Product inhibition, fermentation

The use of membrane separation as a component of continuous fermentation systems is growing in interest. In this case it is possible to increase the productivity of product-inhibited fermentation, for example, by the continuous removal of the low molecular weight products. The enzymatic degradation of cellulose to alcohol, where the microorganisms used are inhibited at an alcohol concentration higher than 12%, might be improved by the use of this concept. 

Many industrial processes are driven forward by separation of at least one of the reaction products. For many of the reactions in aqueous solutit ns one of the products has low solubility in the reaction medium. Its precipitation or evaporation pushes the reaction forward. If, on the other hand, all the products are soluble, an equilibrium is reached and a means of separation has to be applied to recover the product and to recycle the reagents (shift the equilibrium). The economics of such processes is strongly affected by the concentration of the product at equilibrium, which determines the load on the separation and the cost of concentrating the separated product. This could be the case in many processes such as acid production through displacement from its salts production of water-soluble salts, e.g., fertilizers, through metathetic ( double decomposition ) reactions and product-inhibited fermentation,... 

Cydodextrins and their derivatives enhance the solubility of complexed substrates in aqueous media and reduce their toxicity, but they do not damage the microbial cells or the enzymes. As a result, the enzymatic conversion of lipophilic substrates can be intensified (accelerated, or performed at higher substrate concentrations) in industrial processes and in diagnostic reagents, the yield of product-inhibited fermentation can be improved, organic toxic compounds can be tolerated and metabolized by microbial cells at higher concentrations, and compounds in small amounts can be isolated simply and economically from complicated mixtures. 

The batch experiment had neither incoming fresh media nor any product stream leaving the fermentation vessel. A complete experimental set up with a B. Braun Biostat, is shown in the above laboratory experimental set up. The continuous flow of media requires a feed tank and product reservoir. The batch process has many disadvantages such as substrate and product inhibition, whereas in the continuous process the fresh nutrients may remove any toxic by-product formed. 

Example 12.7 Develop a model for the anaerobic batch fermentation of glucose to ethanol and coproduct CO2 using Saccharomyces cerevisiae. The starting mixture contains 10% glucose. The inoculum is 0.0005 w/w. Product inhibition stops cell growth at 14% ethanol. Assume ka = 0 but include the cannibalization of cellular material beginning when the substrate is completely consumed. 

Product inhibition in the glycerol fermentation has been studied in several investigations [12, 38,43-45]. The strongest inhibitor seems to be 3-hydroxy-propionaldehyde. This compound is normally an intracellular intermediate that does not accumulate. However, under conditions of high glycerol excess, it may be excreted into the medium. Whereas K. pneumoniae is able to reduce the accumulated 3-hydroxypropanal further to 1,3-PD, Enterobacter agglomerans is killed by the aldehyde as soon as a concentration of 2.2 g/1 has been reached [44], C. butyricum excretes only very small amounts of 3-hydroxypropanal [46]. 

Apart from public health impacts, residual antimicrobials in animal products can bring about technoeconomic losses in the food processing industry. It has long been known that the presence of some antimicrobial compounds in milk can dramatically affect the production of fermented dairy products such as yogurt, cheese, buttermilk and sour cream (72, 73). As shown in Table 10.2, even minute concentrations of antibiotics in milk can cause inhibition of the growth of commonly used dairy starter cultures (74). 

Aiba, S., Shoda, M. and Nagatani M. Biotechnoi. Bioeng. 10 (1968) 84S. Kinetics of product inhibition in alcohol fermentation. 

Simultaneous L-lactic acid fermentation (by Rhizopus oryzae immobilized in calcium alginate beads) and separation was carried out using a three-phase fluidized-bed bioreactor as a fermenter (F), an external electrodialyzer as a separator, and a pump to recycle the fermentation broth between the bioreactor and the separator. In this way, the experimental specific lactate productivity and yield practically coincided with those obtained in the CaC03-buffered fermentation process (Xuemei et al., 1999), thus confirming the capability of the combined system to alleviate product inhibition without any addition of alkali or alkali salts. It was also shown that the adoption of ED-F for the production of inoculum reduced variability in inoculum quality, thus shortening the length of the lag phase of L-lactate production practically to zero as compared to that observed using an inoculum... 

Fed-batch production of pyruvic acid [CH3COCOOH] from an engineered strain (Escherichia coli YYC202) was optimized by resorting to ED to prevent potential product inhibition in the bioreactor (Zelic et al., 2004). In this way, by continuous separation of pyruvate from the fermentation medium, high values of the pyruvate-to-glucose molar yield (1.78 mol/mol), volumetric productivity (145 kg m 3day ), and pyruvate concentration (79 kg/m3) were achieved by the repeated fed-batch mode. 

Ishizaki, A., Nomura, Y., and Iwahara, M. 1990. Built-in electrodialysis batch culture, a new approach to release of end product inhibition. J. Ferment. Bioeng. 70, 108-113. 

Lactic acid can be produced from a petrochemical route or from fermentation (6,7). The petrochemical route can only produce racemic mixtures of lactic acid, whereas fermentation canproduce optically pure isomer. D(-)-Lactic acid is toxic and must be limited in animal feeds (8), and an optically pure lactic acid is required to produce a specific PLA (9). In addition, fermentation utilizes renewable resources thatmake fermentationmore attractive than the petrochemical route. Extractive fermentation, which couples fermentation with on-line product removal, can eliminate end product inhibition and increase product yield, final product concentration, and reactor productivity. A number of extractive fermentation methods have been reported in the literature, including solvent extraction (10-12), precipitation (13), electrodialysis (14,15), adsorptionby ion-exchange resin (16-18), and an aqueous two-phase system (19-20). 

Aiba S, Shoda M (1969), Reassessment of product inhibition in alcohol fermentation, J. Ferment. Technol. 47 790-794. 

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