Rhubarb (Rheum rhabarbarum) is a species of plant in the family Polygonaceae. They are herbaceous perennials growing from short, thick rhizomes. They have large leaves that are somewhat triangular, with long fleshy petioles. They have small flowers grouped in large compound leafy greenish-white to rose-red inflorescences.
In culinary use, fresh raw petioles (leaf stalks) are crisp (similar to celery) with a strong, tart taste. Most commonly, the plant's leaf stalks are cooked with sugar and used in pies and other desserts. A number of varieties have been domesticated for human consumption, most of which are recognised as Rheum x hybridum by the Royal Horticultural Society.
Rhubarb is usually considered a vegetable. In the United States, however, a New York court decided in 1947 that since it was used in the United States as a fruit, it counted as a fruit for the purposes of regulations and duties. A side effect was a reduction on imported rhubarb tariffs, as tariffs were higher for vegetables than fruits.
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 in many areas. 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, more tender 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/May in the Northern Hemisphere, October/November in the Southern Hemisphere), and the season for field-grown plants lasts until September. 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. 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 noted "Rhubarb Triangle" of Wakefield, Leeds, and Morley.
Rhubarb grows year-round in warm climates, but in temperate climates the above ground portion of the plant completely withers away at the onset of freezing temperature. The plant grows from the root at the return of warm weather. Rhubarb growth can be 'forced' or encouraged to grow early by raising the local temperature, usually by placing an upturned bucket over the new shoots.
Because rhubarb is a seasonal plant, obtaining fresh rhubarb out of season is difficult in colder climates, such as in the UK, Ireland, Russia, etc. Rhubarb thrives in areas of direct sunlight and can successfully be planted in containers if they are large enough to accommodate a season's growth.
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 green-stalked rhubarb is more robust and has a higher yield, but the red-coloured stalks are much more popular with consumers.
Rhubarb has been used for medical purposes by the Chinese for thousands of years, and 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, when the usual route lay through Russia, "Russian rhubarb" became the familiar term.
For centuries, the plant has grown wild along the banks of the River Volga, for which the ancient Scythian hydronym was Rhā. 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 term "rhubarb" is a combination of the Ancient Greek rha and barbarum; rha refers both to the plant and to the River Volga. Rhubarb first came to the United States in the 1820s, entering the country in Maine and Massachusetts and moving westwards with the European American settlers.
Rhubarb is grown primarily for its fleshy stalks, technically known as petioles. The use of rhubarb stems 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 savory 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.
Rhubarb root produces a rich brown dye similar to walnut husks. It is used in northern regions where walnut trees do not survive.
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For cooking, the stalks are often cut into small pieces and stewed (boiled in water) with added sugar. Little water is needed, as rhubarb stalks contain a great deal of water on their own. Spices such as cinnamon, nutmeg, and ginger are sometimes added. The sliced stalks are boiled until soft. Stewed rhubarb, rhubarb sauce, like applesauce, is usually eaten cold.
A similar preparation thickened with cornstarch is used as filling for rhubarb pie, tarts, and crumbles. Sometimes stewed strawberries are mixed with the rhubarb in strawberry-rhubarb pie. This common use has led to the slang term for rhubarb, pie plant, by which name it was more commonly known in the United States in the late 19th century. In her novella The First Four Years, American author Laura Ingalls Wilder refers to rhubarb as "pie plant". It can also be used to make a fruit wine.
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 and Norway, Iceland and 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, not sugar.
In traditional Chinese medicine, rhubarb roots have been used 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 carbon-based 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. Such poisoning was a particular problem in World War I, when the leaves were recommended as a food source in Britain.
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 kg (~140 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%, so a rather unlikely 5 kg (for a 70 kg human) of the extremely sour leaves would have to be consumed to reach an LD50 of oxalic acid. Cooking the leaves with soda can make them more poisonous by producing soluble oxalates. However, 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.
In British theatre and early radio drama, the words "rhubarb rhubarb" were repeated for the effect of unintelligible conversation in the background. This usage lent its title to the 1969 film Rhubarb and its 1980 remake Rhubarb Rhubarb.
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