. However, this is different in biotechnological production in large -scale fermenters. Due to mechanical and economical limitations of the power input, the mixing time of an STR increases 10 -fold or more when reaching liquid volumes of several cubic meters [1 -3] . This leads to a faster conversion by the microorganisms near the feeding zone, while far away from it production of the substrate ceases with increasing cell concentration. The resulting gradients near the feeding point alter the process performance [4,5] . In particular, substrate excess, which thwarts the idea of a fed -batch process, can lead to the synthesis of unwanted byproducts [6] . Due to the movement of cells in the reactor, they are exposed to ongoing oscillating environmental conditions where they fl ip between substrate excess and 16 Biopharmaceutical Production Technology, First Edition. Edited by Ganapathy Subramanian. Glucose metabolism at high density growth of E. coli B and E. coli K: differences in metabolic pathways are