Theo Nash
Ethyl alcohol, also known as ethanol, can be manufactured from starch through the process of fermentation. This biological process involves the conversion of sugars such as glucose, fructose, and sucrose into cellular energy, producing ethanol and carbon dioxide as by-products. The starting material for ethanol production is typically a form of starchy plant material, such as maize, wheat, barley, potatoes, or cassava. The first step in the fermentation process is to break down the complex carbohydrates in these starch sources into simpler ones, such as glucose, through hydrolysis. This is followed by glycolysis, where glucose is converted into pyruvate, and then decarboxylation, where pyruvate is converted into acetaldehyde. Finally, the reduction of acetaldehyde yields ethanol. This process is widely used in industries like brewing and biofuel production, showcasing the versatility of ethanol fermentation.
| Characteristics | Values |
|---|---|
| Starting material | Starchy plant material such as maize (corn), wheat, barley, potatoes, cassava, or rice |
| First step | Breaking down complex carbohydrates into simpler ones, e.g. by heating grain with hot water to extract starch and then warming it with malt |
| Fermentation process | Conversion of starch to glucose through hydrolysis, followed by glycolysis to produce pyruvate, then conversion of pyruvate to acetaldehyde through decarboxylation, and finally reduction of acetaldehyde to ethanol |
| Fermentation conditions | Anaerobic (absence of oxygen) |
| By-products | Heat, carbon dioxide, livestock feed, water, methanol, fuels, fertilizer, alcohols |
| Single-step ethanol production | Combination of raw starch hydrolysis and fermentation |
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What You'll Learn
Starch to glucose conversion
The conversion of starch to glucose is the first step in the process of manufacturing ethyl alcohol through the fermentation of starch. Starch is a complex polysaccharide, a long chain of glucose molecules bonded together. To produce ethanol, we first need to convert this starch into individual glucose units. This process is called hydrolysis and involves the addition of water and an acid or enzyme. The enzyme used in this process is amylase, which is present in grain kernels that have been malted (i.e. germinated). The grain is heated with hot water to extract the starch and then warmed with malt. The amylase in the malt breaks the starch into a simpler carbohydrate called maltose.
The use of carbon-supported heteropoly acid as a catalyst for the selective conversion of starch to glucose has also been explored. Carbon-supported heteropoly acids are environmentally friendly, reusable, and economically viable. One such catalyst is HSiW/C, which is easily separated and reused for multiple cycles. This catalyst has been shown to produce a high yield of glucose (94 wt. %), close to the theoretical yield (111 wt. %) possible from starch.
Another method for the conversion of starch to glucose involves the use of activated carbon. Activated carbon can suppress, but not completely eliminate, the formation of byproducts. One study used simple activated carbon with a trace amount of HCl to produce glucose from cellulose. Another study produced glucose from cassava starch factory residue (CSFR) using a combination of hydrothermal pretreatment followed by enzymatic saccharification.
Once the starch has been converted to glucose, the glucose undergoes glycolysis, a crucial energy-releasing pathway used by all forms of life. During glycolysis, glucose is systematically disassembled through a series of ten reactions in the cytoplasm of cells, without requiring oxygen. The result of these reactions is the formation of pyruvate, which is then converted to acetaldehyde through decarboxylation. Decarboxylation is an anaerobic process that occurs in the absence of oxygen, typically in microbes during fermentation. Finally, acetaldehyde is reduced to ethanol, with NADH and hydrogen ions acting as reducing agents.
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Glucose to pyruvate conversion
The process of converting starch to ethyl alcohol (ethanol) involves four main steps. The first step is the conversion of starch to glucose through hydrolysis. Starch is a polysaccharide, which means it is a long chain of glucose molecules bonded together. To produce ethanol, we first need to convert this starch into individual glucose units. This process is called hydrolysis.
The second step is the conversion of glucose to pyruvate via glycolysis. Glycolysis is a metabolic pathway that converts glucose (C6H12O6) into pyruvate. It is a crucial energy-releasing pathway used by all forms of life. During glycolysis, glucose undergoes a series of ten reactions in the cytoplasm of cells, which do not require oxygen, making glycolysis an anaerobic process. The energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH). The simplified equation representing glycolysis is:
> C6H12O6 → 2C3H4O3 + 2NADH + 2ATP
The third step is the conversion of pyruvate to acetaldehyde through decarboxylation, which occurs under anaerobic conditions. Pyruvate can either enter the mitochondria to undergo oxidative phosphorylation under aerobic conditions or stay in the cytoplasm and be converted to lactate through anaerobic glycolysis. The fourth and final step is the reduction of acetaldehyde to ethanol. This reduction reaction is the heart of the alcohol production phase, turning the somewhat reactive acetaldehyde into the more stable, energy-rich molecule of ethanol (ethyl alcohol).
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Pyruvate to acetaldehyde conversion
The conversion of pyruvate to acetaldehyde is a crucial step in the production of ethanol, commonly known as alcoholic fermentation. This process involves the decarboxylation of pyruvate, catalysed by the enzyme pyruvate decarboxylase. Pyruvate decarboxylase is an essential enzyme in yeast, particularly the genus Saccharomyces, facilitating ethanol production through fermentation. Pyruvate decarboxylase is also present in some fish species, such as goldfish and carp, enabling ethanol fermentation under oxygen-scarce conditions.
During the decarboxylation process, pyruvate decarboxylase converts pyruvate into acetaldehyde and carbon dioxide. This enzyme relies on specific cofactors, namely thiamine pyrophosphate (TPP) and magnesium (Mg2+). The TPP cofactor plays a pivotal role in stabilising carbanion intermediates and facilitating the nucleophilic attack on the keto group of pyruvic acid. The active site of pyruvate decarboxylase includes amino acids such as Glu-51, Glu-477, Asp-444, and Asp-28, which interact with the TPP ring and contribute to its stabilisation.
The decarboxylation of pyruvate is a fundamental step in the transformation of starch into ethyl alcohol. Starch, commonly found in plants like potatoes and corn, is a complex carbohydrate composed of numerous glucose units bonded together. To initiate the process, starch undergoes hydrolysis in the presence of water and an acid or enzyme, breaking down into simpler carbohydrates like maltose. This step is crucial for converting starch into individual glucose units, which then undergo glycolysis to produce pyruvate.
Glycolysis is an energy-releasing pathway utilised by all forms of life, involving a series of ten reactions in the cytoplasm of cells. These reactions occur independently of oxygen, further emphasising the anaerobic nature of the process. The outcome of glycolysis is the formation of pyruvate, setting the stage for the subsequent decarboxylation step.
The decarboxylation of pyruvate, facilitated by pyruvate decarboxylase, bridges the gap between sugar breakdown in glycolysis and the final reduction that yields alcohol. This reduction phase involves the conversion of acetaldehyde to ethanol, with acetaldehyde serving as an electron acceptor. The presence of NADH and hydrogen ions is essential for this reduction reaction, transforming the relatively reactive acetaldehyde into the more stable and energy-rich ethanol molecule.
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Acetaldehyde to ethanol conversion
The conversion of acetaldehyde to ethanol is a crucial step in the production of ethyl alcohol (ethanol) from starch. This process involves the reduction of acetaldehyde, a somewhat reactive molecule, into the more stable and energy-rich ethanol.
The preparation of ethanol from starch can be broken down into four main steps. Firstly, starch is converted to glucose through hydrolysis, which involves breaking down the complex carbohydrate starch into simpler carbohydrates. Secondly, glucose is converted to pyruvate through glycolysis, an energy-releasing pathway used by all forms of life. Thirdly, pyruvate undergoes decarboxylation to become acetaldehyde. This step occurs under anaerobic conditions, without the presence of oxygen. Finally, acetaldehyde is reduced to ethanol through the addition of electrons, facilitated by the reducing agent NADH and hydrogen ions. This reduction reaction is a fundamental process in industries such as brewing and biofuel production.
The equation reflecting the reduction of acetaldehyde to ethanol is as follows:
C_2H_4O + NADH + H^+ → C_2H_5OH + NAD^+
In this equation, C_2H_4O represents acetaldehyde, and C_2H_5OH represents ethanol. NADH and NAD^+ denote the reduced and oxidized forms of the coenzyme nicotinamide adenine dinucleotide, respectively, while H^+ represents a hydrogen ion.
It is important to note that acetaldehyde is also produced by the partial oxidation or dehydrogenation of ethanol. This process involves passing ethanol vapour over a copper-based catalyst at high temperatures, typically between 260–290 °C. While this method was once attractive due to the value of the coproduct hydrogen, it is no longer economically viable.
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Fermentation by-products
Fermentation is a biological process that converts sugars such as glucose, fructose, and sucrose into cellular energy, producing ethanol and carbon dioxide as by-products. The process of preparing ethyl alcohol (ethanol) from starch involves four main steps:
- Conversion of starch to glucose through hydrolysis: Starch is a complex carbohydrate, and the hydrolysis process breaks it down into simpler carbohydrates.
- Conversion of glucose to pyruvate via glycolysis: Glycolysis is an energy-releasing pathway that occurs in the cytoplasm of cells and does not require oxygen, making it an anaerobic process.
- Conversion of pyruvate to acetaldehyde through decarboxylation: This step is an anaerobic process that typically occurs in microbes during fermentation.
- Reduction of acetaldehyde to ethanol: In this final step, acetaldehyde is reduced to ethanol using NADH and hydrogen ions.
The by-products of fermentation depend on the feedstock used. For example, when corn is used, some of it yields by-products such as distillers dried grains with solubles (DDGS), which can be used as feed for livestock. A bushel of corn produces about 18 pounds of DDGS (320 kilograms of DDGS per metric ton of maize). In the case of cassava, fermentation produces by-products such as heat, carbon dioxide, food for livestock, water, methanol, fuels, fertilizer, and alcohols.
It is important to note that the fermentation process must take place in a controlled environment. The vessel used should allow carbon dioxide to escape and prevent outside air from entering, as this could contaminate the brew due to bacteria or mold, and a build-up of carbon dioxide could cause the vessel to rupture.
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