Gluten

Wheat seed, sectioned to reveal endosperm and embryo

Wheat — a prime source of 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 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, but 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 wheat gluten by lacking gliadin. The glutenin in wheat flour gives kneaded dough its elasticity, allows leavening and contributes chewiness to baked products like bagels.

Although wheat supplies much of the world's dietary protein and food supply, as many as 0.5% to 1% of the population of the United States has celiac disease, a condition which results from an inappropriate immune system response to gluten.^[1] The manifestations of celiac disease range from no symptoms to malabsorption of nutrients with involvement of multiple organ systems. The only effective treatment is a lifelong gluten-free diet.

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. 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,^[2] 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. ^[3]

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.^[4] 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.^[5]

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.^[6][7] 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.^[8] 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,^[9]. 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.^[10] The added gluten provides supplemental protein to what would otherwise be high-carbohydrate preparations. The protein content of pet foods is also enhanced by adding gluten.^[11] 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.

Gluten as an additive

The "Codex Alimentarius" set of international standards for food labeling has a standard relating to the labelling of products as "gluten free", however this standard does not apply to "foods which in their normal form do not contain gluten".^[12] Gluten is used as a stabilizing agent in products like ice-cream and ketchup, where it may be unexpected.^{[13] [14]} 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 may 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).^[15] Requirements for proper labeling are being formulated by the USDA. In the United Kingdom, only cereals currently need to be labelled, while other products are voluntary.^[16] In fact, in the UK most gluten free food is clearly labelled so, as is non-gluten-free food.

Adverse reactions

Main article: Gluten sensitivity

Main article: Gluten free diet

Between 0.5 and 1.0 percent of people in the United States are sensitive to gluten. ^{[17] [18]} Coeliac disease (or celiac disease, also called gluten sensitive enteropathy (GSE)), caused by an abnormal immune reaction to partially digested gliadin, is the predominant disorder. It probably occurs with comparable frequencies among all wheat-eating populations in the world.^[19] Certain allergies and neuropathies are also caused by gluten consumption and inhalation.^[20]

Note that wheat allergy and gluten intolerance are not the same thing.^[21]

See also

• Textured vegetable protein
• Zein
• Triticeae glutens

References

Further reading

• Curtis, B.C.; Rajaram, S.; Macpherson, H.G., Bread Wheat, Improvement and production — FAO Plant Production and Protection Series No. #30., http://www.fao.org/DOCREP/006/Y4011E/y4011e05.htm, retrieved on 2007-08-21 
• Pfluger, Laura, Marker Assisted Selection in Wheat, Quality traits. Gluten Strength, Coordinated Agricultural Project (funded by USDACREES), http://maswheat.ucdavis.edu/protocols/gluten/index.htm, retrieved on 2007-09-29 
• Agricultural Databases, Statistics, etc., USDA Cereal Disease Laboratory, http://www.ars.usda.gov/Main/docs.htm?docid=11017 
• Wieser, H. (2007), Cereal Chemistry and Dough Properties, KL1-Gluten Chemistry, German Research Centre of Food Chemistry and Cereal Chemistry and Hans-Dieter-Belitz-Institute for Cereal Grain Research, D-85748 Garching, Germany, http://fp.cerealsandeurope.net/events/sourdough/session1.htm, retrieved on 2007-09-27 
• Wrigley, C.W; Bekes, F.; Bushuk, W (2006), The Gluten Composition of Wheat Varieties and Genotypes, AACC International, ISBN 1-891127-51-9, http://aaccnet.org/grainbin/gluten_gliadin.asp, retrieved on 2007-09-21