Fermentation- Principle, Types, Applications, Limitations

Fermentation is a process in which sugars are transformed into a new product through chemical reactions carried out by microorganisms. Fermentation is one of the oldest food processing technologies, dating back to at least 7000 BC. Since ancient times, humans have taken advantage of the natural fermentation process to develop many products, including foods, beverages, medicines, and fuels.

Fermentation has played a vital role in human civilization, as it has enabled people to preserve food, enhance its nutritional value and flavor, produce alcoholic drinks, and obtain useful substances. Fermentation can be considered the first use of biotechnology, which is the use of living systems or organisms to develop a technology.

The history of fermentation is closely linked to the history of human culture and science. Some of the earliest civilizations, such as the Sumerians and Egyptians, used fermentation to produce bread, wine, beer, cheese, and vinegar. They did not have the scientific knowledge to explain how fermentation worked, but they observed its effects and attributed them to divine intervention. Fermentation was often associated with religious rituals and ceremonies.

It was not until the nineteenth century that the scientific basis of fermentation was discovered by Louis Pasteur, who proved that fermentation is caused by microorganisms such as bacteria and yeast. He also found out that different microorganisms produce different types of fermentation and different end products. Pasteur`s work laid the foundation for modern microbiology and biotechnology.

Today, fermentation is still widely used in various industries and fields. Fermentation can produce a variety of products, such as ethanol, lactic acid, acetic acid, butyric acid, propionic acid, diacetyl, 2,3-butanediol, antibiotics, hormones, vaccines, enzymes, proteins, biofuels, biopolymers, biosurfactants, and bioremediation agents. Fermentation can also improve the quality and safety of food by inhibiting spoilage microorganisms and producing beneficial compounds.

Fermentation is a fascinating and versatile process that has shaped human history and culture for thousands of years. In this article, we will explore the principle of fermentation and its energy production, the types of fermentation and their processes, the applications of fermentation in medicine and the food industry, and other applications of fermentation including waste management and bioremediation. We will also discuss the limitations of fermentation including low scale production and possibilities of contamination.

Fermentation is a natural process that involves the conversion of organic substances into simpler compounds by the action of microorganisms, such as bacteria, yeasts, and molds. Fermentation usually occurs in the absence of oxygen and produces energy (ATP) and various end products, such as alcohols, acids, gases, and other substances. Fermentation can be used for different purposes, such as food production and preservation, industrial biotechnology, medicine, and biofuel generation.

Some examples of fermentation are:

• Yeast fermenting the sugars in grapes to produce wine
• Lactic acid bacteria fermenting the lactose in milk to produce yogurt and cheese
• Acetobacter fermenting the ethanol in wine to produce vinegar
• Clostridium acetobutylicum fermenting the starch in corn to produce acetone and butanol
• Saccharomyces cerevisiae fermenting the sugars in barley to produce beer
• Zymomonas mobilis fermenting the sugars in sugarcane to produce ethanol
• Leuconostoc mesenteroides fermenting the glucose in cabbage to produce kimchi
• Aspergillus oryzae fermenting the starch in rice to produce sake

Fermentation can also occur within the gastrointestinal tracts of animals and humans, where it helps digest food and produce vitamins.

The main principle of fermentation is to derive energy from carbohydrates in the absence of oxygen. Carbohydrates, such as glucose, are partially oxidized to pyruvate by a process called glycolysis. Pyruvate is then converted to different end products, such as alcohol or acid, depending on the type of fermentation and the microorganisms involved. During this conversion, NAD+ is regenerated from NADH, which allows glycolysis to continue and produce more ATP (energy). Fermentation yields only about 5% of the energy obtained by aerobic respiration, which uses oxygen as the final electron acceptor.

Fermentation can be represented by the following general equation:

In this equation, glucose is fermented to ethanol and carbon dioxide by yeasts. The ATP is generated by substrate-level phosphorylation, which transfers a phosphate group from an organic molecule to ADP. The NADH produced by glycolysis is re-oxidized to NAD+ by reducing acetaldehyde to ethanol. This maintains the redox balance of the fermentation process and enables glycolysis to proceed.

Different types of fermentation produce different end products from pyruvate. For example, lactic acid fermentation converts pyruvate to lactate, propionic acid fermentation converts pyruvate to propionate and acetic acid, and butyric acid fermentation converts pyruvate to butyrate and acetone. These end products have various applications in medicine and the food industry, as well as other fields.

Fermentation is an ancient and widespread metabolic pathway that allows microorganisms to survive and thrive in anaerobic environments. It is also used by humans to produce foodstuffs and beverages, such as cheese, wine, beer, and bread. Fermentation has many advantages over other methods of food preservation, such as enhancing the flavor, texture, and nutritional value of the food, inhibiting the growth of spoilage and pathogenic microorganisms, and producing beneficial substances such as vitamins and enzymes.

Fermentation processes can be classified based on different criteria, such as the end products, the media, the type of fermenter, and the oxygen requirement. The most common types of fermentation are alcohol fermentation and lactic acid fermentation, which involve breaking down carbohydrates to produce ethyl alcohol and CO2 gas or lactic acid, respectively. Other types of fermentation include propionic acid fermentation, butanoic acid fermentation, mixed acid fermentation, and acetic acid fermentation, which produce different organic acids or alcohols. Fermentation is used for various purposes, such as making alcoholic beverages, bread, yogurt, vinegar, antibiotics, and vitamins.

Alcohol fermentation

Alcohol fermentation is the best known of the fermentation processes. It is carried out by yeasts and some other fungi and bacteria. The first step of the alcohol fermentation pathway involves pyruvate, which is formed by yeast via the Embden–Meyerhof-Parnas (EMP) pathway or by bacteria via the Entner–Doudoroff (ED) pathway. The pyruvate is then converted to ethanol and carbon dioxide in two steps. First, the pyruvate releases carbon dioxide to form a two-carbon compound called acetaldehyde. Next, acetaldehyde is reduced by NADH to ethanol, thereby regenerating the NAD+ for use in glycolysis. Overall, one molecule of glucose is converted into two molecules of carbon dioxide and two molecules of ethanol.

The following figure depicts the processes of glycolysis and alcohol fermentation.

Lactic acid fermentation

Lactic acid fermentation is carried out by certain bacteria and by animal muscle cells when oxygen is in short supply. There are two main types of lactic acid fermentation: homolactic and heterolactic. In homolactic fermentation, pyruvate is directly reduced by NADH to form lactate (or lactic acid) as the only end product. This process occurs in bacteria belonging to the genera Lactococcus, Enterococcus, Streptococcus, and Pediococcus, and by some species of the genus Lactobacillus. Homolactic fermentation is used to make yogurt, cheese, sauerkraut, and pickles.

In heterolactic fermentation, pyruvate is first converted to an intermediate compound called lactyl-CoA, which is then cleaved into lactate and acetyl-CoA. The acetyl-CoA is further metabolized to produce ethanol and carbon dioxide or acetate and hydrogen gas. This process occurs in bacteria of the genera Leuconostoc, Oenococcus, and Weissella, and by heterofermentative lactobacilli. Heterolactic fermentation is used to make kefir, sourdough bread, and kimchi.

The following figure depicts the processes of glycolysis and homolactic fermentation.

Other types of fermentation

Besides alcohol and lactic acid fermentation, there are other types of fermentation that produce different end products from pyruvate or its derivatives. Some examples are:

• Propionic acid fermentation: This type of fermentation is carried out by several bacteria that belong to the genus Propionibacterium and the species Clostridium propionicum. During propionic acid fermentation, both sugar and lactate can be used as the initial substrate. When sugar is available, these bacteria use the EMP pathway to produce pyruvate; the pyruvate is carboxylated to oxaloacetate and then reduced to propionate via malate, fumarate, and succinate. The other end products of propionic fermentation are acetic acid and CO2. Propionic acid fermentation is used to make Swiss cheese and vitamin B12.
• Butanoic acid fermentation: This type of fermentation is characteristic of several obligate anaerobic bacteria that mainly belong to the genus Clostridium. Pyruvate is in turn oxidized to acetyl-CoA, with the production of CO2 and H2. Part of the acetyl-CoA is converted into acetic acid, with ATP production. The rest of the acetyl-CoA is reduced to butyrate (or butanoic acid) by the transfer of electrons from NADH and H2. Some bacteria, such as Clostridium acetobutylicum, produce fewer acids and more neutral products, thus carrying out acetone butanol fermentation. Butanoic acid fermentation is used to make butter and biofuels.
• Mixed acid fermentation: This type of fermentation is carried out by many bacteria that belong to the family Enterobacteriaceae, such as Escherichia coli and Salmonella. These bacteria use the ED pathway to produce pyruvate from glucose. The pyruvate is then converted into a mixture of acids, such as lactate, acetate, formate, and succinate, as well as ethanol, CO2, and H2. The ratio of these products depends on the environmental conditions and the availability of electron acceptors. Mixed acid fermentation is used to make vinegar and to test for the presence of enteric bacteria.
• Acetic acid fermentation: This type of fermentation is carried out by acetic acid bacteria that belong to the genera Acetobacter and Gluconobacter. These bacteria oxidize ethanol or sugars to acetic acid (or vinegar) in the presence of oxygen. The oxidation of ethanol involves two steps: first, ethanol is oxidized to acetaldehyde by alcohol dehydrogenase; then, acetaldehyde is oxidized to acetic acid by aldehyde dehydrogenase. The oxidation of sugars involves the ED pathway to produce pyruvate, which is then oxidized to acetic acid by pyruvate dehydrogenase. Acetic acid fermentation is used to make vinegar and kombucha.

Fermentation has many applications in medicine and the food industry, as it can produce valuable substances from simple and cheap raw materials. Some of the applications are:

• Production of antibiotics: Fermentation is used to produce antibiotics such as penicillin, streptomycin, erythromycin, and tetracycline by growing specific strains of fungi or bacteria in large-scale fermenters. The antibiotics are then extracted and purified from the fermentation broth.
• Production of insulin: Fermentation is used to produce human insulin by genetically engineering bacteria or yeast to express the insulin gene. The insulin is then harvested and purified from the fermentation culture.
• Production of growth hormones: Fermentation is used to produce human growth hormone by genetically engineering bacteria or yeast to express the growth hormone gene. The growth hormone is then collected and purified from the fermentation medium.
• Production of vaccines: Fermentation is used to produce vaccines by growing attenuated or killed microorganisms that can elicit an immune response in humans or animals. The vaccines are then isolated and formulated from the fermentation culture.
• Production of interferon: Fermentation is used to produce interferon, a protein that can inhibit viral infections and modulate immune responses, by genetically engineering bacteria or yeast to express the interferon gene. The interferon is then extracted and purified from the fermentation broth.
• Production of fermented foods: Fermentation is used to produce fermented foods such as cheese, wine, beer, bread, yogurt, sauerkraut, apple cider vinegar, and kombucha by using different types of microorganisms to transform carbohydrates, proteins, or fats in food materials. Fermented foods have enhanced flavor, texture, shelf-life, and nutritional value compared to their raw counterparts.
• Production of food grade bio preservatives: Fermentation is used to produce food grade bio preservatives such as nisin, natamycin, and bacteriocins by growing specific strains of bacteria that can inhibit the growth of spoilage or pathogenic microorganisms in food products. The bio preservatives are then separated and applied to the food products.
• Production of functional foods/neutraceuticals: Fermentation is used to produce functional foods/neutraceuticals such as probiotics, prebiotics, vitamins, enzymes, antioxidants, and phytochemicals by using selected microorganisms or enzymes to modify or enhance the bioactive components in food materials. Functional foods/neutraceuticals have beneficial effects on human health beyond basic nutrition.
• Production of single-cell protein: Fermentation is used to produce single-cell protein by growing microorganisms such as algae, fungi, bacteria, or yeast on cheap and abundant substrates such as agricultural wastes or industrial effluents. Single-cell protein can be used as a source of protein for animal feed or human consumption.

Fermentation is not only used for food and medicine production, but also for other purposes such as:

• Biofuel production: Fermentation can be used to produce biofuels such as ethanol, butanol, and biogas from renewable sources such as corn, sugarcane, and biomass. Biofuels can reduce greenhouse gas emissions and dependence on fossil fuels.
• Wastewater treatment: Fermentation can be used to treat wastewater by using aerobic or anaerobic bacteria to degrade organic pollutants and produce carbon dioxide, water, and mineral salts. Fermentation can also generate biogas that can be used as an energy source.
• Preservation of food: Fermentation can be used to preserve food by inhibiting the growth of spoilage microorganisms and enhancing the flavor, texture, and nutritional value of food. Fermented foods such as cheese, yogurt, sauerkraut, kimchi, and miso are rich in probiotics that can benefit the digestive system.
• Production of biopolymers: Fermentation can be used to produce biopolymers such as bacterial cellulose, xanthan gum, and polyhydroxyalkanoates from renewable sources such as glucose, sucrose, and starch. Biopolymers are biodegradable and have applications in various fields such as medicine, cosmetics, textiles, and packaging.
• Bioremediation: Fermentation can be used to remediate contaminated soils and water by using microorganisms or their enzymes to degrade toxic substances such as pesticides, petroleum hydrocarbons, and heavy metals. Fermentation can also enhance the availability of nutrients and oxygen for plant growth.

Fermentation is a useful and versatile process that has many applications in food, medicine, and industry. However, it also has some limitations that need to be considered. Some of the main limitations of fermentation are:

• Low scale production: Fermentation usually requires large volumes of culture media and fermenters to produce a relatively small amount of product. This can increase the cost and energy consumption of the process, as well as the environmental impact. Fermentation also depends on the availability and quality of the raw materials, which can vary depending on the season and location.
• Possibilities of contamination: Fermentation involves the growth of microorganisms, which can be susceptible to contamination by other microbes or substances. Contamination can affect the quality, safety, and yield of the product, as well as cause spoilage and waste. Therefore, fermentation requires strict hygiene and sterilization measures to prevent contamination and ensure product consistency.
• Natural variations over time: Fermentation is a dynamic and complex process that can be influenced by many factors, such as temperature, pH, oxygen, nutrients, and microbial interactions. These factors can cause variations in the product characteristics over time, such as flavor, texture, color, and nutritional value. Therefore, fermentation requires careful monitoring and control to maintain product quality and stability.
• Impure product: Fermentation usually produces a mixture of products and by-products, which can be difficult to separate and purify. For example, ethanol fermentation produces not only ethanol but also carbon dioxide, acetic acid, glycerol, and other compounds. Some of these by-products can be beneficial or harmless, but some can be undesirable or harmful. Therefore, fermentation may require additional steps to remove or reduce the by-products and obtain a pure product.
• Undesirable and unexpected end products: Fermentation can sometimes produce end products that are not intended or desired by the producer or consumer. For example, some fermented foods can produce biogenic amines, such as histamine and tyramine, which can cause adverse reactions in some people. Some fermented foods can also produce toxins or carcinogens if they are not prepared or stored properly. Therefore, fermentation requires knowledge and expertise to ensure product safety and quality.

These are some of the limitations of fermentation that should be taken into account when using this process for different purposes. However, these limitations can also be overcome or minimized by using advanced techniques and technologies that can improve the efficiency and effectiveness of fermentation.

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