Optimal energy dissipation in growing microorganisms and rectification columns

This paper proposes a new point of view in analyzing the optimal Gibbs energy dissipation in growing microorganisms. Small Gibbs energy dissipation in growth would be of biological advantage because less resource is consumed and the biomass yield on these resources could be maximized. It is however not so clear why microorganisms still dissipate considerable amounts of Gibbs energy while growing. Distillation columns are used as a simple qualitative model system in order to gain a qualitative understanding of the question. In both growing microorganisms and continuously operated distillation columns, small energy dissipation values result in small process driving forces and therefore in tentatively slow operation. In microorganisms this entails relatively higher maintenance energy requirements, increasing thereby the resource cost for growth and decreasing the biomass yield. In distillation columns, it results in higher capital and maintenance cost, thereby increasing overall process costs. A simple model is proposed to calculate the biomass yield as a function of Gibbs energy dissipation. It shows that this function goes through a maximum because of maintenance requirements at small dissipations. Experimental data confirm that growing microorganisms dissipate an amount of Gibbs energy that is associated with this maximu

The role of thermodynamics in biochemical engineering

This article is an adapted version of the introductory chapter of a book whose publication is imminent. It bears the title "Biothermodynamics - The role of thermodynamics in biochemical engineering.” The aim of the paper is to give a very short overview of the state of biothermodynamics in an engineering context as reflected in this book. Seen from this perspective, biothermodynamics may be subdivided according to the scale used to formalize the description of the biological system into three large areas: (i) biomolecular thermodynamics (most fundamental scale), (ii) thermodynamics of metabolism (intermediary scale), and (iii) whole-cell thermodynamics ("black-box” description of living entities). In each of these subareas, the main available theoretical approaches and the current and the potential applications are discussed. Biomolecular thermodynamics (i) is especially well developed and is obviously highly pertinent for the development of downstream processing. Its use ought to be encouraged as much as possible. The subarea of thermodynamics of live cells (iii), although scarcely applied in practice, is also expected to enhance bioprocess research and development, particularly in predicting culture performances, for understanding the driving forces for cellular growth, and in developing, monitoring, and controlling cellular cultures. Finally, there is no question that thermodynamic analysis of cellular metabolism (ii) is a promising tool for systems biology and for many other applications, but quite a large research effort is still needed before it may be put to practical us

Optimal energy dissipation in growing microorganisms and rectification columns

This paper proposes a new point of view in analyzing the optimal Gibbs energy dissipation in growing microorganisms. Small Gibbs energy dissipation in growth would be of biological advantage because less resource is consumed and the biomass yield on these resources could be maximized. It is however not so clear why microorganisms still dissipate considerable amounts of Gibbs energy while growing. Distillation columns are used as a simple qualitative model system in order to gain a qualitative understanding of the question. In both growing microorganisms and continuously operated distillation columns, small energy dissipation values result in small process driving forces and therefore in tentatively slow operation. In microorganisms this entails relatively higher maintenance energy requirements, increasing thereby the resource cost for growth and decreasing the biomass yield. In distillation columns, it results in higher capital and maintenance cost, thereby increasing overall process costs. A simple model is proposed to calculate the biomass yield as a function of Gibbs energy dissipation. It shows that this function goes through a maximum because of maintenance requirements at small dissipations. Experimental data confirm that growing microorganisms dissipate an amount of Gibbs energy that is associated with this maximum

Control of yeast fed-batch process through regulation of extracellular ethanol concentration

At high growth rates, the biomass yield of baker's yeast (Saccharomyces cerevisiae) decreases due to the production of ethanol. For this reason, it is standard industrial practice to use a fed-batch process whereby the specific growth rate,μ, is fixed at a level below the point of ethanol production, i.e.,μcrit. Optimally, growth should be maintained atμcrit, but in practice, this is difficult becauseμcrit is dependent upon strain and culture conditions. In this work, growth was maintained at a point just aboveμcrit by regulating ethanol concentration in the bioreactor. The models used for control design are shown, as are the experimental results obtained when this strategy was implemented. This technique should be applicable to all microorganisms that exhibit an "overflow” type metabolis

Biothermodynamics of live cells: a tool for biotechnology and biochemical engineering

The aim of this contribution is to review the application of thermodynamics to live cultures of microbial and other cells and to explore to what extent this may be put to practical use. A major focus is on energy dissipation effects in industrially relevant cultures, both in terms of heat and Gibbs energy dissipation. The experimental techniques for calorimetric measurements in live cultures are reviewed and their use for monitoring and control is discussed. A detailed analysis of the dissipation of Gibbs energy in chemotrophic growth shows that it reflects the entropy production by metabolic processes in the cells and thus also the driving force for growth and metabolism. By splitting metabolism conceptually up into catabolism and biosynthesis, it can be shown that this driving force decreases as the growth yield increases. This relationship is demonstrated by using experimental measurements on a variety of microbial strains. On the basis of these data, several literature correlations were tested as tools for biomass yield prediction. The prediction of other culture performance characteristics, including product yields for biorefinery planning, energy yields for biofuel manufacture, maximum growth rates, maintenance requirements, and threshold concentrations is also briefly reviewe

Optimization of the medium perfusion rate in a packed-bed bioreactor charged with CHO cells

In the present study, the optimal medium perfusion rate to be used for the continuous culture of a recombinant CHO cell line in a packed-bed bioreactor made of Fibra-Cel® disk carriers was determined. A first-generation process had originally been designed with a high perfusion rate, in order to rapidly produce material for pre-clinical and early clinical trials. It was originally operated with a perfusion of 2.6vvd during production phase in order to supply the high cell density (2.5×107cellml−1 of packed-bed) with sufficient fresh medium. In order to improve the economics of this process, a reduction of the medium perfusion rate by −25% and −50% was investigated at small-scale. The best option was then implemented at pilot scale in order to further produce material for clinical trials with an improved second-generation process. With a −25% reduction of the perfusion rate, the volumetric productivity was maintained compared to the first-generation process, but a −30% loss of productivity was obtained when the medium perfusion rate was further reduced to −50% of its original level. The protein quality under reduced perfusion rate conditions was analyzed for purity, N-glycan sialylation level, abundance of dimers or aggregates, and showed that the quality of the final drug substance was comparable to that obtained in reference conditions. Finally, a reduction of −25% medium perfusion was implemented at pilot scale in the second-generation process, which enabled to maintain the same productivity and the same quality of the molecule, while reducing costs of media, material and manpower of the production process. For industrial applications, it is recommended to test whether and how far the perfusion rate can be decreased during the production phase - provided that the product is not sensitive to residence time - with the benefits of reduced cost of goods and to simplify manufacturing operation

Calorimetry and thermodynamic aspects of heterotrophic, mixotrophic, and phototrophic growth

A simple stoichiometric model is proposed linking the biomass yield to the enthalpy and Gibbs energy changes in chemo-heterotrophic, mixotrophic, and photo-autotrophic microbial growth. A comparison with calorimetric experiments on the algae Chlorella vulgaris and Chlorella sorokiniana confirmed the trends but revealed large calorimetric measurement inaccuracies. The calorimetric data on purely photo-autotrophic growth was, however, in fair agreement with calculations. The thermodynamic characteristics of photosynthetic growth, including an estimation of the Gibbs energy dissipation, are compared with similar data for chemotrophic microbe

Endothermic microbial growth. A calorimetric investigation of an extreme case of entropy-driven microbial growth

Life is almost always associated with the generation of heat. Thus far, all chemotrophic life forms that have been studied in calorimeters were found to be exothermic. Certain literature reports have even cast doubt on the existence of endothermic growth, even though thermodynamic principles do not rule it out. The present report describes the first experiments demonstrating the actual existence of chemotrophic life forms that take up heat rather than produce i

CHO immobilization in alginate/poly- l -lysine microcapsules: an understanding of potential and limitations

Microencapsulation offers a unique potential for high cell density, high productivity mammalian cell cultures. However, for successful exploitation there is the need for microcapsules of defined size, properties and mechanical stability. Four types of alginate/poly-l-Lysine microcapsules, containing recombinant CHO cells, have been investigated: (a) 800μm liquid core microcapsules, (b) 500μm liquid core microcapsules, (c) 880μm liquid core microcapsules with a double PLL membrane and (d) 740μm semi-liquid core microcapsules. With encapsulated cells a reduced growth rate was observed, however this was accompanied by a 2-3 fold higher specific production rate of the recombinant protein. Interestingly, the maximal intracapsular cell concentration was only 8.7×107cell mL-1, corresponding to a colonization of 20% of the microcapsule volume. The low level of colonization is unlikely to be due to diffusional limitations since reduction of microcapsule size had no effect. Measurement of cell leaching and mechanical properties showed that liquid core microcapsules are not suitable for continuous long-term cultures (>1month). By contrast semi-liquid core microcapsules were stable over long periods with a constant level of cell colonization (ϕ=3%). This indicates that the alginate in the core plays a predominant role in determining the level of microcapsule colonization. This was confirmed by experiments showing reduced growth rates of batch suspension cultures of CHO cells in medium containing dissolved alginate. Removal of this alginate would therefore be expected to increase microcapsule colonizatio