M. Thoma

Effect of the dilution rate on the mode of oscillation in continuous cultures of Saccharomyces cerevisiae.

The growth properties of the asymmetric budding yeast Saccharomyces cerevisiae were analysed during spontaneous oscillations in continuous cultures at varying dilution rates D. The length of the oscillation period changed between 1.4 and 14 h in response to the decrease of dilution rate from 0.15 to 0.05 h-1. The distribution of parent and daughter cells in the population was determined microscopically after staining the bud scars and DNA. Most of the data obtained fits a theoretical population balance model assuming two-classes of subpopulations and integer ratios between the generation times of both classes. Some data has to be described by an extended population model assuming there is one parent and two daughter cell classes. How changes of dilution rate may cause an accidental switch of the mode of oscillation is demonstrated. Glucose consumption and metabolite production were measured off-line by enzymatic methods and gas exchange was monitored on-line. All these data of one period point to internal and external signals responsible for the synchronisation of the cell cycle.

Constant dissolved oxygen concentrations in cephalosporin C fermentation: Applicability of different controllers and effect on fermentation parameters

SummaryThe behaviour and applicability of several controllers for maintaining a constant dissolved oxygen concentration (DO) during the cephalosporin C production with Cephalosporium acremonium in a laboratory fermentor is described. The process controllers were realized on a MC 68000 based process computer using the real-time language PEARL. The discrete signum integral controller showed the best control action. In addition some derived fermentation data were calculated on-line by the process computer.The results obtained by comparison of fermentations carried out at DO between 10% and 40% saturation during ideophase indicate that high DO leads to a high specific production rate for cephalosporin C and a low specific production rate for penicillin N and vice versa. In the range of DO investigated the production of deacetyl and deacetoxy cephalosporin C is not affected by DO. A direct correlation between DO and the yield coefficients YP/S and YP/X could be established. The yield coefficient YP/O for cephalosporin C is constant in the DO range from 10%–40%.

Oxygen, pH value, and carbon source induced changes of the mode of oscillation in synchronous continuous culture of Saccharomyces cerevisiae

Oscillations of measured process parameters occur in continuous cultures of Saccharomyces cerevisiae owing to a partial synchronization of budding. Intentional changes of the oxygen concentration, pH value, and carbon source cause effects on the period length similar to those known from variations of the dilution rate. The generation times of parent and daughter cells frequently differ in synchronous culture. To analyze the oscillation the term mode IJ of oscillation is used, which is defined as the ratio IJ of the generation times of parent and daughter cells. When the dissolved oxygen concentration was reduced to zero, the mode of oscillation changed within two periods from mode 12 to mode 11, caused by a decrease of the generation time of daughter cells and an increase of that of the parent cells. When the pH value was slowly reduced from 5.0 to 3.9, a change from mode 112 to mode 13 was observed. Mode 13, representing one parent and three daughter cell populations (the start of budding of each of the three being delayed by one period), denotes an elongated generation time of the daughter cells compared to mode 112, marked by one parent and two different daughter cell classes. When the carbon source galactose was replaced by glucose a mode change from mode 12 to mode 11 was observed. This alteration of the mode was found to be dependent on the status of the cell cycle at the time when the carbon source is changed. The population distribution in batch cultures with glucose or galactose as a substrate was analysed by dyeing the DNA and counting the bud scars. Galactose provoked higher growth rates for the older cells. According to the model for stationary synchronous growth parameters like DO, pH value or the type of carbon source can be varied within a certain range without effecting the period length. If the variation imposes a certain stress, the culture switches to a new mode. These kinds of parameters therefore provide selective measures to influence the period lengths and the modes of oscillation.

Model development, parameter identification, and simulation of the SCP production process in air lift tower bioreactors with external loop—I: Generalized distributed parameter model

Oxygen, substrate and cell mass balance in the liquid phase as well as oxygen and CO2 in the gas phase are used to described air lift tower reactors. The validity of dispersion models in both of the phases and of Monod kinetics with regard to oxygen and substrate were assumed. It is necessary to consider the longitudinal variations of pressure and model parameters to obtain a good coordination between simulated and measured data.

Modelling of the enzymatic hydrolysis of cellobiose and cellulose by a complex enzyme mixture of Trichoderma reesei QM 9414

SummaryThe enzymatic hydrolysis of cellobiose and cellulose by the cell-free culture filtrate of Trichoderma reesei QM 9414 was investigated. The concentrations of cellobiose and glucose were measured as a function of time for different initial concentrations of cellobiose. It was not possible to describe these concentration variations by a model which considers only the cellobiase hydrolysis with competitive and noncompetitive substrate and product inhibition; it is necessary that the endo-β-1.4-glucanase with competitive product inhibition is also taken into account.The enzymatic hydrolysis of cellulose (Avicel) was described with a mathematical model by using the results of the decomposition of cellobiose by the same enzyme mixture.the identified model parameters are presented. A sensitivity analysis of the parameter was carried out also.

A model for penicillin production with and without temperature shift after the growth phase

SummaryA strain of Penicillium chrysogenum producting about 8 g/l of penicillin V, was cultivated in a 10-1 bioreactor. Under carbon (C)-limitation during the production phase a glucose/ammonium sulphate mixture was fed using microprocessor control. When the temperature was shifted from 25° C to 30° C at the end of the active growth phase, the specific penicillin production rate was increased by 30%, while the yield remained constant. Maximal productivity without sporulation was obtained when the net growth rate of the active (respiring and producing) biomass, estimated by measuring the respiration rate under defined conditions, was equal to or higher than 0.004 h−1. A model was developed for penicillin fermentation during C-limitation possessing the following properties: (1) the model is based on ordinary differential equations; (2) the influence of different nutrients is considered; (3) the model recognizes two cell types (active and inactive); (4) the model describes the influence of a temperature shift at the end of the vigorous growth phase.

Model development, parameter identification, and simulation of SCP production processes in air lift tower reactors with external loop—II: Extended culture modelling and quasi steady state parameter identification

Abstract Yeast was cultivated in extended culture in a bench-scale 275 cm high air lift tower reactor 15 cm dia. with an external loop. Longitudinal dissolved oxygen concentration profiles, substrate and cell mass concentrations in the medium, O2 and CO2 concentrations in the gas phase, as well as gas flow rates and liquid recirculation rates were measured. A distributed parameter model was used to describe the cultivation process variation along the column, cell mass, substrate and oxygen balances in the medium, O2 and CO2 balances in the gas phase, variation of the volumetric mass transfer coefficient along the column due to bubble coalescence, as well as double substrate Monod kinetics. Based on simulation runs it was assumed that under non limited and oxygen transfer limited growth conditions, the cell mass and substrate concentrations are uniform in the reactor. The simulation was carried out by a hybrid computer. The unknown model parameters (volumetric mass transfer coefficient at the gas entrance, kLaE, and coalescence factor KST) and two kinetic parameter ROmax and KO were identified by means of experimental results with quasi steady state simulation methods.

Dynamics of growth in batch and continuous cultures of Saccharomyces cerevisiae during shifts from aerobiosis to anaerobiosis and reverse

SummarySaccharomycescerevisiae H 1022 was cultivated in batch and continuous culture on a glucose substrate. The yeast was subjected to a sudden change from aerobic to anaerobic growth conditions by switching the inlet-gas stream from air to dinitrogen. The dynamics of growth and product formation during the periods of adaptation were studied. A structured growth model based on Monod-Blackman-kinetics was applied to simulate these shift-experiments. The immediate switch of the yeast to maximum anaerobic growth and ethanol production predicted by this model was confirmed very well in the chemostat-experiments. However, a slow adaptation was evident for the switch from anaerobiosis back to aerobiosis.

The adaptive control of an airlift tower loop fermenter : an application of model-based control employing a sophisticated process model

Abstract The contribution focuses on the application of model-based control techniques to a biotechnological process described by a complicated reactor model. Treated is the control of the aeration rate during the batch production of single cellular protein in an airlift tower loop reactor. To obtain an accurate description of this process, a spatially distributed model consisting of coupled algebraic and ordinary differential equations is employed. The first technique discussed is an adaptive strategy known as the open-loop-feedback-optimal (OLFO) control. To further the practical applicability of this strategy, methods are presented for simulation and optimization of the process in a computationally very efficient manner. At the same time, the spatially distributed nature of the process model is retained in order to ensure sufficient accuracy in performing the tasks of the control algorithm. In a second technique, a time-variant, adaptive feedback control law is suggested as an alternative means to optimally control the treated process. The feedback law is base on a criterion formulated from the necessary conditions of optimality according to variational principles and the results of a sensitivity analysis with respect to the model parameters.

Growth of Chaetomium cellulolyticum on glucose substrate

SummaryThe batch cultivation of Chaetomium cellulyticum on glucose was described by a structured model based on the experimental investigations of Hecht et al. (1982) and Rosen (1982). The Monod model did not give satisfactory results. The structured model takes into account the adaptation of the transport system of cells to the substrate supply. The model parameters identified are presented. The agreement between calculated and measured courses of cell growth is excellent.