Gluten

Gluten is a mixture 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 populace, including those with coeliac disease, is gluten-intolerant and cannot consume it safely.

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, 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.

Uses
When cooked in broth, gluten absorbs some of the surrounding liquid (including the taste) and becomes firm to the bite, so is often used in vegetarian, vegan and Buddhist cuisines as a meat substitute. In China, as miàn jīn , it is the basis for imitation meats resembling chicken, duck, fish, pork and beef. The Japanese variants, called namafu, yakifu, or seitan is used in the same way.

When dough made with wheat flour is kneaded, the gluten molecules cross-link to form a sub-microscopic network. If such dough is leavened with yeast, sugar fermentation produces bubbles of carbon dioxide which are trapped by the gluten network, causing 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.

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. More development leads to chewier products like pizza and bagels, while less development yields tender baked goods such as pie crust. Several other factors affect the development of gluten in baked goods:


 * The amount of gluten in the flour. For examples, bread flour has high gluten content, while cake flour is low in gluten.
 * Fat inhibits the formation of long gluten strands, so increased shortening yields a more tender product.
 * Kneading develops the gluten strands, so a baked product is chewier in proportion to how much the dough is worked.
 * Water is essential to gluten development, so more of it is used in doughs when a chewier texture is desired.


 * As a practical application of these rules, pie crust, which should be very tender, is made with low-gluten flour, much shortening, and little liquid, then mixed only enough to combine the ingredients.

Gluten can be dried and milled into a flour or powder, which, added to ordinary flour dough, makes for higher rising and increases the bread's structural stability and chewiness. . Since such doughs must be worked vigorously if they are to rise to their full capacity, a bread machine or food processor may be required for their kneading.

Gluten is used as a protein supplement, especially in low-carbohydrate baked goods where it replaces flour. It is also added to many pet foods to increase their protein content.

Occurrence
Strictly speaking, gluten is the non-starch (proteinaceous) component of wheat that allows bread dough to rise. Wheat gluten is found to contain albumin, globulins, glutelins and prolamins. The 'sticky' proteins in the gluten that facilitate bread making are known as gliadin (wheat prolamins) and glutenin (wheat glutelins). Glutenin from wheat possesses one additional property that makes bread gluten unique; it is an elastic protein, but on heat denaturation, fixes it shape. This prevents bread from collapsing after cooking. Because wheat is made of three grass genomes (AA, BB, DD allohexaploid), the amount of glutinous protein is higher than most related species. Wheat grown in countries with cold growing seasons, e.g. Canada, tends to have a higher gluten content than wheat grown in countries with mild climates. High levels of glutens are important for some baking procedures, others require less gluten. Wheat flour with a high gluten content is called "strong" or "hard" flour, and is used for breads, whereas flour with a lower gluten content is called "soft" flour, and is used for cakes.

Glutinous cereals
Gluten is derived from the Latin word glūten, meaning glue. Such 'sticky' proteins are found over the seed producing species and include other seeds like corn, rice, and soy. Glutinous ('sticky') strains and preparations (for example: glutinous rice (aka sticky rice), or corn gluten feed), are used widely to describe the products enriched with the seed storage proteins of cereals -- although in the case of glutinous rice, the stickiness is inherent in the natural grain of at least some types. Some of these proteins are useful in making gluten-free breads but most require some form of supplementation (xanthum gum or gum arabic). As a result gluten is widely used to describe similar proteins (particularly prolamins) from other cereal species.

Adverse reactions
Between 0.5 and 1.0 percent of the United States populace is sensitive to gluten. 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.

Patients with conditions associated with GSE 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.

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. These T-cell activating sites are also found in other the Triticeae genera including Aegilops. 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, strain specific or weakly stimulatory. For some 2% of coeliacs, 'gluten-free' extends to the foods free of oats glutens.