Rhubarb (Rheum rhabarbarum or Rheum × hybridum according to the British Royal Horticultural Society) is a species of plant in the family Polygonaceae. It is a herbaceous perennial growing from short, thick rhizomes. The fleshy, edible stalks (petioles), are used in cooking, but the large, triangular leaves contain high levels of oxalic acid, making them inedible. The small flowers are grouped in large compound leafy greenish-white to rose-red inflorescences.
Although rhubarb is a vegetable, it is often put to the same culinary uses as fruits. The leaf stalks can be used raw, when they have a crisp texture (similar to celery, although it is in a different family), but are most commonly cooked with sugar and used in pies, crumbles and other desserts. They have a strong, tart taste. Several varieties have been domesticated for human consumption, most of which are recognised as Rheum x hybridum by the Royal Horticultural Society.
Rhubarb contains anthraquinones including rhein, and emodin and their glycosides (e.g. glucorhein), which impart cathartic and laxative properties. It is hence useful as a cathartic in case of constipation.
Rhubarb is grown widely, and with greenhouse production it is available throughout much of the year. Rhubarb grown in hothouses (heated greenhouses) is called "hothouse rhubarb", and is typically made available at consumer markets in early spring, before outdoor cultivated rhubarb is available. Hothouse rhubarb is usually brighter red, tenderer and sweeter-tasting than outdoor rhubarb. In temperate climates, rhubarb is one of the first food plants harvested, usually in mid- to late spring (April or May in the Northern Hemisphere, October or November in the Southern Hemisphere), and the season for field-grown plants lasts until the end of summer. In the northwestern US states of Oregon and Washington, there are typically two harvests, from late April to May and from late June into July; half of all US production is in Pierce County, Washington. Rhubarb is ready to consume as soon as harvested, and freshly cut stalks are firm and glossy.
In the United Kingdom, the first rhubarb of the year is harvested by candlelight in forcing sheds where all other light is excluded, a practice that produces a sweeter, more tender stalk. These sheds are dotted around the "Rhubarb Triangle" between Wakefield, Leeds, and Morley.
The colour of rhubarb stalks can vary from the commonly associated crimson red, through speckled light pink, to simply light green. Rhubarb stalks are poetically described as "crimson stalks". The colour results from the presence of anthocyanins, and varies according to both rhubarb variety and production technique. The colour is not related to its suitability for cooking:
The Chinese call rhubarb "the great yellow" (dà huáng 大黃), and have used rhubarb root for medicinal purposes for thousands of years. It appears in The Divine Farmer's Herb-Root Classic which is thought to have been compiled about 2,700 years ago. Though Dioscurides' description of ρηον or ρά indicates that a medicinal root brought to Greece from beyond the Bosphorus may have been rhubarb, commerce in the drug did not become securely established until Islamic times. During Islamic times, it was imported along the Silk Road, reaching Europe in the 14th century through the ports of Aleppo and Smyrna, where it became known as "Turkish rhubarb". Later, it also started arriving via the new maritime routes, or overland through Russia. The "Russian rhubarb" was the most valued, probably because of the rhubarb-specific quality control system maintained by the Russian Empire.
The cost of transportation across Asia made rhubarb expensive in medieval Europe. It was several times the price of other valuable herbs and spices such as cinnamon, opium, and saffron. The merchant explorer Marco Polo therefore searched for the place where the plant was grown and harvested, discovering that it was cultivated in the mountains of Tangut province. The value of rhubarb can be seen in Ruy Gonzáles de Clavijo's report of his embassy in 1403–05 to Timur in Samarkand: "The best of all merchandise coming to Samarkand was from China: especially silks, satins, musk, rubies, diamonds, pearls, and rhubarb..."
The high price as well as the increasing demand from apothecaries stimulated efforts to cultivate the plant on European soil. These were unsuccessful in producing rhubarb root with the necessary medicinal qualities, but the variety then grown in Russia became the ancestor of the common modern rhubarb. The availability of this "Siberian rhubarb", together with the increasing abundance and decreasing price of sugar in the 18th century, galvanised its culinary adoption.
Though it is often asserted that rhubarb first came to the United States in the 1820s, John Bartram was growing medicinal and culinary rhubarbs in Philadelphia from the 1730s, planting seeds sent him by Peter Collinson. From the first, the familiar garden rhubarb was not the only Rheum in American gardens: Thomas Jefferson planted R. undulatum at Monticello in 1809 and 1811, observing that it was "Esculent rhubarb, the leaves excellent as Spinach."
The advocate of organic gardening Lawrence D. Hills listed his favourite rhubarb varieties for flavour as ‘Hawke's Champagne’, ‘Victoria’, ‘Timperley Early’, and ‘Early Albert’, also recommending ‘Gaskin's Perpetual’ for having the lowest level of oxalic acid, allowing it to be harvested over a much longer period of the growing season without developing excessive sourness.
"Rhubarb" originally comes from the two Greek words for rhubarb. Rheon from the Persian rewend, later became the Latin word rheum, meaning rhubarb. The other Greek word is rha, an ancient name for the Volga River in Russian, where rhubarb was cultivated, having been introduced there from China via the Silk Road. In medieval Latin, rheon and rha became rheum barbarum, literally "foreign rhubarb" or "strange rhubarb", and evolved later into rheubarbarum. Most likely, the English word "rhubarb" entered the language via the French word rhubarbe brought over by the conquering Normans and from the Latin rheubarbarum.
Rhubarb is grown primarily for its fleshy leafstalks, technically known as petioles. The use of rhubarb stalks as food is a relatively recent innovation. This usage was first recorded in 17th-century England after affordable sugar became available to common people, and reached a peak between the 20th century's two world wars.
Commonly, it is stewed with sugar or used in pies and desserts, but it can also be put into savoury dishes or pickled. Rhubarb can be dehydrated and infused with fruit juice. In most cases, it is infused with strawberry juice to mimic the popular strawberry rhubarb pie.
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For cooking, the stalks are often cut into small pieces and stewed (boiled in water) with added sugar, until soft. Little water is added, as rhubarb stalks already contain a great deal of water. Rhubarb should be processed and stored in containers which are unaffected by residual acid content, such as glass or stainless steel. Spices such as cinnamon, nutmeg, and ginger are sometimes added. Stewed rhubarb or rhubarb sauce, like applesauce, is usually eaten cold. Pectin, or sugar with pectin, can be added to the mixture to make jams.
A similar preparation, thickened with cornstarch or flour, is used as filling for rhubarb pie, tarts, and crumbles, leading to the nickname "pie plant", by which it is referred to in many 19th-century cookbooks, as well as by American author Laura Ingalls Wilder in her short novel The First Four Years.
In former days, a common and affordable sweet for children in parts of the United Kingdom and Sweden was a tender stick of rhubarb, dipped in sugar. It is still eaten this way in western Finland, Norway, Canada, Iceland, the Faroe Islands and Sweden, and also some other parts of the world. In Chile, Chilean rhubarb, which is only very distantly related, is sold on the street with salt or dried chili pepper.
In traditional Chinese medicine, rhubarb roots have been thought of as a laxative for several millennia. Rhubarb also appears in medieval Arabic and European prescriptions. It was one of the first Chinese medicines to be imported to the West from China.
The roots and stems are rich in anthraquinones, such as emodin and rhein. These substances are cathartic and laxative, explaining the sporadic use of rhubarb as a dieting aid. The anthraquinone compounds have been separated from powdered rhubarb root for medicinal purposes.
Rhubarb contains quinone molecules which are capable of carrying an electrical charge. In 2014, a Harvard-based team of scientists published results describing the use of the quinone AQDS, almost identical to a form found in rhubarb, in flow-batteries.
Rhubarb leaves contain poisonous substances, including oxalic acid, which is a nephrotoxic and corrosive acid that is present in many plants. Humans have been poisoned after ingesting the leaves, a particular problem during World War I when the leaves were mistakenly recommended as a food source in Britain. The toxic rhubarb leaves have been used in flavouring extracts, after the oxalic acid is removed by treatment with precipitated chalk (i.e., calcium carbonate). Oxalic acid can also be found in the stalks of rhubarb, but the levels are too low to cause any bodily harm.
The LD50 (median lethal dose) for pure oxalic acid in rats is about 375 mg/kg body weight, or about 25 grams for a 65-kilogram (143 lb) human. Other sources give a much higher oral LDLo (lowest published lethal dose) of 600 mg/kg. While the oxalic acid content of rhubarb leaves can vary, a typical value is about 0.5%,  meaning a 65 kg adult would need to eat 4 to 8 kg (9 to 18 lbs) to obtain a lethal dose, depending on which lethal dose is assumed. Cooking the leaves with baking soda can make them more poisonous by producing soluble oxalates. The leaves are believed to also contain an additional, unidentified toxin, which might be an anthraquinone glycoside (also known as senna glycosides).
Hungry wildlife may dig up and eat rhubarb roots in the spring, as stored starches are turned to sugars for new foliage growth.
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