Gluten (from Latin gluten, "glue") is a protein composite found in wheat and related grains, including barley and rye. Gluten gives elasticity to dough, helping it rise and keep its shape and often gives the final product a chewy texture.
Gluten is the composite of the storage proteins, gliadin and a glutenin, and is conjoined with starch in the endosperm of various grass-related grains. Worldwide, gluten is a source of protein, both in foods prepared directly from sources containing it, and as an additive to foods otherwise low in protein.
The fruit of most flowering plants have endosperms with stored protein to nourish embryonic plants during germination. True gluten, with gliadin and glutenin, is limited to certain members of the grass family. The stored proteins of maize and rice are sometimes called glutens, but their proteins differ from true gluten.
Gluten is extracted from flour by kneading the flour, agglomerating the gluten into an elastic network, a dough, and then washing out the starch. Starch granules disperse in cold/low-temperature water, and the dispersed starch is sedimented and dried. If a saline solution is used instead of water, a purer protein is obtained, with certain harmless impurities departing to the solution with the starch. Where starch is the prime product, cold water is the favored solvent because the impurities depart from the gluten.
In home or restaurant cooking, a ball of wheat flour dough is kneaded under water until the starch disperses out. In industrial production, a slurry of wheat flour is kneaded vigorously by machinery until the gluten agglomerates into a mass. This mass is collected by centrifugation, then transported through several stages integrated in a continuous process. About 65% of the water in the wet gluten is removed by means of a screw press; the remainder is sprayed through an atomizer nozzle into a drying chamber, where it remains at an elevated temperature a short time to evaporate the water without denaturing the gluten. The process yields a flour-like powder with a 7% moisture content, which is air cooled and pneumatically transported to a receiving vessel. In the final step, the collected gluten is sifted and milled to produce a uniform product.
Gluten forms when glutenin molecules cross-link to form a submicroscopic network attached to gliadin, which contributes viscosity (thickness) and extensibility to the mix. If this dough is leavened with yeast, fermentation produces carbon dioxide bubbles, which, trapped by the gluten network, cause the dough to rise. Baking coagulates the gluten, which, along with starch, stabilizes the shape of the final product. Gluten content has been implicated as a factor in the staling of bread, possibly because it binds water through hydration.
The development of gluten (i.e., enhancing its elasticity) affects the texture of the baked goods. Gluten's attainable elasticity is proportional to its content of glutenins with low molecular weights as this portion contains the preponderance of the sulfur atoms responsible for the cross-linking in the network. More refining (of the gluten) leads to chewier products such as pizza and bagels, while less refining yields tender baked goods such as pastry products.
Generally, bread flours are high in gluten (hard wheat); pastry flours have a lower gluten content. Kneading promotes the formation of gluten strands and cross-links, creating baked products that are chewier (in contrast to crumbly). The "chewiness" increases as the dough is kneaded for longer times. An increased moisture content in the dough enhances gluten development, and very wet doughs left to rise for a long time require no kneading (see no-knead bread). Shortening inhibits formation of cross-links and is used, along with diminished water and less kneading, when a tender and flaky product, such as a pie crust, is desired.
The strength and elasticity of gluten in flour is measured in the baking industry using a farinograph. This gives the baker a measurement of quality for different varieties of flours in developing recipes for various baked goods.
Gluten, when dried and milled to a powder and added to ordinary flour dough, improves a dough's ability to rise and increases the bread's structural stability and chewiness. Gluten-added dough must be worked vigorously to induce it to rise to its full capacity; an automatic bread machine or food processor may be required for kneading. Generally, higher gluten levels are associated with higher amounts of overall protein.
Gluten, especially wheat gluten, is often the basis for imitation meats resembling beef, chicken, duck (see mock duck), fish, and pork. When cooked in broth, gluten absorbs some of the surrounding liquid (including the flavor) and becomes firm to the bite.
Other consumer products
Gluten is often present in beer and soy sauce, and can be used as a stabilizing agent in more unexpected food products, such as ice cream and ketchup. Foods of this kind may raise a problem for a small number of consumers because the hidden gluten constitutes a hazard for people with celiac disease.
In individuals with coeliac disease (American English: celiac), consumption of gluten causes adverse health issues ranging from abdominal bloating, gas, diarrhea, and vomiting to migraine headaches and joint pain. Coeliac disease, an autoimmune disorder, affects the digestive process of the small intestine. In 2009, research showed between 0.5 and 1.0% of people in the US and UK are sensitive to gluten due to coeliac disease. It probably occurs with comparable frequency among all wheat-eating populations in the world. Coeliac disease has no cure, but is manageable with a gluten-free diet.
Non-coeliac gluten sensitivity (gluten intolerance) is a condition arising from a syndrome of gastrointestinal responses to gluten different from the immune response characteristic of coeliac disease. The global prevalence of gluten-related disorders is estimated to be 5%. However, no scientific consensus exists to confirm gluten intolerance is a definable pathological condition, and the mechanism by which this could occur is unknown. Frequently, symptoms arise in individuals as a result of undiagnosed coeliac disease or due to a reaction to other components of wheat, such as short-chain, fermentable carbohydrates called FODMAPs.
People can also experience adverse effects of wheat as result of a wheat allergy. As with most allergies, a wheat allergy causes the immune system to abnormally respond to a component of wheat that it treats as a threatening foreign body. This immune response is often time-limited and does not cause lasting harm to body tissues. Wheat allergy and coeliac disease are different disorders.
The Codex Alimentarius international standards for food labelling has a standard relating to the labelling of products as "gluten-free". It only applies to foods that would normally contain gluten.
By law in Brazil, all food products must display labels clearly indicating whether or not they contain gluten.
One in 133 Canadians experiences adverse symptoms from gluten in celiac disease. Labels for all food products sold in Canada must clearly identify the presence of gluten if it is present at a level greater than 20 parts per million.
In the United Kingdom, only cereals must be labelled; labelling of other products is voluntary.
In the United States, gluten is not listed on labels unless added as a stand-alone ingredient. Wheat or other allergens are listed after the ingredient line. The US Food and Drug Administration (FDA) has historically classified gluten as "generally recognized as safe" (GRAS). In August 2013, FDA issued a final rule, effective August 2014, that defined the term "gluten-free" for voluntary use in the labeling of foods as meaning that the amount of gluten contained in the food is below 20 parts per million.
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