Bioconversion

Bioconversion

	Bioconversion To obtain liquid fuels from sources other than fossil fuel, the procurement or production of fermentable sugar from nature's biomass is the first step. Biomass includes renewable resources such as residues of crops, sugarcane bagasse, forestry residues, etc., which are estimated to yield 40 billion gallons of ethanol/year. The major constituents of biomass are hemicellulose, cellulose and lignin (3:4:3). Hemicellulose hyrolysis gives xylose, a sugar not so easily fermented to ethanol. Glucose is the major end product of cellulose hydrolysis. Lignin protects cellulose and is difficult to degrade enzymatically.
By mechanical methods cellulosic residues are shredded to fine pieces followed by acid or alkali hydrolysis. The hydrolysate is neutralized to yield a substrate to be acted upon by cellulolytic microorganisms to provide a syrup very poor in glucose content. Trichoderma spp. have been known to be good producers of cellulases but some of the commercially available cellulases are not very potent. Trichoderma reesei produces three classes of enzymes: endoglucanases, exo-cellobiohydrolases and B-glucosidaases. It is generally accepted that the first two classes of enzymes act cooperatively and synergistically in deploymerizing cellulose to glucose and oligosaccharides, which are then converted by B-glucosidase to glucose. Cellulose biosynthesis Trichoderma is under multiple regulatory control of induction and repression.
Several attempts have been made to obtain mutants of Trichoderma and later selecting them for high cellulase production by Cetus Corporation, Berkeley, California and U.S. Army Natick Rand D. Laboratories, Natick, Massachusetts, U.S.A. The use of ethanol as a fuel has come to the forefront because admixture of ethnol with gasoline (2:8) has been practised in Brazil and used as an automobile fuel. Brazil has surplus of sugarcane production and can afford to spare sugarcane juice for ethanol production by conventional Saccharomyces cerevisiae fermentation whereas other developing countries cannot afford to do so. A question has often been asked whether biomass could be used for production of ethanol.
To accomplish this, research is required to evaluate the genetic transfer and regulation of genes for glycolysis and hexose monophosphate shunt pathway. Most of the structural genes involved in the reactions between glucose and pyruvate have been identified and mapped. Attempts at cloning of glycolysis genes in E. coli have been made and genes for most of the glycolysis enzymes of yeasts have been cloned. However, the solution to scaling up problems in alcohol production clearly lie in devising a prototype microorganism which overcomes product (ethanol) inhibition and tolerates high temperature. The ethanol sensitivity could also be overcome by continuous dialysis of the culture or by some other continuous process involving immobilization of the yeast and separation, perhaps by magnetic means.
	Another factor in the utilization of biomass is the expensive physical process of shredding the materials and the chemical hydrolysis process which could only be overcome by ethanol production directly by using cellulose and yeast concurrently. Attempts in this direction have to be combined with efforts to concentrate the glucose syrup by inexpensive means so as to enrich the substrate for yeast fermentation.

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