Fermentation is a chemical process by which carbohydrates, such as starch and glucose, are broken down anaerobically. Fermentation has many health benefits and is used in the production of alcoholic beverages, bread, yogurt, sauerkraut, apple cider vinegar and kombucha. It is also used in industry to generate ethanol as a source of biofuel.
A Brief History of Fermentation
Over the course of human history, different cultures have produced fermented beverages by leaving grains and fruits in covered containers, without understanding why the recipe worked.
It was not until Joseph Louis Gay-Lussac experimented with a method for keeping grape juice unfermented for a long time that he found yeast was indispensable for alcoholic fermentation. It was Pasteur, however, who demonstrated that yeast is responsible for the transformation of glucose to ethanol in fermented beverages. He also discovered microorganisms that cause milk to sour, which was later found to be the action of bacteria in lactic acid fermentation.
Definition of Fermentation
Fermentation is a metabolic process in which the activity of microorganisms brings about a desirable change to a food or beverage. For example, in the production of alcoholic beverages or acidic dairy products. In this chemical process, molecules such as glucose are broken down under anaerobic conditions.
The word “ferment” stems from the Latin word “fervere,” which means to boil. The science of fermentation is known as zymology, from Greek for “the workings of fermentation”, and is a study of the biochemical process of fermentation and its applications.
Fermentation occurs under anaerobic conditions (absence of oxygen), with the action of microorganisms (yeasts, bacteria and molds) that extract energy from the process.
Some species of yeast, such as Saccharomyces cerevisiae, prefers fermentation to aerobic respiration, even when oxygen is abundant, as long as there is an adequate supply of sugar. Fermentation is not limited to yeast but can also be carried out in muscles, in which the muscles catalyze the conversion of glucose to lactate.
Glycolysis, which is the metabolic pathway that converts glucose to pyruvate, is the first step in fermentation. During the process of glycolysis, one molecule of glucose, a six carbon sugar, breaks down into two pyruvate molecules. This exothermic reaction releases energy for the phosphorylation of ADP to ATP and conversion of NAD+ to NADH.
In the presence of oxygen, the pyruvate may then be oxidized through the tricarboxylic acid cycle, a process known as aerobic respiration. Conversely, the pyruvate may be reduced to alcohol, lactic acid or other products in the absence of oxygen, in the fermentation process.
Types of Fermentation
There are many types of fermentation, distinguished mainly by the end products. Two of the most important and commonly used types are ethanol/alcoholic fermentation and lactic acid fermentation.
Ethanol fermentation is used in the production of alcoholic beverages. Lactic acid fermentation is used to flavor or preserve dairy and vegetables. Lactic acid fermentation also occurs in muscle cells under strenuous activity. In this case, muscles consume energy (ATP) faster than oxygen can be supplied, resulting in an anaerobic environment and thus lactic acid buildup and sore muscles.
There are other types of fermentation such as acetic acid fermentation, acetone-butanol-ethanol fermentation and mixed acid fermentation.
Ethanol fermentation is defined as the biological process that turns sugar (glucose, fructose and sucrose) into ethanol, carbon dioxide and energy.
After the initial glycolysis step that converts one glucose molecule to two pyruvate molecules, the pyruvate molecules further break down into two acetaldehyde and two carbon dioxide molecules, a step catalyzed by pyruvate decarboxylase. Alcohol dehydrogenase then facilitates the conversion of the two acetaldehyde molecules to two ethanol molecules, utilizing the energy and hydrogen from NADH.
Lactic Acid Fermentation
Lactic acid fermentation is another type of fermentation and is described as the metabolic process that transforms sugar into the metabolite lactate and energy. It is the only respiration process that does not produce a gas and occurs in some bacteria (such as lactobacilli) and muscle cells.
This type of fermentation converts the two molecules of pyruvate from glycolysis to two lactic acid molecules and regenerates the NAD+ in the process, continuing the cycle. This redox reaction is catalyzed by lactic acid dehydrogenase.
Lactic acid bacteria can carry out either homolactic fermentation, where lactic acid is the major product, or heterolactic fermentation, where some lactate is further metabolized into ethanol, carbon dioxide and other byproducts.
Importance and Benefits of Fermentation
Rich in probiotics, fermented foods contain microorganisms that can help maintain a healthy gut system, so it can extract nutrients from food more efficiently. They are beneficial for human health in a number of ways.
The probiotics, enzymes and lactic acid in fermented foods can facilitate the intake of vitamins and minerals by the body. Fermentation increases vitamins B and C and enhances folic acid, riboflavin, niacin, thiamin and biotin, making them more accessible for absorption.
Fermentation can also neutralize phytic acid, a substance in grains, nuts, seeds and legumes that causes mineral deficiencies. Phytates, the ionized form of phytic acid, also make starch, proteins and fats less digestible.
The microorganisms, or probiotics, in fermented food can help maintain a healthy gut in producing antibiotic, antiviral, antifungal and antitumor agents, as well as creating an acidic environment that pathogens do not thrive in.
Daily Uses of Fermentation
Fermentation is widely used for the production of alcoholic beverages, for instance, wine from fruit juices and beer from grains. Potatoes, rich in starch, can also be fermented and distilled to make gin and vodka.
Fermentation is also extensively used in bread making. When sugar, yeast, flour and water are combined to form dough, yeast breaks down the sugar and gives off carbon dioxide, which causes the bread to rise. Specialty bread such as sourdough uses both yeast and lactobacilli. This combination gives the dough its stretchy texture and distinctive sour taste.
Lactic acid fermentation is used to flavor or preserve dairy products and vegetables, for example yogurt, sauerkraut, pickles and kimchi.
Acetic acid fermentation can also be used to turn starches and sugars from grains and fruit into sour tasting vinegar and condiments including apple cider vinegar and kombucha.
Industrial Application of Fermentation
Fermentation is used in industry to generate ethanol for the production of biofuel. It is an attractive renewable resource because it originates from feedstocks including grains and crops such as corn, sugar cane, sugar beets and cassava. It can also come from trees, grasses, agricultural and forestry residues.
In the United States, which is the largest ethanol fuel producer, the main feedstock for ethanol fuel is corn given its abundance and low price. Approximately 0.42 liter of ethanol can be produced from one kilogram of corn. The second largest producer is Brazil, and most of its ethanol fuel comes from sugar cane. Most cars in Brazil run on pure ethanol or a blend of gasoline and ethanol.
Fermentation is also capable of producing hydrogen gas, for example in Clostridium pasteurianum, where glucose is converted to butyrate, acetate, carbon dioxide and hydrogen gas. In acetone-butanol-ethanol fermentation, carbohydrates such as starch and glucose are broken down by bacteria to produce acetone, n-butanol and ethanol. This process was developed by Chaim Weizmann as a primary method for making acetone in World War I.
About the Author
Lan Luo has a PhD in Organic Chemistry from University of Chicago and a BS in Chemistry from Worcester Polytechnic Institute. She has years of research experience in asymmetric catalysis, natural product synthesis, drug discovery and drug delivery. She has served as a contributor for Synfacts and a reviewer for journal articles.