Converting sugar to alcohol, or more specifically, glucose to ethanol, is one of the oldest chemical processes known. Glucose is naturally converted directly to ethanol in a variety of organisms through a series of chemical reactions that start with a process called glycolysis and end with fermentation.
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Glycolysis is the first step in the process of converting sugar to ethanol. Glycolysis requires a small input of energy, but results in a net gain of energy for the organism. It is one way an organism can convert sugar into energy. At the end of the glycolysis pathway, a 6-carbon glucose molecule has been converted to two 3-carbon pyruvate molecules. Pyruvate, in the absence of oxygen, can then either be converted to lactate or ethanol, depending upon the organism.
Yeast and other microorganisms can convert the end product of glycolysis, pyruvate, into ethanol in a two-step process. First, pyruvate is decarboxylated; that is, a carbon dioxide molecule from pyruvate is removed, creating acetaldehyde. The enzyme responsible for this reaction is called pyruvate decarboxylase. Acetaldehyde is then converted to ethanol by the actions of another enzyme, alcohol dehydrogenase.
Yeast and certain kinds of bacteria produce ethanol from glucose naturally, in the absence of oxygen. Alcohol-producing bacteria include strains of escherichia and salmonella. Many of these bacteria also generate additional products including other alcohols, organic acids, polyols and various gases. Yeast strains such as Saccharomyces cerevisiae produce ethanol from glucose as well.
Yeast convert glucose to ethanol to obtain energy. Ethanol is actually the waste product of the reaction from the organism's point of view. At a certain point, the amount of ethanol produced by yeast negatively affects the enzymes required for the process and eventually becomes toxic to the yeast cells themselves. Different yeast strains are sensitive at different concentrations of ethanol. The toxic level of ethanol for brewers yeast is about 5 or 6 percent ethanol by volume, whereas ethanol is toxic to wine yeast at a range of about 10 to 15 percent. There are specially cultured strains of yeast that can withstand alcohol levels up to 21 percent alcohol.
Because ethanol has gained popularity as a possible biofuel, natural sources of materials containing high levels of glucose are sought as starting materials for industrial fermentation processes designed to create large scale quantities of ethanol. However, many of these starting materials are too expensive to use in most countries.
Brazil, however, has developed a fuel ethanol program from sugarcane that is both efficient and economical. In the past, Brazil has had to rely heavily on imported oil for fuel, leading to significant economic issues. Due to various economic developments, Brazil has had excess sugarcane stores. Because sugarcane is an excellent source of glucose, and can be produced cheaply and in abundance, Brazil has been able to use it to produce ethanol on an industrial scale.
Many different types of bacteria and yeast microorganisms can be used for fermentation. However, the specific yeast strain Saccharomyces cerevisiae, also known as bakers’ yeast, is the most common organism used. In theory, S. cerevisiae could convert 100 grams of glucose into 51.4g of ethanol and 48.8g of carbon dioxide. However, in practice, because the yeast need some of that glucose for growth, the actual yield is much less.