This article focuses on the recent world-wide advances of the biotechnology of the increasingly important fed-batch (or semi-batch) cultivation technique used in microbial processes. The history and characteristics of the fed-batch technique in microbial reactions are reviewed and examples of those fed-batch operations are cited which have greatly increased the productivity of microbial conversions in comparison to conventional batch operation; the various fed-batch techniques will be classified according to the mode of nutrient-feeding. Theoretical mathematical models are compared with experimental results. Fed-batch cultures with automatic feedback control are discussed in detail. Cultivation of high microbial cell concentrations, repeated fed-batch operations, and the start-up of microbial processes are described and, finally, some future prospects of fed-batch techniques in microbial processes are discussed.

Fed-batch techniques in microbial processes

A mixed culture derived from soil and activated sludge organisms was used to degrade phenol which was inhibitory to microorganisms at higher concentrations. The purpose of the experiments was to determine the kinetic parameters governing growth of the organisms by measuring growth rates in batch culture. To maintain a constant inoculum for the experiments inoculum was taken from a continuously operating continuous culture. Two populations were studied corresponding to two separate residence times in the continuous culture apparatus. One contained predominantly filamentous organisms, the other nonfilamentous. Five kinetic models were applied to the data and the best kinetic parameters for each model were determined by nonlinear least squares techniques. The models were then evaluated for best relative fit to the data. No significant differences were found between the models on the basis of fit and so a choice was made on the grounds of simplicity. A model proposed by Haldane was chosen as the best. No function however gave a satisfactory fit at the highest growth rates obtained. This experimental maximum in the plot of growth rate against substrate concentration was very sharp.

Mixed culture biooxidation of phenol. I. Determination of kinetic parameters

Publisher Summary This chapter discusses the microbial production of gibberellins. Gibberellins (GAS), a large family of closely related diterpenoid acids biologically derived from tetracyclic diterpenoid hydrocarbon, represent an important group of potent plant growth hormones. This chapter discusses the chemistry and biosynthetic pathways of GAS. It illustrates the mode of action, structure–activity relationship of GAS. The uses of GA are also discussed. GA is a high-value industrially important biochemical in the international market, depending on the purity and potency. Therefore, its use at present is limited to high-premium crops. The industrial process currently used for fermentative production of GA3 is based on submerged fermentation (SmF) techniques. This chapter critically examines the existing as well as the potential techniques of fermentation and product recovery in the production of GAS.

Microbial Production of Gibberellins: State of the Art

Modeling, optimization and control of semi-batch bioreactors

Inhibitory substrate levels are common in industrial fermentations and in biological waste-water treatment of many industrial wastes. Continuous microbial cultures are unstable to certain disturbances, such as shock loading by inhibitory substrates. Two feedback proportional control strategies are analyzed and compared for a simple model culture assumed represent able by the culture concentrations of biomass and a single rate-limiting and growth-limiting nutrient (substrate). One control strategy, the well known turbidostat, consists of adjusting culture holding time (e.g., by flow rate adjustment) in response to deviations in turbidity or some other measure of culture biomass concentration. The other control strategy is to adjust holding time in response to deviations in limiting nutrient concentrations in the culture. This second control strategy, termed the nutristat, can be superior to the turbidostat in many applications. The sign and magnitude of the dimensionless group {(X/YD)[dμ/dS]s}, is shown to be an important determinant, in the behavior of the open loop and the two closed loop processes. This characteristic group is positive when the specific growth rate is increased by increases in the nutrient concentration, zero when the growth rate is unaffected by the nutrient concentration, and negative in the presence of nutrient or substrate inhibition.

The effects of process modifications and of modeling assumptions on the control of the process are discussed and more sophisticated control schemes are also proposed.

Dynamics and control of continuous microbial propagators to subject substrate inhibition

Extended culture, a special type of semicontinuous culture, permits prolonged maintenance of a constant or programmed environment in a growing culture by a controlled addition of one or more substrates. Differences between extended culture and continuous culture data are a measure of differences in the properties of cell populations with different cell age distributions but identical steady-state environments. Both extended culture and continuous culture were used to study the growth kinetics of Candida utilis (ATCC 9226) under conditions of substrate inhibition at controlled concentrations of sodium acetate in a carbon-limited mineral salts medium supplemented with 0.01 g/1 yeast extract. Acetate concentrations ranged from 1.2 g/l to 10.8 g/l (expressed as acetic acid), while yeast concentrations varied from 0.3 to 7.8 (g dry cells)/1. Rate parameters such as growth yields (Y), specific growth rates (μ), and linear growth rates (K), were calculated by computer from the data and theory presented herein. Specific growth rates as high as 0.54/hr were observed, although extended culture growth was more nearly linear than exponential in these experiments. Growth yields usually varied between 0.2 and 0.4 (g dry cells)/(g acetate), although values were as high as 0.8 for a brief period during one experiment. Growth yields at a given acetate concentration were correlated by an equation of the form 1/Y = 1/YG + m/μ. A maintenance coefficient (m) of 0.17 (g acetate)/(g dry cell-hr) was observed at acetate concentrations of 4.5 and 10. g/1. A typical maximum growth yield (YG) of 0.51 (g dry cell)/(g acetate) was obtained at 4.5 g/1 acetate, but an unusually high YG of 1.33 was found at 10. g/1 acetate. Oxygen uptake measurements are compared with these cell yield measurements. Linear growth rates in expended culture were correlated by the equation K = 0.89–0.70 (S/S0) where K has units of (g dry cell)/(l-hr), S is the instantaneous acetate concentration, and S0 is the initial acetate concentration. The extended culture kinetic data are shown to be substantially different from continuous culture kinetic data. Reason for these differences are discussed in light of diffrences in the cell age distributions, as well as possible differences in experimental conditions.

A. Y. Noojin

Papers

Extended culture: the growth of Candida utilis at controlled acetate concentrations.