Product inhibition, fermentation processes, extractive

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). 

All biochemical reactions are enzyme-mediated. The rate of an enzyme reaction depends on the substrate concentration at the location of the enzyme and thereby on the diffusion rate of a substrate to the enzyme. It is therefore important to permanently obtain an intimate contact between a cell or enzyme and substrate molecules. Additionally, the product generated in the bioreactor has to be extracted because it may under certain conditions inhibit its own production. In some processes there may also be even a prepurification in the bioreactor itself. If living micro-organisms have to be applied, it is necessary to provide sufficient nutrition and respiration gases in case of aerobic fermentation. All other reaction parameters such as temperature, pH-value and reaction time have to be controlled precisely. In many cases (generally with modem processes) the maintenance of microbiological integrity (sterile process) is absolutely mandatory for a successful fermentation. 

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. 

High concentrations of ethanol inhibit the fermentation process, ptirticularly when a fermentative medium with high suhstrate concentration is used, as is the case in the majority of the industrial processes. Considering this, Silva et al. [1] studied a process of fermentation combined with a flash vessel, which selectively extracts ethanol firom the medium as soon as it is produced. These authors have shown that this scheme presents many positive features and better performance than conventional industrial processes [2]. Cardona and Sanchez [3] point out that the reaction-separation integration is a particularly attractive alternative for the intensification of bioethanol production. When bioethanol is removed fiom the culture broth, its inhibition effect on the growth rate is diminished or neutralized. However, the performance of the whole process is significantly influenced by separation unit, and that means that thermod)mamic knowledge of the mixture is required. 

Key to the proposal appears to be the use of new technology being developed at the Oak Ridge National Laboratory under Dr. Brian Davison. The new process is based on a three-phase, biparticle, fluidized-bed bioreactor, in which lactic acid, produced continuously in a fluidized bed of immobilized Lactobacillus delhreuckii, is simultaneously adsorbed onto a solid polyvinylpyridine resin moving countercurrent to the fermenter beer. In this way, the pH can be maintained at the optimum 5.5 and product inhibition of the fermentation is minimized. As a result, fermentation rates have been increased 4- to 10-fold higher than the conventional fermentation process and the acid product can be recovered by methanol extraction. 

The effect of detoxification of the medium by removal of toxic compounds with UF membranes was demonstrated by Boyaval et al. [36] in the fermentation of propionic acid. UF runs led to an eightfold increase in volumetric productivity relative to fed batch experiments. The effectiveness of membrane bioreactors in the lowering of toxicity of the compounds involved in the bioconversion system was demonstrated by Edwards and co-workers [159]. An eightfold increase in the removal of phenoHc compounds from effluents was observed when polyphenoloxidase was immobilized in a capillary poly(ether)sulfone membrane as compared to the use of the free enzyme. Butanol recovery from the fermentation medium with organic solvent extraction or membrane solvent extraction led to similar results, both processes leading to decreased product inhibition. Due to the low toxicity of the extractive solvent used (isopropyl myristate) on Clostridium beyerinckii cells, no protective effect of the membrane was observed. However, precipitates observed in two-Hquid phase extraction were not observed... 

Because ethanol fermentation is inhibited by product, the selective extraction of ethanol during fermentation is also important to improve process performance. Silva et al. (3) have reported that the scheme that combines a fermentor with a vacuum flash vessel presents many positive features and good performance when compared to a conventional process (4). 

Christen et al. (2004) developed an SLM system for the extraction of EtOH during semicontinuous fermentation of Saccharomyces bayanus. The membrane was a porous Teflon sheet as support, soaked with isotridecanol. The removal of EtOH from the cultures led to decreased inhibition and, thus, to gain in conversion of 452 g/1 glucose versus 293 g/1 glucose without extraction. At the same time, the EtOH volumetric productivity was enhanced 2.5 times, due to an improvement of yeast viability, while the substrate conversion yield was maintained above 95% of its theoretical value. In addition to these improvements in the fermentation performances, the process resulted in EtOH purification, since the separation was selective toward microbial cells and carbon substrate and likely selective to mineral ions present in the fermentation broth. For PV, a concentration of EtOH four times greater was obtained in the collected permeates. 

---