Gluten

    Gluten is a composite of the proteins gliadin and glutenin. These exist, conjoined with starch, in the endosperms of some grass-related grains, notably wheat, rye, and barley. Gliadin and glutenin comprise 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 an important source of nutritional protein, both in foods prepared directly from foods 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, but true gluten, with gliadin and glutenin, is limited to certain members of the grass family. The stored proteins of corn and rice are sometimes called glutens, but their proteins differ importantly from wheat gluten by lacking glutenin. The glutenin in wheat flour gives kneaded dough its elasticity, allowing leavening and contributing chewiness to baked products like bagels.

Although wheat supplies much of the world's dietary protein, a small percentage of the population, including those with celiac disease, is gluten-intolerant and cannot consume it safely. ^[1]

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Extraction

Legend attributes the discovery of gluten to Buddhist monks in 7th century China who sought meat-like ingredients for use in their vegetarian diet.^[2] With easily available wheat flour and water they made a dough which they submerged in cold water and kneaded. The water dissolved the starchy components of the dough and left behind an insoluble, gummy mass, 70% to 80% of which was gluten.

Gluten is still extracted from flour by washing out the starch by means not fundamentally different from the ancient way, which exploited the fact that starch is water-soluble while gluten is not -- also, that gluten binds together strongly, while starch dissolved in cold water is mobile. If a saline solution is used instead of water a purer protein is obtained, with certain harmless impurities going into solution with the starch. However, on an industrial scale, starch is the prime product, so cold water is the favored solvent. To effect the separation, a slurry of wheat flour is stirred vigorously by machinery until the starch dissolves and the gluten consolidates into a mass, which is collected by centrifugation, then carried, by complex machinery,^[3] through several stages combined into a continuous process: Approximately 65% of the water in the wet gluten is removed by means of a screw press, and the residue is sprayed through an atomizing nozzle into a drying chamber, where it remains at an elevated temperature only long enough to evaporate the water without denaturing the gluten. This yields a flour-like powder with a 7% moisture content, which is quickly air-cooled and pneumatically transported to a receiving vessel. In the final step, the collected gluten is sifted and milled to make the product uniform. ^[4]

Uses

When dough made with wheat flour is kneaded, gluten forms when glutenin molecules cross-link to make a sub-microscopic network and associates with gliadin, which contributes viscosity and extensibility to the mix.^[5] If such dough is leavened with yeast, sugar fermentation produces bubbles of carbon dioxide which, trapped by the gluten network, cause the dough to swell or 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 by hydration.^[6]

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 because that fraction contains the preponderance of the sulfur atoms responsible for the cross-linking in the network.^[7][8] More development leads to chewier products like pizza and bagels, while less development yields tender baked goods. In general, bread flours are high in gluten while cake flours are low. Kneading promotes the formation of gluten strands and cross-links, so a baked product is chewier in proportion to how much the dough is worked. Increased wetness of the dough also enhances gluten development.^[9] Shortening inhibits formation of cross-links, so it is used, along with diminished water and minimal working, when a tender and flaky product, such as pie crust, is desired.

Gluten, dried and milled to powder and added to ordinary flour dough, improves rising and increases the bread's structural stability and chewiness,^[10]. Such doughs must be worked vigorously if they are to rise to their full capacity, so a bread machine or food processor may be required for their kneading.^[11] The added gluten provides suppplemental protein to what would otherwise be high-carbohydrate preparations. The protein content of pet foods is also enhanced by adding gluten.^[12] When cooked in broth, gluten absorbs some of the surrounding liquid (including the taste) and becomes firm to the bite, so is widely used in vegetarian, vegan and Buddhist cuisines as a meat substitute. In China, as miàn jin, it is the basis for imitation meats resembling chicken, duck, fish, pork and beef. The Japanese variants, called namafu, yakifu, and seitan, are used in the same way.

Adverse reactions

Between 0.5 and 1.0 percent of the United States populace is sensitive to gluten.^{[13] [14]} Coeliac disease (or celiac disease, also called gluten sensitive enteropathy (GSE)), is the predominant disorder caused by gluten sensitivity. GSE is an abnormal immune reaction to digestive breakdown products of gliadin. This process damages the lining of the small intestine, which results in chronic malnutrition. Treatment requires a lifelong gluten-free diet and avoiding exposure to air-borne gluten-containing particles such as wheat flour. Gluten allergies and gluten-sensitive idiopathic neuropathies are two other adverse reactions to gluten.^[15]

Patients with conditions associated with GSE^[13] also benefit from a gluten-free diet and avoiding gluten inhalation. An example of gluten-related skin sensitivity is dermatitis herpetiformis, an intensely itchy skin eruption, which is nearly always accompanied by coeliac disease. This dermatitis usually develops in young adults, predominantly in males; people of North European ethnicity are especially susceptible.^[16]

A different, clinical definition of gluten has developed as a result of the determination of wheat gluten as the fraction of wheat that caused coeliac disease. Purification of the gluten proteins first revealed wheat gliadin as the culprit. Proteins from other cereals, even in small amounts, can cause the pathology to persist. Wheat gluten, and the similar proteins within the pure cereals of the grass tribe Triticeae (cultivars are wheats, barleys and ryes) can mediate enteropathy for the majority of affected individuals.^[17][18] These T-cell activating sites are also found in other the Triticeae genera including Aegilops.^[19] Therefore, these relatives of wheat are also commonly considered as having gluten.

Studies of the oat gluten avenin have revealed that pathogenic prolamins are either not present^[20], strain specific^[21] or weakly stimulatory.^[22] For some 2% of coeliacs, 'gluten-free' extends to the foods free of oats glutens.

See also

• Textured vegetable protein
• Zein
• Triticeae glutens

References

Further reading

• Curtis, B.C.; Rajaram, S. & Macpherson, H.G., , . Retrieved on 2007-8-21
• Pfluger, Laura, , . Retrieved on 2007-9-29
• , USDA Cereal Disease Laboratory,
• Wieser, H. (2007), , . Retrieved on 2007-9-27
• Wrigley, C.W; Bekes, F. & Bushuk, W (2006), , AACC International, ISBN 1-891127-51-9, be-x-old:Глютэн