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

amilton's VisiFerm Arc 120 dissolved oxygen and Polilyte Plus Arc 120 pH sensors are valuable tools when used in a plug flow reactor (PFR) to measure the metabolic response of Bacillus subtilis to oxygen availability. The PFR is connected to a common stirred tank reactor (STR) to simulate conditions in large-scale industrial processes. The impact of inhomogeneities of oxygen, and potentially pH, on Bacillus subtilis cultivations were observed. This study leads to the determination that such inhomogeneities will also exist, and can be measured and corrected, in industrial processes. HTo achieve conditions that mimic those in large scale, determination of the hydrodynamic behavior by ascertaining the residence time distribution is of basic interest. Therefore, a reliable measurement of the pH along the PFR is necessary.During cultivation, the distribution of oxygen and pH along the PFR allows an exact description of the process conditions and enables the user to adopt them to large-scale conditions. The PFR is characterized by high inhomogeneities in the axial direction with respect to oxygen concentration. Since oxygen limitation in industrial fedbatch processes occurs near the feed line, the feed is introduced at the entrance of the PFR. Unlike the first lab-scale two-compartment reactors, the system in this study is equipped with five pH and optical dissolved oxygen (DO) sensors along the height of the PFR (Figure 1).