Rhubarb

Rhubarbis the fleshy, edible stalks (petioles) of species and hybrids (culinary rhubarb) ofRheumin the familyPolygonaceae, which are cooked and used for food.The plant is a herbaceousperennialthat grows from short, thickrhizomes. Historically, different plants have been called "rhubarb" in English. The large, triangularleavescontain high levels ofoxalic acidandanthroneglycosides, making them inedible. The smallflowersare grouped in large compound leafy greenish-white to rose-redinflorescences.
The precise origin of culinary rhubarb is unknown. The speciesRheum rhabarbarum(syn.R. undulatum) andR. rhaponticumwere grown in Europe before the 18th century and used for medicinal purposes. By the early 18th century, these two species and a possiblehybridof unknown origin,R.×hybridum, were grown as vegetable crops in England and Scandinavia. They readily hybridize, andculinary rhubarbwas developed by selecting open-pollinated seed, so its precise origin is almost impossible to determine.In appearance, samples of culinary rhubarb vary on a continuum betweenR. rhaponticumandR. rhabarbarum. However, modern rhubarb cultivars aretetraploidswith 2n= 44, in contrast to 2n= 22 for the wild species.
Rhubarb is avegetableand is often put to the same culinary uses asfruits.The leaf stalks can be used raw while they have a crisp texture, but are most commonly cooked with sugar and used inpies,crumbles, and otherdesserts. They have a strong,tart taste. Manycultivarshave been developed for human consumption, most of which are recognised asRheum×hybridumby theRoyal Horticultural Society.
The wordrhubarbis likely to have derived in the 14th century from theOld Frenchrubarbe, which came from the LatinrheubarbarumandGreekrha barbaron, meaning 'foreign rhubarb'.The Greek physicianDioscoridesused the Greek wordῥᾶ(rha), whereasGalenlater usedῥῆον(rhēon), Latinrheum. These in turn derive from aPersianname for species ofRheum.The specific epithetrhaponticum, applying to one of the presumed parents of the cultivated plant, means 'rha from theregion of the Black Sea'or the riverVolga, Rha being its ancient name.
Rhubarb is grown widely, and withgreenhouseproduction it is available throughout much of the year. It needs rainfall and an annual cold period of up to 7–9 weeks at 3 °C (37 °F), known as 'cold units', to grow well. The plant develops a substantial underground storage organ (rhubarb crowns) and this can be used for early production by transferring field-grown crowns to warm conditions.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.After forcing for commercial production, the crowns are usually discarded.Intemperate climates, rhubarb is one of the first food plantsharvested, usually in mid- to late spring (April or May in theNorthern Hemisphere, October or November in theSouthern Hemisphere), and the season for field-grown plants lasts until the end of summer.
In the United Kingdom, the first rhubarb of the year is harvested by candlelight inforcing shedswhere all other light is excluded, a practice that produces a sweeter, more tender stalk.These sheds are dotted around the "Rhubarb Triangle" inYorkshirebetweenWakefield,Leeds, andMorley.
In the United States rhubarb is primarily produced in the states ofOregon,Washington, andWisconsinwith approximately 1,200 acres in production, of which 175 are covered in hothouses.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 commercial production is inPierce County, Washington.Rhubarb is ready to consume as soon as harvested, and freshly cut stalks are firm and glossy.
Rhubarb damaged by severe cold should not be eaten, as it may be high inoxalic acid, which migrates from the leaves and can cause illness.
The colour of rhubarb stalks can vary from the commonly associated crimson red, through speckled lightpink, to simply light green. Rhubarb stalks are poetically described as "crimson stalks". The colour results from the presence ofanthocyanins, 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.It appears inThe Divine Farmer's Herb-Root Classic, which is thought to have been compiled about 1,800 years ago.ThoughDioscurides' description ofρηονorράindicates that a medicinal root brought to Greece from beyond theBosphorusmay have been rhubarb, commerce in the plant did not become securely established untilIslamic times. During Islamic times, it was imported along theSilk Road, reaching Europe in the 14th century through the ports ofAleppoandSmyrna, where it became known as "Turkish rhubarb".Later,it began to arrive via new maritime routes and 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 2020 edition ofPharmacopoeia of the People's Republic of Chinalists the following species as medicinally acceptable:Rheum officinale,Rheum palmatum, andRheum tanguticum.Grieve describes "Turkish rhubarb" as a mixture ofR. palmatumandR. rhaponticum.
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 ascinnamon,opium, andsaffron. The merchant explorerMarco Polotherefore searched for the place where the plant was grown and harvested, discovering that it was cultivated in the mountains ofTangutprovince.The value of rhubarb can be seen inRuy Gonzáles de Clavijo's report of his embassy in 1403–1405 toTimurinSamarkand: "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 different species of rhubarb on European soil.R. rhaponticum×R. officinalecame to be grown in England to produce the roots.R. alpinuswas also allowed to grow wild.The local availability of the plants grown for medicinal purposes, together with the increasing abundance and decreasing price of sugar in the 18th century, galvanised its culinary adoption.Grieve claims a date of 1820 in England.Rhubarb was harvested inScotlandfrom at least 1786, having been introduced to the Botanical Garden inEdinburghby the travellerBruce of Kinnairdin 1774. He brought the seeds fromAbyssiniaand they produced 3,000 plants.
Though it is often asserted that rhubarb first came to the United States in the 1820s,John Bartramwas growing medicinal and culinary rhubarbs inPhiladelphiafrom the 1730s, planting seeds sent to him byPeter Collinson.From the first, the familiar garden rhubarb was not the onlyRheumin American gardens:Thomas JeffersonplantedR. undulatumat Monticello in 1809 and 1811, observing that it was "Esculent rhubarb, the leaves excellent as Spinach."
The advocate of organic gardeningLawrence D. Hillslisted 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.
The Royal Horticultural Society has the UK's national collection of rhubarb that comprises 103 varieties. In 2021–2022 this was moved from southern England to the more northern gardenRHS Bridgewaterwhere winter cold and rainfall are better suited for rhubarb.The following cultivars have gained theRoyal Horticultural Society'sAward of Garden Merit:
Rhubarb is grown primarily for its fleshy leafstalks, technically known aspetioles. The use of rhubarb stalks as food is a relatively recent innovation. This usage was first recorded in 18th- to 19th-century England after affordablesugarbecame more widely available.
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 the United States, it is usually infused with strawberry juice to mimic the popularstrawberry rhubarb pie.
The speciesRheum ribeshas been eaten in theIslamic worldsince the 10th century.
In Northern Europe and North America, the stalks are commonly cut into pieces and stewed with added sugar until soft.The resultingcompote, sometimes thickened with corn starch, can then be used in pies, tarts and crumbles. Alternatively, greater quantities of sugar can be added withpectinto makejams. A paired spice used isginger, althoughcinnamonandnutmegare also common additions.
In the United Kingdom, as well as being used in the typical pies, tarts and crumbles, rhubarb compote is also combined with whipped cream or custard to make rhubarbfool. In the United States, the common usage of rhubarb in pies has led to it being nicknamed "pie plant", by which it is referred to in 19th-century cookbooks.Rhubarb in the US is also often paired with strawberries to make strawberry-rhubarb pie, though some rhubarb purists jokingly consider this "a rather unhappy marriage".
Rhubarb can also be used to make alcoholic drinks, such asfruit winesor Finnish rhubarbsima (mead). It is also used to makeKompot.
Raw rhubarb is 94% water, 5%carbohydrates, 1%protein, and contains negligible fat (table). In a100-gram (3+1⁄2-ounce) reference amount, raw rhubarb supplies 88 kilojoules (21 kilocalories) offood energy, and is a rich source ofvitamin K(28% of theDaily Value, DV), a moderate source ofvitamin C(10% DV), and contains no othermicronutrientsin significant amounts (table).
Intraditional Chinese medicine, rhubarb roots of several species were used as alaxativefor several millennia,although there is noclinical evidenceto indicate such use is effective.
The roots and stems containanthraquinones, such asemodinandrhein.Emodin"represents agenotoxicrisk for humans" whilerheinis "a compound devoid of genotoxic capabilities".The anthraquinones have been separated from powdered rhubarb root for purposes intraditional medicine,although long-term consumption ofanthraquinoneshas been associated withacute kidney failure.
The rhizomes containstilbenoidcompounds (includingrhaponticin), and theflavanolglucosides(+)-catechin-5-O-glucosideand (−)-catechin-7-O-glucoside.
Rhubarb leaves containpoisonoussubstances, includingoxalic acid, anephrotoxin.The long term consumption of oxalic acid leads to kidney stone formation in humans. Humans have been poisoned after ingesting the leaves, a particular problem duringWorld War Iwhen 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).
TheLD50(median lethal dose) for pure oxalic acid in rats is about 375 mg/kgbody weight,or about 25 grams for a 65-kilogram (143 lb) human. Other sources give a much higher oralLDLo(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 withbaking sodacan make them more poisonous by producing solubleoxalates.The leaves are believed to also contain an additional, unidentified toxin,which might be an anthraquinoneglycoside(also known assenna glycosides).
In thepetioles(leaf stalks), the proportion of oxalic acid is about 10% of the total 2–2.5% acidity, which derives mainly frommalic acid.Serious cases of rhubarb poisoning are not well documented.Both fatal and non-fatal cases of rhubarb poisoning may be caused not by oxalates, but rather by toxicanthraquinoneglycosides.
Rhubarb is a host to the rhubarb curculio,Lixus concavus, which is aweevil. Damage is mainly visible on leaves and stalks, withgummosisand oval or circular feeding and egg-laying sites.
Hungry wildlife may dig up and eat rhubarb roots in the spring, as stored starches are turned to sugars for new foliage growth.