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Fermentation is an important process for the regeneration of NAD+ in living cells. During metabolism, nutrients are oxidized by glycolysis and released electrons reduce NAD+ to NADH. Since the pool of NAD+ is small, glycolysis would stop if the NADH is not oxidised back to NAD+. In fermentation, NADH transfers its electrons to a product of the catabolism, whereas in respiration, NADH transfers electrons to molecules that are abundant in the organism's ecological niche (often oxygen).
During fermentation, pyruvate produced in glycolysis is metabolised to various different compounds. Homolactic fermentation is the production of lactic acid from pyruvate; alcoholic fermentation is the conversion of pyruvate into ethanol and carbon dioxide; and heterolactic fermentation is the production of lactic acid as well as other acids and alcohols.
Pasteur performed careful research and concluded, "I am of the opinion that alcoholic fermentation never occurs without simultaneous organization, development and multiplication of cells.... If asked, in what consists the chemical act whereby the sugar is decomposed ... I am completely ignorant of it.".
The research efforts undertaken by the Danish Carlsberg scientists greatly accelerated the gain of knowledge about yeast and brewing. The Carlsberg scientists are generally acknowledged with jump-starting the entire field of molecular biology.
The reaction differs according to the sugar being used in the process of fermentation, as well as the particular organism performing it. Below, the sugar is glucose (C6H12O6), the most common sugar, and the process is the alcoholic fermetation (as in S. cerevisiae)
- C6H12O6 → 2 C2H5OH + 2CO2 + 2 ATP (Energy Released:118 kJ mol−1)
- Sugar (glucose) → Alcohol (ethanol) + Carbon Dioxide + Energy (ATP)
Energy source in anaerobic conditions
Fermentation 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.
Fermentation products contain chemical energy (they are not fully oxidized) but are considered waste products since they cannot be metabolised further without the use of oxygen (or other more highly-oxidized electron acceptors). A consequence is that the production of ATP by fermentation is less efficient than oxidative phosphorylation, where pyruvate is fully oxidized to carbon dioxide. Fermentation produces two ATP molecules per molecule of glucose compared to approximately 36 by aerobic respiration.
Aerobic glycolysis is a method employed by muscle cells for the production of lower-intensity energy over a longer period of time when oxygen is plentitful. Under low-oxygen conditions, however, vertebrates use the less-efficient but faster anaerobic glycolysis to produce ATP. The speed at which ATP is produced is about 100 times that of oxidative phosphorylation. While fermentation is helpful during short, intense periods of exertion, it is not sustained over extended periods in complex aerobic organisms. In humans, for example, lactic acid fermentation provides energy for a period ranging from 30 seconds to 2 minutes.
The final step of fermentation, the conversion of pyruvate to fermentation end-products, does not produce energy. However, 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.
Products produced by fermentation are actually waste products produced during the reduction of pyruvate to regenerate NAD+ in the absence of oxygen. Bacteria generally produce acids. Vinegar (acetic acid) is the direct result of bacterial metabolism (Bacteria need oxygen to convert the alcohol to acetic acid). In milk, the acid coagulates the casein, producing curds. In pickling, the acid preserves the food from pathogenic and putrefactive bacteria.
- Ethanol fermentation (performed 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.
In vertebrates, during intense exercise, cellular respiration will deplete oxygen in the muscles faster than it can be replenished. An associated burning sensation in muscles has been attributed lactic acid causing a decrease in the pH during a shift to anaerobic glycolysis. While this does partially explain acute muscle soreness, lactic acid may also help delay muscle fatigue, although, eventually the lower pH will inhibit enzymes involved in glycolysis. Contrary to currently popular belief, the lactic acid is not the primary causes for the drop in pH, but rather ATP-derived hydrogen ions. Inorganic phosphate that increases during fatigue due to breakdown of creatine phosphate, appears to be a major cause of muscle fatigue . The advantages and disadvantages of lactic acid accumulation during muscle activity are discussed in a recent article .
Delayed onset muscle soreness cannot be attributed to the lactic acid and other waste products as they are quickly removed after exercise. It is actually due to microtrauma of the muscle fibres. Eventually the liver metabolises the lactic acid back to pyruvate.
Zymology is the scientific study of fermentation. It deals with the biochemical processes involved in fermentation, with yeast selection and physiology, and with the practical issues of brewing. Zymology is occasionally known as zymurgy.
- Fermentation (food)
- Industrial fermentation
- Fermentation lock
- Cellular respiration
- Anaerobic respiration
Voet & Voet "Fundamentals of Biochemistry"