Gluten (from Latin gluten, "glue") is a protein composite found in foods processed from wheat and related grain species, including barley and rye. It gives elasticity to dough, helping it to rise and to keep its shape, and often giving the final product a chewy texture.
Gluten is the composite of a gliadin and a glutelin, which is conjoined with starch in the endosperm of various grass-related grains. The prolamin and glutelin from wheat (gliadin, which is alcohol-soluble, and glutenin, which is only soluble in dilute acids or alkalis) compose about 80% of the protein contained in wheat seed. Being insoluble in water, they can be purified by washing away the associated starch. 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 seeds 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 gluten.
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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 water, and the dispersed starch will be sedimented and dried. If a saline solution is used instead of water, a purer protein is obtained, with certain harmless impurities going into solution with the starch. Where starch is the prime product, cold water is the favored solvent because the impurities stay with the gluten.
In home or play-dough 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.[1] Approximately 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.[2]
Gluten forms when glutenin molecules cross-link to form a sub-microscopic network attached to gliadin, which contributes viscosity (thickness) and extensibility to the mix.[3] If this dough is leavened with sugar, 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.[4]
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.[5][6] 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 product that is chewier in proportion to the length of kneading. An increased moisture content in the dough enhances gluten development,[7] 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.[8] 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.[9] The added gluten provides supplemental protein to products with low or nonexistent protein levels.
Gluten, especially wheat gluten, is often the basis for imitation meats resembling chicken, duck (mock duck), fish, pork and beef. When cooked in broth, gluten absorbs some of the surrounding liquid (including the taste) and becomes firm to the bite.
The "Codex Alimentarius" international standards for food labeling has a standard relating to the labeling of products as "gluten-free", but this standard does not apply to foods that "...in their normal form do not contain gluten."[10] Gluten is used as a stabilizing agent in products like ice cream and ketchup, where it might be unexpected.[11][12] Foods of this kind present a problem because the hidden gluten constitutes a hazard for people with celiac disease: In the United States, at least, gluten might not be listed on the labels of such foods because the U.S. Food and Drug Administration has classified gluten as GRAS (generally recognized as safe).[13] Requirements for proper labeling are being formulated by the USDA. In the United Kingdom, only cereals currently must be labeled, while labeling of other products is voluntary.[14]
The protein content of some pet foods may also be enhanced by adding gluten.[15]
Between 0.5 and 1.0 percent of people in the United States are sensitive to gluten due to Celiac disease.[16][17] Celiac disease constitutes an abnormal immune reaction to partially digested gliadin. It probably occurs with comparable frequencies among all wheat-eating populations in the world.[18] Certain allergies and neuropathies are also caused by gluten consumption and inhalation. In some instances what is known as cross contamination can occur without the person even being aware that they are ingesting gluten. Known examples occur when people share silverware or other eating instruments. There have also been confirmed reports of cross contamination through oral sex. Doctors have warned that this is possible for as many as 30 days after a person ingests gluten containing products. People known to be allergic to gluten often cite this as an area of "extreme frustration". [19] Wheat allergy and celiac disease are different disorders.[20]
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