| Kingdom: | Plantae |
| (unranked): | Angiosperms |
| (unranked): | Eudicots |
| (unranked): | Rosids |
| Order: | Fabales |
| Family: | Fabaceae |
| Subfamily: | Faboideae |
| Genus: | Glycine |
| Species: | G. max |
| Binomial name | |
|---|---|
| Glycine max (L.) Merr. | |
| Synonyms[1] | |
The soybean (US) or soya bean (UK) (Glycine max)[2] is a species of legume native to East Asia, widely grown for its edible bean which has numerous uses. The plant is classed as an oilseed rather than a pulse by the UN Food and Agricultural Organization (FAO).
Fat-free (defatted) soybean meal is a significant and cheap source of protein for animal feeds and many prepackaged meals; soy vegetable oil is another product of processing the soybean crop. For example, soybean products such as textured vegetable protein (TVP) are ingredients in many meat and dairy analogues.[3] Soybeans produce significantly more protein per acre than most other uses of land.[4]
Traditional nonfermented food uses of soybeans include soy milk, and from the latter tofu and tofu skin. Fermented foods include soy sauce, fermented bean paste, natto, and tempeh, among others. The oil is used in many industrial applications. The main producers of soy are the United States (35%), Brazil (27%), Argentina (19%), China (6%) and India (4%).[5][6] The beans contain significant amounts of phytic acid, alpha-linolenic acid, andisoflavones.
Classification
The genus name Glycine was originally introduced by Carl Linnaeus (1737) in his first edition of Genera Plantarum. The word glycine is derived from the Greek – glykys (sweet) and likely refers to the sweetness of the pear-shaped (apios in Greek) edible tubers produced by the native North American twining or climbing herbaceous yambean legume, Glycine apios, now known as Apios americana. The cultivated soybean first appeared in Species Plantarum, by Linnaeus, under the name Phaseolus max L. The combination Glycine max (L.) Merr., as proposed by Merrill in 1917, has become the valid name for this useful plant.
The genus Glycine Willd. is divided into two subgenera, Glycine and Soja. The subgenus Soja (Moench) F.J. Herm. includes the cultivated soybean,Glycine max (L.) Merr., and the wild soybean, Glycine soja Sieb. & Zucc. Both species are annuals. Glycine soja is the wild ancestor of Glycine max, and grows wild in China, Japan, Korea, Taiwan and Russia.[9] The subgenus Glycine consists of at least 16 wild perennial species: for example, Glycine canescens F.J. Herm. and G. tomentella Hayata, both found in Australia and Papua New Guinea.[10][11]
Like some other crops of long domestication, the relationship of the modern soybean to wild-growing species can no longer be traced with any degree of certainty. It is a cultural variety with a very large number of cultivars.
Description and physical characteristics
Soy varies in growth and habit. The height of the plant varies from less than 0.2 to 2.0 m (0.66 to 6.6 ft).
The pods, stems, and leaves are covered with fine brown or gray hairs. The leaves are trifoliolate, having three to four leaflets per leaf, and the leaflets are 6–15 cm (2.4–5.9 in) long and 2–7 cm (0.79–2.8 in) broad. The leaves fall before the seeds are mature. The inconspicuous, self-fertile flowers are borne in the axil of the leaf and are white, pink or purple.
The fruit is a hairy pod that grows in clusters of three to five, each pod is 3–8 cm long (1–3 in) and usually contains two to four (rarely more) seeds 5–11 mm in diameter.
Soybeans occur in various sizes, and in many hull or seed coat colors, including black, brown, blue, yellow, green and mottled. The hull of the mature bean is hard, water-resistant, and protects the cotyledon and hypocotyl (or "germ") from damage. If the seed coat is cracked, the seed will not germinate. The scar, visible on the seed coat, is called the hilum (colors include black, brown, buff, gray and yellow) and at one end of the hilum is the micropyle, or small opening in the seed coat which can allow the absorption of water for sprouting.
Remarkably, seeds such as soybeans containing very high levels of protein can undergo desiccation, yet survive and revive after water absorption. A. Carl Leopold, son of Aldo Leopold, began studying this capability at the Boyce Thompson Institute for Plant Research at Cornell University in the mid 1980s. He found soybeans and corn to have a range of soluble carbohydrates protecting the seed's cell viability.[12] Patents were awarded to him in the early 1990s on techniques for protecting "biological membranes" and proteins in the dry state. Compare to tardigrades.
[edit]Nitrogen-fixing ability
Many legumes (alfalfa, clover, peas, beans, lentils, soybeans, peanuts and others) contain symbiotic bacteria called Rhizobia within nodules of their root systems. These bacteria have the special ability of fixing nitrogen from atmospheric, molecular nitrogen (N2) into ammonia (NH3).[13] The chemical reaction is:
- N2 + 8 H+ + 8 e− → 2 NH3 + H2
Ammonia is then converted to another form, ammonium (NH4+), usable by (some) plants by the following reaction:
- NH3 + H+ → NH4+
This arrangement means that the root nodules are sources of nitrogen for legumes, making them relatively rich in plant proteins.
Chemical composition of the seed
| Nutritional value per 100 g (3.5 oz) | |
|---|---|
| Energy | 1,866 kJ (446 kcal) |
| Carbohydrates | 30.16 g |
| - Sugars | 7.33 g |
| - Dietary fiber | 9.3 g |
| Fat | 19.94 g |
| - saturated | 2.884 g |
| - monounsaturated | 4.404 g |
| - polyunsaturated | 11.255 g |
| Protein | 36.49 g |
| - Tryptophan | 0.591 g |
| - Threonine | 1.766 g |
| - Isoleucine | 1.971 g |
| - Leucine | 3.309 g |
| - Lysine | 2.706 g |
| - Methionine | 0.547 g |
| - Cystine | 0.655 g |
| - Phenylalanine | 2.122 g |
| - Tyrosine | 1.539 g |
| - Valine | 2.029 g |
| - Arginine | 3.153 g |
| - Histidine | 1.097 g |
| - Alanine | 1.915 g |
| - Aspartic acid | 5.112 g |
| - Glutamic acid | 7.874 g |
| - Glycine | 1.880 g |
| - Proline | 2.379 g |
| - Serine | 2.357 g |
| Water | 8.54 g |
| Vitamin A equiv. | 1 μg (0%) |
| Thiamine (vit. B1) | 0.874 mg (76%) |
| Riboflavin (vit. B2) | 0.87 mg (73%) |
| Niacin (vit. B3) | 1.623 mg (11%) |
| Pantothenic acid (B5) | 0.793 mg (16%) |
| Vitamin B6 | 0.377 mg (29%) |
| Folate (vit. B9) | 375 μg (94%) |
| Choline | 115.9 mg (24%) |
| Vitamin C | 6.0 mg (7%) |
| Vitamin E | 0.85 mg (6%) |
| Vitamin K | 47 μg (45%) |
| Calcium | 277 mg (28%) |
| Iron | 15.7 mg (121%) |
| Magnesium | 280 mg (79%) |
| Manganese | 2.517 mg (120%) |
| Phosphorus | 704 mg (101%) |
| Potassium | 1797 mg (38%) |
| Sodium | 2 mg (0%) |
| Zinc | 4.89 mg (51%) |
| Link to USDA Database entry Percentages are relative to US recommendations for adults. Source: USDA Nutrient Database | |
Together, soybean oil and protein content account for about 60% of dry soybeans by weight (protein at 40% and oil at 20%). The remainder consists of 35% carbohydrate and about 5% ash. Soybean cultivars comprise approximately 8% seed coat or hull, 90% cotyledons and 2% hypocotyl axis or germ.
Most soy protein is a relatively heat-stable storage protein. This heat stability enables soy food products requiring high temperature cooking, such astofu, soy milk and textured vegetable protein (soy flour) to be made.
The principal soluble carbohydrates of mature soybeans are the disaccharide sucrose (range 2.5–8.2%), the trisaccharide raffinose (0.1–1.0%) composed of one sucrose molecule connected to one molecule of galactose, and the tetrasaccharide stachyose (1.4 to 4.1%) composed of one sucrose connected to two molecules of galactose. While the oligosaccharides raffinose and stachyose protect the viability of the soybean seed from desiccation (see above section on physical characteristics) they are not digestible sugars, so contribute to flatulence and abdominal discomfort in humans and other monogastric animals, comparable to the disaccharide trehalose. Undigested oligosaccharides are broken down in the intestine by native microbes, producing gases such as carbon dioxide, hydrogen, and methane.
Since soluble soy carbohydrates are found in the whey and are broken down during fermentation, soy concentrate, soy protein isolates, tofu, soy sauce, and sprouted soybeans are without flatus activity. On the other hand, there may be some beneficial effects to ingesting oligosaccharides such as raffinose and stachyose, namely, encouraging indigenous bifidobacteria in the colon against putrefactive bacteria.
The insoluble carbohydrates in soybeans consist of the complex polysaccharides cellulose, hemicellulose, and pectin. The majority of soybean carbohydrates can be classed as belonging to dietary fiber.
Within soybean oil or the lipid portion of the seed is contained the phytosterols: stigmasterol (17–21%), sitosterol(53–56%) and campesterol (20–23%) accounting for 2.5% of the lipid fraction.
Saponins, a class of natural surfactants (soaps), are sterols that are present naturally in a wide variety of food-plants: vegetables, legumes, and cereals–ranging from beans and spinach to tomatoes, potatoes and oats. Whole soybeans contain from 0.17 to 6.16% saponins, 0.35 to 2.3% in defatted soy flour and 0.06 to 1.9% in tofu. Legumes such as soybean and chickpeas are the major source of saponins in the human diet. Sources of non-dietary saponins include alfalfa, sunflower, herbs and barbasco. Recent studies have shown that saponins are potential functional food ingredients because of their physiological properties.[14]
Soy contains isoflavones like genistein and daidzein. It also contains glycitein, an O-methylated isoflavone which accounts for 5–10% of the total isoflavones in soy food products. Glycitein is a phytoestrogen with weak estrogenic activity, comparable to that of the other soy isoflavones.[15]
[edit]Nutrition
Further information: Soy protein
For human consumption, soybeans must be cooked with "wet" heat to destroy the trypsin inhibitors (serine protease inhibitors). Raw soybeans, including the immature green form, are toxic to humans, swine, chickens, and in fact, all monogastric animals.[16]
Soybeans are considered by many agencies to be a source of complete protein.[17] A complete protein is one that contains significant amounts of all the essential amino acids that must be provided to the human body because of the body's inability to synthesize them. For this reason, soy is a good source of protein, amongst many others, for vegetarians and vegans or for people who want to reduce the amount of meat they eat. According to theUS Food and Drug Administration:
Soy protein products can be good substitutes for animal products because, unlike some other beans, soy offers a 'complete' protein profile. ... Soy protein products can replace animal-based foods—which also have complete proteins but tend to contain more fat, especially saturated fat—without requiring major adjustments elsewhere in the diet.[17]
The gold standard for measuring protein quality, since 1990, is the Protein Digestibility Corrected Amino Acid Score (PDCAAS) and by this criterion soy protein is the nutritional equivalent of meat, eggs, and casein for human growth and health. Soybean protein isolate has a biological value of 74, whole soybeans 96, soybean milk 91, and eggs 97.[18]
Soy protein is essentially identical to the protein of other legume seeds and pulses.[19][20][21] Moreover, soybeans can produce at least twice as much protein per acre than any other major vegetable or grain crop besides hemp, five to 10 times more protein per acre than land set aside for grazing animals to make milk, and up to 15 times more protein per acre than land set aside for meat production.[4]
Consumption of soy may also reduce the risk of colon cancer, possibly due to the presence of sphingolipids.[22]
[edit]Comparison to other major staple foods
The following table shows the nutrient content of green soybean and other major staple foods, each in respective raw form. Raw staples, however, aren't edible and can not be digested. These must be sprouted, or prepared and cooked for human consumption. In sprouted and cooked form, the relative nutritional and anti-nutritional contents of each of these grains is remarkably different from that of raw form of these grains reported in this table. The nutritional value of soybean and each cooked staple depends on the pre-processing and the method of cooking: boiling, frying, roasting, baking, etc.
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