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14 декабря, 2021
Yeasts are capable of converting sugar into alcohol by a biochemical process called fermentation. The yeasts of primary interest to industrial fermentation of ethanol include Saccharomyces cerevisiae, Saccharomyces uvarum, Schizosaccharomyces pombe, and Kluyveromyces spp. Under anaerobic conditions, yeasts metabolize glucose to ethanol primarily via the Embden — Meyerhof pathway. The Embden-Meyerhof pathway of glucose metabolism is the series of enzymatic reactions in the anaerobic conversion of glucose to lactic acid (or ethanol in this case), resulting in energy in the form of adenosine triphosphate (ATP) [17]. The overall net reaction represented by a stoichiometric equation involves the production of two moles of ethanol from each mole of glucose as shown below. However, the yield attained in practical fermentation attempts does not usually exceed 90-95% of the theoretical value. In this case, the theoretical value (i. e., 100% of yield) means that exactly two moles of ethanol are produced from each mole of glucose input to the fermenter. Therefore, this 100% yield is equivalent to the mass conversion efficiency of 51%, which is defined later in this section. The following stoichiometric equation shows the basic biochemical reaction in the conversion by fermentation of glucose to ethanol, carbon dioxide, and endothermic heat.
C6H12O6 = 2 C2H5OH + 2 CO2
ДИ°298 = 92.3 kJ/mol
Theoretically, the maximum conversion efficiency of glucose to ethanol is 51% on a weight basis, which comes from a stoichiometric calculation of:
2 * (Molecular wt of Ethanol)/(Molecular wt of Glucose) = (2 * 46)/(180)
= 0.51
However, some glucose is inevitably used by the yeast for production of cell mass and for metabolic products other than ethanol, thus reducing the conversion efficiency from its theoretical maximum of 51%. In practice, 40 to 48% of glucose, on a weight basis, is actually converted to ethanol. With a 46% fermentation efficiency, 1,000 kilograms of fermentable sugar would produce about 583 liters of pure ethanol, after taking into account the density of ethanol (specific gravity at 20°C = 0.789), or
(1,000 kg sugar) * (0.46 kg ethanol/kg sugar)/
(0.789 kg ethanol/L) = 583 L ethanol
Conversely, about 1,716 kilograms of fermentable sugar are required to produce 1,000 liters of ethanol, when a 46% mass conversion efficiency is assumed. Mash typically contains between 50 and 100 grams of ethanol per liter (about 5 to 10% by weight) when the fermentation step is complete. This is called distilled mash or stillage, which still contains a large amount of nonfermentable portions of fibers or proteins.