Lactic acid
Lactic acid is a naturally occurring organic compound produced during the metabolism of glucose in animals and by specific bacteria during fermentation. It appears as a clear, slightly yellowish liquid and is commonly found in fermented foods such as yogurt, cheese, and sauerkraut, where it contributes to their tangy taste and texture. The body generates lactic acid during anaerobic exercises when glucose is metabolized without sufficient oxygen, often leading to muscle soreness post-exercise. While lactic acid can be utilized by the heart, liver, and kidneys to produce energy or glucose, excessive production can lead to a rare condition known as lactic acidosis, often associated with health issues like heart attack or liver disease. Beyond its role in metabolism and food production, lactic acid is also used in cosmetics for its exfoliating properties and serves as an antibacterial and descaling agent in various household products. Historically, lactic acid was first identified in the 18th century, and its fermentation properties have been exploited for thousands of years in food preservation.
Lactic acid
Lactic acid is a naturally occurring substance. It is created both as a by-product of glucose, or sugar, metabolism in animals and by bacteria in dairy products, wine, and other fermented foods. In addition, lactic acid may be reproduced chemically and used as an ingredient in food, cosmetics, and other products.
Lactic Acid in Metabolism
Animals and human beings require both fuel and oxygen to move and function. The process by which fuel and oxygen are used to generate the energy needed to make the body's systems function properly is called metabolism. As fuel is metabolized by various parts of the body, a complex series of reactions takes place. Some of these processes create lactic acid, a clear or slightly yellowish water-soluble, syrupy liquid.
When the body uses stored glucose as fuel but does not have sufficient oxygen to completely use the glucose—as in anaerobic exercise, which is done in short, intense bursts—glucose converts to lactic acid. Aching muscles following an intense workout can be attributed to lactic acid. The ache eventually diminishes as blood removes excess lactic acid over time.
Some parts of the body, namely the heart, liver, and kidneys, can convert excess lactic acid into carbon dioxide, which the body removes via the respiratory system through exhalation. The kidneys and liver can use lactic acid to produce more glucose through a process known as the Cori cycle. Sometimes, however, the body produces more lactic acid than it can process and remove, or neutralize. When this happens, a condition known as lactic acidosis occurs. This condition is rare in otherwise healthy people and usually results from system impairment caused by conditions such as carbon monoxide or cyanide poisoning, diabetes, heart attack, or the excessive consumption of alcohol.
Blood tests that detect lactic acidosis are often part of the process used to diagnose conditions such as heart failure, diseases of the liver or lungs, and sepsis infections.
Lactic Acid in Dairy, Wine, and Other Foods
Lactic acid is part of the natural fermentation process that results in products such as yogurt, kefir, cheese, buttermilk, sour cream, some sausages, olives, and fermented vegetable dishes such as sauerkraut and kimchee. It was first identified in 1780 by Swedish chemist Carl Wilhelm Scheele (1742–1786). Louis Pasteur (1822–1895), the famed French scientist, discovered the first lactobacillus bacteria in 1856, and by the late 1800s, researchers were able to produce lactic acid commercially. However, researchers have determined that humans have taken advantage of lactic acid–generating bacteria to culture foods for at least four thousand years and possibly much longer.
Many types of bacteria produce lactic acid as part of their digestive process. Also known as lactobacilli, or LAB, these bacteria are very common in nature. They are found in many of the mucosal membranes of the human body, such as the mouth and intestines. Lactobacilli are considered probiotics that produce beneficial effects in the digestive system. Lactobacilli are so efficient at this that people were able to harness their power to ferment and preserve food long before the bacteria were identified or the process was understood.
One early discovery was the bacteria’s ability to convert highly perishable milk to products that could be stored and used longer. In this process, the lactobacilli that occur naturally in the environment around the milk transform the lactose, or milk sugar, to lactic acid. As more and more lactic acid is produced, the original protein in the milk curdles and changes, and the product develops a different taste and texture. It is this lactic acid process that gives these dairy products their characteristic tangy taste and thicker texture. The same bacteria serve as probiotics in the human digestive system, enhancing those that naturally occur within the body. Probiotics have been proven to help with the treatment of intestinal inflammation and the prevention of urinary tract infections, among other conditions.
Winemakers rely on a process known as malolactic fermentation (MLF) to help wines develop a proper level of acidity. In this process, malic acid—a tart, naturally occurring acid found in grapes—is converted into mellower lactic acid by lactobacilli.
Bacteria also are responsible for generating the lactic acid that plays a key role in fermenting various food products, such as beer, ciders, olives, pickles, sourdough bread, and more. Lactic acid is helpful in preventing the growth of more dangerous organisms that cause food spoilage and lead to food-borne illnesses.
Other Uses
Lactic acid can be reproduced chemically and is an additive in many products. Creams and lotions containing lactic acid are applied topically to the skin to treat a range of conditions, such as dry skin, eczema, and keratosis. Lactic acid, along with salicylic acid and urea, are keratolytics, which means they help to break down keratin. Keratin is what holds the top layer of human skin together. This layer is made up of dead skin cells, which lactic acid and other keratolytics help dissolve and remove, allowing the next layer to absorb water and other emollients that help moisturize the skin.
Other applications include use as a natural descaling and anti–soap scum agent, a detergent, and an antibacterial agent.
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