Lena Savoie
Alcoholic fermentation, a process known to humanity for over 10,000 years, involves the conversion of sugars into ethanol and carbon dioxide. The biochemical process is complex, but the end products are simple: ethanol, carbon dioxide, and a variety of other compounds. In alphabetical order, the products of alcoholic fermentation include acetaldehyde, carbon dioxide, ethanol, esters, glycerol, higher alcohols, and succinic acid.
| Characteristics | Values |
|---|---|
| Alcoholic fermentation process | Microorganisms consume organic compounds under anaerobic conditions to produce gas and ethanol |
| The conversion of sugar to alcohol is achieved through yeast metabolism | |
| Yeast breaks down molecules of pyruvate, leading to the metabolism of glucose into carbon dioxide and alcohol | |
| Alcoholic fermentation occurs within the cytoplasm | |
| Alcoholic fermentation improves the nutritional value of food and increases its shelf life | |
| Alcoholic fermentation produces ethanol, carbon dioxide, glycerol, succinic acid, and amylic alcohol | |
| Alcoholic fermentation produces bread, ethanol for fuel, and pharmaceutical and medical products | |
| Alcoholic fermentation produces wine, beer, mead, cider, and perry |
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What You'll Learn
Carbon dioxide
The production of carbon dioxide during alcoholic fermentation has several important applications. One example is in bread-making, where carbon dioxide forms bubbles in the dough, causing it to rise. Additionally, the observation of bubbles during fermentation led to the process being named "fermentation" from the Latin word "fervere", which means "to boil".
Furthermore, carbon dioxide production during alcoholic fermentation has implications for the production of alcoholic beverages. For instance, wine producers have traditionally used their feet to soften and grind grapes before leaving the mixture to stand, inadvertently transferring microorganisms from their feet and facilitating the fermentation process. This process results in the production of alcohol and carbon dioxide, which was later understood by the French chemist Louis Pasteur to be caused by the action of living yeast.
In addition to its role in food production and the naming of the fermentation process, carbon dioxide produced during alcoholic fermentation has other applications. For example, ethanol can be produced from cassava, and the carbon dioxide generated as a byproduct can be used as food for livestock or to create fertilizer.
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Ethanol
Alcoholic fermentation, also called ethanol 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. Yeast organisms consume sugars and produce ethanol and carbon dioxide as waste products. The carbon dioxide forms bubbles, expanding the dough to a foam—this is what causes bread dough to rise.
The process of alcoholic fermentation can be broadly divided into two main parts: glycolysis and fermentation. In glycolysis, glucose is broken down into two pyruvate molecules. In fermentation, the pyruvate molecules are converted into two molecules of carbon dioxide and two ethanol molecules. This process occurs within the cytoplasm.
The French chemist Louis Pasteur was the first to demonstrate that fermented beverages are the result of living yeast transforming glucose into ethanol. He also showed that only microorganisms can convert sugars into alcohol from grape juice and that this process occurs in the absence of oxygen.
In addition to ethanol, other compounds are generated during alcoholic fermentation, such as esters, higher alcohols, glycerol, succinic acid, and 2,3-butanediol. These by-products can be used as food for livestock, fertilizer, and fuel.
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Esters
The most significant esters found in beer are isoamyl acetate, with a banana or pear-like aroma, ethyl acetate, which has a light fruity or solvent-like smell, ethyl caprylate and ethyl caproate, both with apple-like aromas, and phenylethyl acetate, which has a rose or honey aroma. The ester isoamyl acetate is also found in wine, where it contributes to a fruity or flowery character. The shape of the fermentation vessel can impact ester production, with tall, narrow fermenters tending to produce lower levels of esters than shallow, open fermentation vessels. Brewers must take these factors into account when brewing specific styles and brands of beer to achieve a consistent ester profile and the desired flavour and aroma.
The rate of ester formation is dependent on the concentration of the two co-substrates (the acyl-coenzyme A (CoA) component and the alcohol) and the activity of the enzymes involved in their synthesis and hydrolysis. The genes involved in ester synthesis have been identified, and it has been found that the expression levels of ATF1 and ATF2 genes affect the production of ethyl acetate and isoamyl acetate. Overexpression of alleles of ATF1 and ATF2 derived from different yeast strains led to different production rates for individual esters, indicating that differences in aroma profiles may be due to specific mutations in their ATF genes.
The physiological state of the yeast can also influence ester production, with higher fermentation temperatures leading to increased ester production. The composition of the wort can also have an impact, with high dissolved oxygen levels tending to inhibit ester formation, while high sugar concentrations increase ester levels.
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Glycerol
The formation of glycerol is undesirable in distilleries as it reduces the efficiency of fermentation. However, it is important for maintaining the metabolic balance of yeast cells. The amount of glycerol produced is influenced by factors such as feeding time, temperature, and yeast concentration.
Optimising the yield of glycerol can benefit the flavour and ethanol reduction in fermented beverages. This can be achieved through fermentation optimisation or yeast genetic modification. For example, screening yeast strains and optimising fermentation parameters can increase the glycerol yield to a maximum of 14 g/L.
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Succinic acid
Alcoholic fermentation is a biological process that has been known to humanity for over 10,000 years. It involves the conversion of sugar sources into ethanol and carbon dioxide through yeast metabolism. This process is used in the production of alcoholic beverages, bread, ethanol fuel, and more. One of the products of alcoholic fermentation is succinic acid.
The metabolic pathway of succinic acid fermentation can be genetically engineered to optimize the production of succinic acid. This can be achieved by using the oxidative section of the tricarboxylic acid cycle (TCA) under anaerobic conditions or by utilizing the glyoxylate bypass. These methods can result in a mass-based succinic acid yield of up to 1.12 g/g, which is higher than the theoretical maximum yield where more succinic acid is formed than glucose consumed.
Microbial production of succinic acid can be performed using wild bacteria or genetically modified organisms. Examples of wild bacteria include Actinobacillus succinogenes, Mannheimia succiniciproducens, and Anaerobiospirillum succiniciproducens. Genetically modified organisms such as Escherichia coli, Corynebacterium glutamicum, and Saccharomyces cerevisiae can also be used. By understanding the central carbon metabolism of these organisms, the maximum obtainable yield of succinic acid can be determined.
Overall, succinic acid is an important product of alcoholic fermentation, contributing to the stimulation of gastric acid output in fermented products and playing a role in optimizing the yield of succinic acid through metabolic pathway engineering and the use of various microorganisms.
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Frequently asked questions
Alcoholic fermentation produces ethanol, carbon dioxide, acetaldehyde, and other compounds such as esters, higher alcohols, glycerol, succinic acid, and 2,3-butanediol.
Alcoholic fermentation is a biological process where sugars such as glucose, fructose, and sucrose are converted into ethanol and carbon dioxide by yeast.
Some examples of products that undergo alcoholic fermentation include wine, beer, cider, perry, mead, brandy, and bread.
