Rhubarb

Rhubarb is a group of plants that belong to the genus Rheum in the family Polygonaceae. They are herbaceous perennial plants growing from short, thick rhizomes. They have large leaves that are somewhat triangular-shaped with long fleshy petioles. They have small flowers grouped in large compound leafy greenish-white to rose-red inflorescences.

Although the leaves are toxic, various parts of the plants have medicinal and culinary uses. The traditional Chinese pharmacopeia features rhubarb In culinary use, fresh raw stalks are crisp (similar to celery) with a strong tart taste; most commonly the plant's stalks are cooked and used in pies and other foods for their tart flavour. 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 to be a vegetable; however, in the United States, a New York court decided in 1947 that since it was used in the United States as a fruit it was to be counted as a fruit for the purposes of regulations and duties. A side effect was a reduction in taxes paid.

Cultivation

Rhubarb is now grown in many areas and thanks to greenhouse production 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 cultivated rhubarb. In temperate climates, rhubarb is one of the first food plants to be ready for harvest, 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: one from late April to May and another from late June into July. Rhubarb is ready to be consumed as soon as it is harvested, and freshly cut stalks will be firm and glossy.

In the United Kingdom, the first rhubarb of the year is harvested by candlelight in dark sheds dotted around the noted "Rhubarb Triangle" of Wakefield, Leeds, and Morley, a practice that produces a sweeter, more tender stalk.

Rhubarb will grow year-round in warm climates, but in temperate climate the aboveground 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.

Rhubarb can successfully be planted in containers, so long as the container is large enough to accommodate a season's growth.

The colour of the 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.

Historical cultivation

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 legend attributes to the mythical Shen Nung, the Yan Emperor, but is thought to have been compiled about 2700 years ago. Though Dioscurides' description of ρηον or ρά, a root brought to Greece from beyond the Bosphorus may have included rhubarb, the commerce did not become securely established until Islamic times, when it was imported along the Silk Road reaching Europe from the 14th century through the ports of Aleppo and Smyrna, and so becoming known as "Turkish rhubarb" before the usual route lay through Russia, and "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 expense of transportation across Asia caused rhubarb to be highly expensive in medieval Europe where it was several times the price of other valuable herbs and spices such as cinnamon, opium and saffron. The merchant explorer, Marco Polo, was therefore much interested to find the plant being grown and harvested in the mountains of Tangut province.

The term rhubarb is a combination of the Ancient Greek rha and barbarum; rha is a term that 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.

Uses

Rhubarb is grown primarily for its fleshy petioles, commonly known as rhubarb sticks or stalks. The use of rhubarb stems as food is a relatively recent innovation, first recorded in 17th century England, after affordable sugar became available to common people, and reaching a peak between the 20th century's two world wars.

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.

Cooking

One way is to cut up the stalks into pieces and stew them (boil in water); it is only necessary to barely cover the stalks with water because rhubarb stalks contain a great deal of water; add 1/2 to 3/4 cup of sugar for each pound of rhubarb,

The roots and stems are rich in anthraquinones, such as emodin and rhein. These substances are cathartic and laxative, which explains the sporadic use of rhubarb as a dieting aid.

Rhubarb roots are used in traditional Chinese medicine; rhubarb also appears in medieval Arabic and European prescriptions.

The rhizomes ('roots') contain stilbenoid compounds (including rhaponticin) which seem to lower blood glucose levels in diabetic mice.

Toxicity

Rhubarb leaves contain poisonous substances, including oxalic acid which is a nephrotoxic and corrosive acid that is present in many plants. The (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. While the oxalic acid content of rhubarb leaves can vary, a typical value is about 0.5%, so a rather unlikely 5 kg of the extremely sour leaves would have to be consumed to reach an 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).

In the petioles, the amount of oxalic acid is much lower, only about 2-2.5% of the total acidity which is dominated by malic acid. This means that the raw stalks may not be hazardous, though they are generally thought to be in the US. The tart taste of raw stalks is so strong as to be unpalatable to many.

Pests

The rhubarb curculio, Lixus concavus, is a member of the weevils group, also called the snout beetles. Rhubarb is a host, damage being mainly visible on the leaves and stalks, with gummosis and oval or circular feeding and/or egg-laying sites.

Hungry wildlife will dig up and eat roots in the spring as stored starches are turned to sugars for new foliage growth.

References

Further reading

Thompson, Fed S. Rhubarb or Pie Plant Culture, 1894. (1894)

Morse, J. E. The New Rhubarb Culture, 1901. (1901) (1903) (1909)

Bland, Reginald. Winter rhubarb, culture and marketing, 1915.

External links

The Rhubarb Compendium

A Rhubarb Wine Recipe

Category:Medicinal plants Category:Polygonaceae Category:Stem vegetables