Ethanol production by yeast

Ethanol Production by Yeast Fermentation. Many organisms have been exploited for ethanol production from saccharified starch or other sugars, however, the yeast Saccharomyces cerevisiae still remains the most important species. 

At higher ethanol concentrations the intracellular alcohol interferes with membrane organization, increasing its fluidity and permeability to ions and small metabolites and inhibiting transport of nutrients. Especially Ca and Mg ions are able to increase the plasma membrane stability. It has been demonstrated that incorporation of unsaturated fatty acids and/or sterol(s) as well as proteolipids into cellular membrane of yeasts helps to alleviate ethanol tolerance. For the synthesis of the unsaturated fatty acids the presence of traces of oxygen under fermentation conditions is required. Further to Ca and Mg ions, other trace elements such as Co, Cu, Mn and Zn " and vitamins, e.g. pantothenate, thiamine, riboflavin, nicotinic acid, pyridoxine, biotin, folic acid and inositol, are essential for the growth and ethanol production by yeasts. 

A classic example of a clarification process used in Brazil is that of ethanol production by yeast Saccharomyces cerevisiae. The viability of ethanol production is based on the efficiency of the clarifying operation, which depends on the recycling of yeast to the fermentation reactor and, consequently, the maintenance of high cell concentrations in the culture medium. " Another relevant aspect for the appropriate process performance is the selectivity afforded by centrifugation, keeping bacteria in suspension while yeasts and other larger solids can sediment. The separation occurs due to the density difference between bacteria and yeast, the latter being removed from the supernatant due to their lower density. The partial removal of bacteria, the main contaminant, is a fundamental factor for an effective fermentation. ... 

The mathematical model of batch beer fermentation used is based on the work of Gee and Ramirez (1988). In mathematical modeling for ethanol production by yeast, biomass growth and ethanol formation are assumed to be limited by three sugars, glucose, maltose and maltotriose. The rates of uptake of the three sugars are given by the equations ... 

The study of microbial production of 1,3-propanediol has an interesting history (reviewed by Biebl et al. 1999). It is one of the oldest fermentation products known and has been studied for over 100 years. For a number of years, interest in the fermentation was due to its potential as an outlet for surplus glycerol. Glycerol can be made via a chemical process, or it can be derived from various agricultural fats during the production of fatty acids and soaps. Increased availability of low-cost glycerol might be expected in the future, as it is a by-product of such processes as transesterification of fats for biodiesel production as well as the process for ethanol production by yeast. 

Gong, C.S., C.S. Chen, L.F. Chen. 1993. Pretreatment of sugarcane bagasse hemicellulose hydrolysate for ethanol production by yeast. Appl. Biochem. Biotechnol. 39-40,83-88. 

Bioethanol production by yeasts is widely used for biodegradation of potato. However, yeasts cannot ferment starch directly, and a two-step enzymatic reaction to glucose is necessary. Different potato wastes such as industrial residues, low-grade potatoes, and spoiled potatoes can be used for acetone/ethanol production (Nimcevic et al., 1998). They used whole potato media... 

A good example is the effort to enhance ethanol production in yeast by overexpression of each enzyme in the pathway, but this has failed to increase the rate significantly (Bailey, 1999). 

The pH affects the ethanol production rate, yeast growth, byproduct formation and bacterial contamination. Sugar fermentation by yeast is relatively insensitive... 

Shabtai Y, Chaimonitz S, Freeman A, Katchalski KE, Linder C, Nemas M, Perry M, and Kedem O. Continuous ethanol production by immobilized yeast reactor with membrane pervaporation unit. Biotech. Bioengg. 1991 38 869-876. 

Varela H, Ferrari MD, Belobradjic L et al. (1997) Skin unhairing proteases of Bacillus subtilis production and partial characterization. Biotechnol Lett 19 755-758 Verduyn C, Zomerdijk TPL, van Etijken JHP et al. (1984) Continuous measurement of ethanol production by aerobic yeast suspensions with an enzyme electrode. J Appl Microbiol Biotechnol 19(3) 181-185... 

Some of the first successes of cell-free systems have been in the use of purified enzymes. In 1985, Welch enabled cell-free ethanol production by reconstituting the yeast glycolytic system in vitro [41]. In 1992, Fessner and Walter successfully produced dihydroxyacetone phosphate (DHAP), a key metabolic intermediate in glycolysis and the Calvin cycle [63]. Fessner s system required high-energy phos-phoenolpyruvate (PEP) for ATP generation and did not quantify DHAP. That said, similar to Welch s work, it demonstrated the feasibility of a multienzymatic pathway in vitro. 

Welch, P. and Scopes, R.K. (1985) Studies on cell-free metabolism ethanol production by a yeast glycolytic system... 

During ethanol fermentations, yeasts suffer from various stresses. Ethanol as the major metabolic product of yeast fermentation accumulates in the cell and acts as a potent chemical stress towards the yeast cell. Further, temperatures higher than 35 °C, pH values below 3.5, acetic acid produced either by the yeast itself or by contaminants such as lactic acid bacteria or wild yeasts, lactic acid and osmotic pressure have a negative impact on the ethanol production by Saccharomyces cerevisiae. 

Yu, J., Xuzhang, Tan, T. (2008). Ethanol production by solid state fermentation of sweet sorghum using thermotolerant yeast strain. Fuel Processing Technology, 89, 1056—1059. 

During alcoholic fermentation, Saccharomyces converts grape sugars to alcohol and carbon dioxide as per the Gay-Lussac equation where 1 mole sugar yields 2 moles each of ethanol and carbon dioxide (Fig. 8.1). The rate of ethanol production by Saccharomyces varies with many factors but can reach 8 X lO to 9 X lO molecules of ethanol per yeast cell per second (Foy, 1994b). 

Lee et al., 2008b) and ranked second only to ethanol fermentative production by yeast in large scale (Ramey et al., 2004). 

Biochemistry resulted from the early elucidation of the pathway of enzymatic conversion of glucose to ethanol by yeasts and its relation to carbohydrate metaboHsm in animals. The word enzyme means "in yeast," and the earfler word ferment has an obvious connection. Partly because of the importance of wine and related products and partly because yeasts are relatively easily studied, yeasts and fermentation were important in early scientific development and stiU figure widely in studies of biochemical mechanisms, genetic control, cell characteristics, etc. Fermentation yeast was the first eukaryote to have its genome elucidated. 

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