Inhibition, fermentation processes

Carbon dioxide generated by the fermentation process must be removed to help maintain the pH of the solution at pH 7.6—8.0. Carbon dioxide also inhibits the activity of the bacteria. The oxidation reduction potential is kept at 100—200 mV. The ideal temperature in the reactor varies with different strains in the bacteria but generally is 25—35°C. 

Use of biofilm reactors for ethanol production has been investigated to improve the economics and performance of fermentation processes.8 Immobilisation of microbial cells for fermentation has been developed to eliminate inhibition caused by high concentrations of substrate and product, also to enhance productivity and yield of ethanol. Recent work on ethanol production in an immobilised cell reactor (ICR) showed that production of ethanol using Zymomonas mobilis was doubled.9 The immobilised recombinant Z. mobilis was also successfully used with high concentrations of sugar (12%-15%).10... 

However, if the raw materials are contaminated or the composting process is incomplete, unfavorable effects must be expected. Heavy metals may be introduced into the compost with communal waste. To ensure that these do not enter the food chain, authorised limit values must be strictly adhered to. The same is true of organic contaminants (particularly polyaromatic or chlorinated hydrocarbons), the effect of which is extremely complex. If the fermentation process is not satisfactory, putrefaction will occur, the by-products of which (S02, NH3, N02, organic acids, cadaveric alkaloids, etc.) inhibit plant growth and attract pests. 

Semisynthetic Penicillins. However useful they may be, natural penicillins have several drawbacks. They have a relatively narrow activity spectrum, primarily inhibiting Gram-positive bacteria only. They are acid- and lactamase-sensitive, and in a small percentage of patients they cause allergie side effects. All of these limitations could potentially be overcome by molecular modifications during the biosynthesis of these drugs. Unfortunately, however, the fermentation process used in penicillin production is not very flexible and does not permit the incorporation of very many amide side chains into the molecule. 

Fig. 24.6. The degree of inhibition measured in fermentation process of white grape juice spiked with (1) 500 ppb of Paraoxon and (2) 500 ppb of Aldicarb. Reprinted from Ref. [55] with kind permission of Springer Science.

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... 

The use of recombinant microorganisms for cofermentation is one of the most promising approaches in the field of bioethanol production, though their use for large-scale industrial processes still requires fine-tuning of the reliability of the entire process (2). The technical hurdles of cofermentation increase when real biomass hydrolysates have to be fermented. In fact, whatever the biomass pretreatment, the formation of degradation byproducts that could inhibit the fermentation usually requires the addition of a further detoxification step. Therefore, the production of ethanol from hydrolysates should be considered in its entirety, from the optimal pretreatment to the choice of the proper fermentation process. 

In principle, the same carbohydrates and their degradation products formed after hydrolysis of wood can be recovered from sulfite spent liquors. However, this requires complicated and expensive separation techniques. The industrial use of sulfite spent liquor components is mainly limited to fermentation processes. The most common product is ethyl alcohol which is produced from hexose sugars by yeast (Saccharomyces cerevisae) and separated from the mixture by distillation. Even the carbon dioxide formed in the process can be recovered. Other fermentation products, including acetone, n-butanol, and lactic acid, can be produced by certain microorganisms. Because some contaminants, for example, sulfur dioxide, inhibit the growth of the yeast, they must be removed from the liquor prior to the fermentation. 

Space-time yield (STY) sets the capital costs of the production facility and an STY of 100 g I, 1 d 1 has often been mentioned as the minimum for the profitable production of a building block of intermediate complexity. Fermentation processes are often much less productive, due to regulation (inhibition) and toxicity problems. 

Wood is about 65—75% carbohydrate and has been considered as a potential source of ethanol for fuel. The carbohydrate material can be hydrolyzed to monomer sugars, which in turn can be fermented to produce ethanol. However, wood carbohydrates are expensive to hydrolyze. Hydrolysis with acids and enzymes is impeded by the crystalline structure of cellulose. Lignin interferes with processing, and hydrolytic by-products such as furfural, acetic acid, and derivatives of lignin and extractives can inhibit fermentation. Research is still being conducted on wood hydrolysis to develop a process that is economically sound. Furfural is a useful chemical feedstock and results from the dehydration of pentose sugars. It can be obtained in 9 to 10% yield from the dilute acid hydrolysis of hardwoods (75). 

Gryta [150] conducted integration of fermentation process with membrane distillation for the production of ethanol. The removal of by products, which tends to inhibit the yeast productivity, from the fermenting broth by MD process increased the efficiency and productivity of the membrane bioreactor. The ethanol concentration in permeate was 2-6 times higher than that in the fermenting broth. The enrichment coefficient was found to increase with decrease of ethanol concentration in the broth. 

The alteration of fermentation conditions, such as pH, drastically affects product concentrations. Research with C. ljungdahlii has shown that at high pH values (5.5-6), acetate was the dominant product, while at a lower pH (4-4.5), there was a drastic shift towards the production of ethanol. " Inhibition by end products or intermediates is the principal factor that limits metabolic rates and final product concentrations in many fermentation processes. Product inhibition can greatly affect the economics of commercialization. With regards to ethanol inhibition, growth of B. methylotrophicum was inhibited at alcohol concentrations of 5g/L. " However, a recently isolated clostridial strain was shown to tolerate ethanol concentrations up to 78g/L. Efforts have been made to eliminate the drawbacks of inhibition by improvement of bacterial strains to tolerate higher product concentrations and/or by use of novel separation coupled fermentation processes such as pervaporation, extraction, and membrane separation. 

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