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Autolysis ((EXCLUSIVE))


In biology, autolysis, more commonly known as self-digestion, refers to the destruction of a cell through the action of its own enzymes. It may also refer to the digestion of an enzyme by another molecule of the same enzyme.




autolysis


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Autolysis is uncommon in living adult organisms and usually occurs in necrotic tissue as enzymes act on components of the cell that would not normally serve as substrates. These enzymes are released due to the cessation of active processes in the cell that provide substrates in healthy, living tissue; autolysis in itself is not an active process. In other words, though autolysis resembles the active process of digestion of nutrients by live cells, the dead cells are not actively digesting themselves as is often claimed, and as the synonym self-digestion suggests. Failure of respiration and subsequent failure of oxidative phosphorylation is the trigger of the autolytic process.[1] The reduced availability and subsequent absence of high-energy molecules that are required to maintain the integrity of the cell and maintain homeostasis causes significant changes in the biochemical operation of the cell.[citation needed]


Molecular oxygen serves as the terminal electron acceptor in the series of biochemical reactions known as oxidative phosphorylation that are ultimately responsible for the synthesis of adenosine triphosphate, the main source of energy for otherwise thermodynamically unfavorable cellular processes.[2] Failure of delivery of molecular oxygen to cells results in a metabolic shift to anaerobic glycolysis, in which glucose is converted to pyruvate as an inefficient means of generating adenosine triphosphate.[2] Glycolysis has a lower ATP yield than oxidative phosphorylation and generates acidic byproducts that decrease the pH of the cell, which enables many of the enzymatic processes involved in autolysis.


Peroxisomes typically are responsible for the breakdown of lipids, particularly long-chain fatty acids. In the absence of an active electron transport chain and associated cellular processes, there is no metabolic partner for the reducing equivalents in the breakdown of lipids.[1] In terms of autolysis, peroxisomes provide catabolic potential for fatty acids and reactive oxygen species, which are released into the cytosol as the peroxisomal membrane is damaged by water retention and digestion by other catabolic enzymes.[1]


In the food industry, autolysis involves killing yeast and encouraging breakdown of its cells by various enzymes. The resulting autolyzed yeast is used as a flavoring or flavor enhancer. For yeast extract, when this process is triggered by the addition of salt, it is known as plasmolysis.[4]


In the making of fermented beverages, autolysis can occur when the must or wort is left on the lees for a long time.In beer brewing, autolysis causes undesired off-flavors. Autolysis in winemaking is often undesirable, but in the case of the best Champagnes it is a vital component in creating flavor and mouth feel.[7]


Autolysis in winemaking relates to the complex chemical reactions that take place when a wine spends time in contact with the lees, or dead yeast cells, after fermentation. While for some wines - and all beers[1] - autolysis is undesirable, it is a vital component in shaping the flavors and mouth feel associated with premium Champagne production.[2][3] The practice of leaving a wine to age on its lees (or sur lie aging) has a long history in winemaking dating back to Roman winemaking. The chemical process and details of autolysis were not originally understood scientifically, but the positive effects such as a creamy mouthfeel, breadlike and floral aromas, and reduced astringency were noticed early in the history of wine.[4]


Ancient Roman writers, such as Marcus Porcius Cato, observed that wine that was left on its lees (or sediment as they knew it) exhibited different characteristics than wine that was quickly separated from its sediment. While the Romans did not understand the full chemical process or details behind the autolysis that took place, they were able to perceive the results of this autolysis in the creamy mouthfeel, reduced astringency and unique flavors and aromas that developed.[4] With modern day understanding of autolysis, winemaking in the Champagne have strict regulation regarding the time Champagne must spend in contact with its lees in order to receive some benefit from autolysis. Under Appellation d'origine contrôlée (AOC) regulations, wines from Champagne cannot legally be sold until it has gone through autolysis in the bottle for at least 15 months with non-vintage Champagne. Vintage Champagne must have a minimum of 3 years aging; some Champagne houses extend the time for autolysis to 7 years or more.[5]


During fermentation, yeast cells convert the sugar in the grape must into ethanol. When the sugar food source for the yeast and necessary nutrients such as nitrogen run out, or the alcohol level of the wine reaches such a point to where it is toxic for the yeast, the cells die and sink to the bottom of the fermentation vessel. These dead cells, or "lees", are normally removed by racking the wine into a clean vessel. If the wine is left in contact with the lees, enzymes start to break the cells down producing mannoproteins (mannose-containing glycoproteins) and polysaccharides that are released into the wine. In sparkling wine production, a wine is made "sparkling" or "bubbly" because a secondary fermentation is introduced when the wine is contained within a sealed wine bottle. During this time the wine is deliberately kept in contact with its lees inside the bottle for a period ranging from a couple of months to several years. Studies have shown that the chemical reaction of autolysis, and its impact on the wine, starts to become noticeable after 18 months and will continue to impart traits for at least 5 years. The sparkling wine is eventually separated from its lees through a technique known as riddling and disgorgement.[2]


The effects of autolysis on wine contributes to a creamy mouthfeel that may make a wine seem to have a fuller body. The release of enzymes inhibits oxidation which improves some of the aging potential of the wine. The mannoproteins improve the overall stability of the proteins in the wine by reducing the amount of tartrates that are precipitated out. They may also bind with the tannins in the wine to reduce the perception of bitterness or astringency in the wine. The increased production of amino acids leads to the develop of several flavors associated with premium Champagne including aromas of biscuits[6] or bread dough, nuttiness[7] and acacia. As the wine ages further, more complex notes may develop from the effects of autolysis.[2]


If not properly managed, wine faults can potentially develop from autolysis. If the layer of lees begins to exceed 4 inches (10 centimeters), the enzymes released from the process of the yeast digesting themselves creates reducing conditions and promotes the development of hydrogen sulfide and mercaptan odors. The process of stirring the lees or bâttonage can help prevent a thick layer of lees forming and promote a smoother autolysis. Poor hygiene of the winemaking equipment or wine made from grapes that had residue from fungicide can create off odors smelling of sulfide.[4]


Many causes may be cited for autolysis, not least simple old age. However, poor handling of yeast and beer will accelerate autolysis. Common examples are high temperatures, particularly above 25C, or sudden changes in temperature at pitching or at chilling, and osmotic shock where yeast is pitched into high gravity worts. Some yeast strains may be inherently sensitive to conditions such as high alcohol, high carbonation, and high acidity and autolyse faster than others. In other conditions extensive re-pitching of yeast from batch to batch may create stress, as can the presence of contaminants including lactic acid bacteria and other yeast species.


A dough that undergoes autolysis will usually require less mixing and kneading time (either by hand or mechanical mixer). By letting the mixture of flour and water rest, gluten bonds will begin forming, and the dough will start to smooth out and soften. Because this happens without mechanical mixing, we can minimize the amount of dough oxidization.


Dying is an important part of life because when things die, they decompose and return to the earth. In this way, they can fuel new life by returning the nutrients and molecules in their body to be used again. While you probably already know that this occurs, you might be surprised to learn that when a body dies, there is an organized process of decomposition that begins almost immediately. One part of this process is autolysis (''auto'' = self and ''lysis'' = breakdown), which is cellular self-digestion. This self-destruction of cells occurs as endogenous or internal cellular enzymes (''endo'' = inside and ''genous'' = originating from) are released and work to break down cellular material.


Because autolysis occurs after the organism dies, the changes that are seen are called postmortem for ''after death.'' While the process of autolysis is an internal one, the rate at which it occurs can be influenced by different factors, both internal and external.


For example, warmer temperatures will speed up autolysis, while colder temperatures will slow it down. This means that a dead animal left in the heat will autolyze faster than one kept in a refrigerator. Similarly, a larger animal will autolyze more quickly than a smaller animal, because the internal body temperature will stay warmer longer than that of a smaller animal with less mass.


The rate of autolysis of an organism may not concern your daily activities very much, but if you study histology, which is the microscopic anatomy of plant and animal tissue, then you should be very interested in autolysis and the things that affect it. Histology is a field that analyzes biological tissues for disease diagnostics, forensic investigations (including autopsies), and even archeological history. 041b061a72


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