Since fruits ferment naturally, fermentation precedes human history. However, humans began to take control of the fermentation process at some point. There is strong evidence that people were fermenting beverages in Babylon circa 5000 BC, ancient Egypt circa 3000 BC, pre-Hispanic Mexico circa 2000 BC, and Sudan circa 1500 BC. There is also evidence of leavened bread in ancient Egypt circa 1500 BC and of milk fermentation in Babylon circa 3000 BC. The Chinese were probably the first to develop vegetable fermentation.
Throughout human history, bacteria and fungi have been intimately involved with both the success and failure of daily life. Microorganisms cause disease, but they also play a wider role in sustaining life. We know that certain bacteria play a major part in recycling chemical elements as well as compounds. For example, bacteria help return the materials of dead organisms back to the earth, so those materials can be used by living organisms. What's more, without bacterial action, living things would not be able to use certain compounds found in soil, water, and even the atmosphere. Modified versions of these microorganisms are now essential in modern day pharmaceutical production.
In fermentation, microorganisms such as bacteria, yeasts, and molds are mixed with ingredients that provide them with food. As they digest this food, the organisms produce two critical by-products, carbon dioxide gas and alcohol.
Discovery of the fermentation process allowed early peoples to produce foods by allowing live organisms to act on other ingredients. But our ancestors also found that, by manipulating the conditions under which the fermentation took place, they could improve both the quality and the yield of the ingredients themselves.
Fermentation is one of the oldest ways humans have used microbes. Fermentation enhances the nutrient content of foods through the biosynthesis of vitamins, essential amino acids and proteins, by improving protein and fibre digestibility, by enhancing micronutrient bioavailability, and by degrading antinutritional factors. It also provides a source of calories when used in the conversion of substrates, unsuitable for human consumption, to human foods.
Fermentation is carried out by both bacteria (prokaryotes) and fungi (eukaryotes) during their metabolism. Both groups of organisms also figure in the world of human disease, as they are both sources of antibiotics, as we'll see throughout this module. Fermentation results in a number of byproducts that have many different uses. As mentioned, the most well-known product of fermentation is ethyl alcohol. This substance is both a beverage as well as a starter molecule for synthesizing other compounds. Since fermentation is carried out in the absence of oxygen, we call it an anaerobic process. It is a method by which organisms such as yeast obtain their energy by converting sugars into other chemical compounds, particularly carbon dioxide and water. Interestingly, our bodies also use this same anaerobic fermentation to obtain energy from sugars when oxygen is in low supply in our blood, such as during vigorous exercise. The products of this process are lactic acid and water rather than the carbon dioxide and water that human metabolism normally produces. In this day and age, pharmaceutical companies utilize the fermentation carried out by microorganisms to produce antibiotics, hormones, and specialized proteins such as antibodies and insulin (Fermentation processes enhance food safety by reducing toxic compounds such as aflatoxins and cyanogens, and producing antimicrobial factors such lactic acid, bacteriocins, carbon dioxide, hydrogen peroxide and ethanol which facilitate inhibition or elimination of food-borne pathogens). This wide range of products is possible because the bacterium or fungus involved in fermentation has been genetically changed to produce a specific substance. Therapeutic properties of fermented foods have also been reported.
In addition to its nutritive, safety and preservative effects, fermentation enriches the diet through production of a diversity of flavors, textures and aromas. It improves the shelf-life of foods while reducing energy consumption required for their preparation. The production of fermented foods is also important in adding value to agricultural raw materials, thus providing income and generating employment.
Traditional fermentation processing is generally a spontaneous, non-aseptic operations which result from the competitive activities of a variety of microorganisms. In a bioreactor - which may consist of clay or metal pots, a basket, or a simply hole in the ground lined with leaves - strains best adapted, and with the highest growth rates, dominate under uncontrolled conditions. Optimization of process controls and of the microbial flora associated with fermentations therefore poses one of the biggest challenges in improving food fermentation technologies. Appropriate quality control methodologies - e.g. use of high quality raw materials, proper hygienic standards in the processing environment, proper packaging - also need to be developed.
Improvements in process control through the development of more appropriate bioreactors, particularly those suitable for solid substrate fermentations, could improve the quality and quantity of fermented foods available in developing countries. The selection and development of more productive microbial strains, and the control and manipulation of culture conditions could also increase the efficiency of fermentation processes.
Fermentation is a process that is important in anaerobic conditions when there is no oxidative phosphorylation to maintain the production of ATP by glycolysis. During fermentation pyruvate is metabolised to various different compounds. Textbook examples of fermentation products are ethanol (drinkable alcohol), lactic acid, and hydrogen. However, more exotic compounds can be produced by fermentation, such as butyric acid and acetone.
Although the final step of fermentation (conversion of pyruvate to fermentation end-products) does not produce energy, it is critical for an anaerobic cell since it regenerates nicotinamide adenine dinucleotide (NAD+), which is required for glycolysis. This is important for normal cellular function, as glycolysis is the only source of ATP in anaerobic conditions.
Fermentation products contain chemical energy (they are not fully oxidised) but are considered waste products since they cannot be metabolised further without the use of oxygen (or other more highly-oxidised electron acceptors). A consequense is that the production of ATP by fermentation is less efficient than oxidative phosphorylation, where pyruvate is fully oxidised to carbon dioxide. Fermentation produces two ATP molecules per molecule of glucose compared to approximately 36 by aerobic respiration. Even in vertebrates, however, it is used as an effective means of energy production during short, intense periods of exertion, where the transport of oxygen to the muscles is insufficient to maintain aerobic metabolism. In humans, for example, lactic acid fermentation provides energy for a period ranging from 30 seconds to 2 minutes. The speed at which ATP is produced is about 100 times that of oxidative phosphorylation. The pH in the cytoplasm quickly drops when lactic acid accumulates in the muscle, eventually inhibiting enzymes involved in glycolysis.
The production of carbon dioxide and alcohol are incidental to the release of energy needed by organisms such as yeast to survive. But these metabolic by-products have been used in human enterprise for centuries. The yeast Saccharomyces cerevisiae is traditionally added to liquids derived from grains and fruits to brew beer and wine. The natural starches and sugars provide food for the yeast and during fermentation the desired alcohol is released. In China for thousands of years, traditional soy sauce or shoyu was brewed by adding the fungus Aspergillus oryzae to a mixture of boiled soybeans and wheat and allowing it to ferment for about a year.
In recent times, yeasts have been used to aid in the production of alternative energy sources. Yeasts are placed in large fermentation vats containing organic material. During fermentation the yeast convert the organic material into ethanol fuel. Researchers are working on developing yeast strains that will convert even larger organic biomasses into ethanol more efficiently.
Products produced by fermentation are actually waste products produced during the reduction of pyruvate to regenerate NAD+ in the absence oxygen.
- Ethanol fermentation (done by yeast and some types of bacteria) breaks the pyruvate down into ethanol and carbon dioxide. It is important in bread-making, brewing, and wine-making. When the ferment has a high concentration of pectin, minute quantities of methanol can be produced. Usually only one of the products is desired; in bread the alcohol is baked out, and in alcohol production the carbon dioxide is released into the atmosphere.
- Lactic acid fermentation breaks down the pyruvate into lactic acid. It occurs in the muscles of animals when they need energy faster than the blood can supply oxygen. It also occurs in some bacteria and some fungi. It is this type of bacteria that convert lactose into lactic acid in yogurt, giving it its sour taste.
The burning sensation in muscles during hard exercise used to be attributed to the production of lactic acid during a shift to anaerobic glycolosis, as oxygen is converted to carbon dioxide by aerobic glycolysis faster than the body can replenish it; but muscle soreness and stiffness after hard exercise is actually due to microtrauma of the muscle fibres. The body falls back on this less-efficient but faster method of producing ATP under low-oxygen conditions. This is thought to have been the primary means of energy production in earlier organisms before oxygen was at high concentration in the atmosphere and thus would represent a more ancient form of energy production in cells. The liver later gets rid of this excess lactate by transforming it back into an important glycolysis intermediate called pyruvate. Aerobic glycolysis is a method employed by muscle cells for the production of lower-intensity energy over a longer period of time.
Bacteria generally produce acids. Vinegar (acetic acid) is the direct result of bacterial fermentation. In milk, the acid coagulates the casein, producing curds. In pickling, the acid preserves the food from pathogenic and putrefactive bacteria.
The primary benefit of fermentation is the conversion, e.g., converting juice into wine, grains into beer, and carbohydrates into carbon dioxide to leaven bread.
According to Steinkraus (1995), food fermentation serves five main purposes:
- Enrichment of the diet through development of a diversity of flavors, aromas, and textures in food substrates
- Preservation of substantial amounts of food through lactic acid, alcoholic, acetic acid, and alkaline fermentations
- Biological enrichment of food substrates with protein, essential amino acids, essential fatty acids, and vitamins
- Detoxification during food-fermentation processing
- A decrease in cooking times and fuel requirements
Fermentation has some benefits exclusive to foods. Fermentation can produce important nutrients or eliminate antinutrients. Food can be preserved by fermentation, since fermentation uses up food energy and can make conditions unsuitable for undesirable microorganisms. For example, in pickling the acid produced by the dominant bacteria inhibit the growth of all other microorganisms.