On-line control of feeding of medium components to attain high cell density

Abstract

On line control coupled with an expert system was constructed for the control of a fed-batch culture with the aim of achieving a high cell density. During the cultivation, the expert system could monitor the extents of suf®ciencies in the amounts of chemical elements in the medium every 10 min, and suggest a modi®cation to the feeding control policy if the amount of a particular component was inadequate for cell growth. However, we often encounter such a kind of cultivations in which some particular carbon source such as glucose and ethanol should be controlled at low concentration in many culture processes, because the excess feeding might cause the growth inhibition by its own accumulation in the culture broth, or the biproduct accumulation like as lactate or acetate, unsuitable substances for smoothed growth. In this study, we developed an online control system based on production rules which managed the glucose feed rate from DO signal. The online control was carried out by the control computer connected with the other computer for expert system, because a relatively long time (several minutes) was needed for the inference of the expert system and the in¯uence of the starvation of carbon source on the cell growth is not negligible even in several minutes. The on-line control system with expert system was applied to the production of cell mass of E. coli W3110 and the ®nal cell concentration reached 137 g-dry cell weight/l. IntroductionIn many bioprocesses, a high cell growth is advantageous for the production of bioproducts [1,2]. In order to achieve a high cell density, the cell growth activity must be kept at a high level for a relatively long period during the cultivation. However, any of several factors, for example, the insuf®ciency of a medium component, the production of a growth-inhibitory substance, a pH change caused by organic acid production, or mechanical damage to cells resulting from strong shear stress, might prevent the longterm continuation of a high level of activity. Recently, several studies on high cell density cultivations have been reported [3±8], and much attention has been focused on substrate supply, that is, the provision of suf®cient amounts of nutrients for cell growth, with even minor mineral components being supplied carefully, and the accumulations of acetate, lactate and the other organic acids that might inhibit cellular growth have been avoided. However, these operations were carried out on the basis of manual supervision, not automatically. The pH and DO values were also kept at suitable levels for cell growth.In actual bioprocesses, it is often the case that manual operation must be employed several times during a cultivation in order to avoid inhibitory factors such as those mentioned above, because even if an operation overcomes a particular growth limiting factor, other inhibitory factors might appear in the culture process soon after. Therefore, a supervisory control system capable of maintaining suitable culture conditions while forestalling the emergence of inhibito...

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