Rhubarb

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For other uses, see Rhubarb (disambiguation).
Rhubarb

Scientific classification
Kingdom: Plantae
Division: Magnoliophyta
Class: Magnoliopsida
Order: Caryophyllales
Family: Polygonaceae
Genus: Rheum
L.
Species

About 60, including:

Rhubarb belongs to genus Rheum, a genus of perennial plants that grows from thick short rhizomes. The genus is in the family Polygonaceae, and includes the vegetable rhubarb (Rheum rhabarbarum or Rheum x hybridum.) The plants have large leaves that are somewhat triangular shaped with long fleshy petioles. The flowers are small, greenish-white to rose-red, and grouped in large compound leafy inflorescences. A number of varieties of rhubarb have been domesticated both as medicinal plants and for human consumption. While the leaves are toxic, the stalks are used in pies and other foods for their tart flavour.

Contents

[edit] Species

The genus is represented by about 60 extant species.[1] Among species found in the wild, those most commonly used in cooking are the garden rhubarb (R. rhabarbarum) and the so-called false rhubarb (R. rhaponticum), which is actually a true rhubarb.[2] The many varieties of cultivated rhubarb more usually grown for eating are recognised as Rheum x hybridum in the Royal Horticultural Society's list of recognised plant names. The drug rheum is prepared from the rhizomes and roots of another species, R. officinale or medicinal rhubarb. This species is also native to Asia, as is the turkey rhubarb (R. palmatum). Another species, the Sikkim rhubarb (R. nobile), is limited to the Himalayas.

Rheum species have been recorded as larval food plants for some Lepidoptera species including brown-tail, buff ermine, cabbage moth, large yellow underwing, and nutmeg moth.

[edit] Description

Rhubarb growing

Rheum species are herbaceous perennials with hermaphrodite flowers, consisting of a coloured perianth, composed of six to nine segments, arranged in two rows. The flowers have nine stamina inserted on the torus at the base of the peranthium, they are free or subconnatent at their base. The ovary is simple and triangular shaped with three styles. The fruits are a three-sided achene with winged sides, the seeds are albuminous and have straight embryos.

[edit] Cultivation and consumption

Rhubarb displayed for sale at a grocery

The plant is indigenous to Asia, and many[who?] suggest that it was often used by the Mongolians; particularly, the Tatars tribes of the Gobi Desert.[citation needed] The plant has grown wild along the banks of the River Volga for centuries. The term rhubarb is a combination of the Greek rha and barbarum; rha is a term that refers both to the plant and to the River Volga.[3] Rhubarb roots are used in traditional Chinese medicine; rhubarb also appears in medieval Arabic and European prescriptions. [4] [5] 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 first came to America in the 1820s, entering the country in Maine and Massachusetts and moving westwards with the European American settlers.[6]

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. This rhubarb is typically made available at consumer markets in February and March, before outdoor cultivated rhubarb is available. The hothouse rhubarb is usually a brighter red than the cultivated rhubarb. Hothouse rhubarb is also more tender and tastes sweeter than cultivated rhubarb. [7] Rhubarb is grown primarily for its fleshy petioles, commonly known as rhubarb sticks or stalks. 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 and into July. Rhubarb is ready to be consumed as soon as it is harvested, and freshly cut stalks will be firm and glossy.

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:[8] The green-stalked rhubarb is more robust and has a higher yield, but the red-coloured stalks are much more popular with consumers.[citation needed]

The stalks, which are petioles, can be cooked in a variety of ways. One way is to cut up the stalks into one-inch pieces and stew them (boil in water); it is only necessary to just barely cover the stalks with water because rhubarb stalks themselves contain a great deal of water); add 1/2 to 3/4 cup of sugar for each pound of rhubarb [9], then add cinnamon and/or nutmeg to taste. Sometimes a tablespoon of lime juice or lemon juice is added. The sliced stalks are boiled until soft.
At this stage, cooked with strawberries or apples as a sweetener, or with stem or root ginger, rhubarb makes excellent jam. Other fruits, with the addition of pectin (or using sugar with pectin already added), can also be added to rhubarb at this stage to make a variety of jams: the fruit is added at a ratio of two parts fruit to one part rhubarb, consisting of strawberries or raspberries, or chopped plums, apricots, or apples. Boiling should continue for at least ten minutes after all fruit is completely softened, depending on whether a simple refrigerated jam is made, or (with longer cooking) jam is being canned for a long shelf life.
To make a "sauce," of rhubarb (to which dried fruit could be added near the end) continue simmering 45 minutes to one hour at medium heat, until the sauce is mostly smooth and the remaining discrete stalks can easily be pierced with a fork, which yield a smooth tart-sweet sauce with a flavor similar to sweet and sour sauce. This sauce is called rhubarbsauce, analogous to applesauce. Like applesauce, this sauce is usually stored in the refrigerator and eaten cold. The sauce, when stewed over medium heat only a short time (about 20 minutes) and with only a little water so that the rhubarb stalks stay mostly discrete, may be used as filling for pies (see rhubarb pie), tarts, and crumbles. Sometimes stewed strawberries are mixed with the rhubarb to make strawberry-rhubarb pie. This common use has led to the slang term for rhubarb, "pie plant". It can also be used to make 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 Norway. In the UK the first rhubarb of the year is harvested by candlelight in dark sheds dotted around the noted "Rhubarb Triangle" of Wakefield, Leeds and Morley[10], a practice that produces a sweeter, more tender stalk.[11]

A homemade rhubarb pie

In warm climates, rhubarb will grow all year round, but in colder climates the parts of the plant above the ground disappear completely during winter, and begin to grow again from the root in early spring. It can be forced, that is, encouraged to grow early, by raising the local temperature. This is commonly done by placing an upturned bucket over the shoots as they come up. 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.

Rhubarb is used as a strong laxative and for its astringent effect on the mucous membranes of the mouth and the nasal cavity.[12]

[edit] Toxic effects

Rhubarb flower.

Rhubarb leaves contain poisonous substances, including oxalic acid which is a nephrotoxic and corrosive acid that is present in many plants. The LD50 (median lethal dose) for pure oxalic acid in rats is about 375 mg/kg body weight,[13] or about 25 g for a 65 kg (~140 lb) human. While the oxalic acid content of rhubarb leaves can vary, a typical value is about 0.5%,[14] so a rather unlikely 5 kg of the extremely sour leaves would have to be consumed to reach an LD50 dose of oxalic acid. Cooking the leaves with soda can make them more poisonous by producing soluble oxalates.[15] However, the leaves are believed to also contain an additional, unidentified toxin,[16] which might be an anthraquinone glycoside (also known as senna glycosides).[17] In the petioles, the amount of oxalic acid is much lower, only about 2-2.5% of the total acidity.[18]

The roots have been used as an aggressive laxative for over 5,000 years.[19] 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 slimming agent.

[edit] References

  1. ^ Ailan Wang, Meihua Yang and Jianquan Liu (2005). "Molecular Phylogeny, Recent Radiation and Evolution of Gross Morphology of the Rhubarb genus Rheum (Polygonaceae) Inferred from Chloroplast DNA trnL-F Sequences". Annals of Botany. http://aob.oxfordjournals.org/cgi/content/abstract/96/3/489. Retrieved on 2006-06-18. 
  2. ^ "Rheum rhaponticum L. Taxonomic Serial Number 21319". Integrated Taxonomic Information System. http://itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=21319. 
  3. ^ McGee, Harold. On Food and Cooking: The Science and Lore of the Kitchen. New York, NY: Scribner, 2004. p 366
  4. ^ Charles Perry, trans. An Anonymous Andalusian Cookbook of the 13th Century
  5. ^ Oxford English Dictionary s.n. rhubarb, n.
  6. ^ Waters, Alice. Chez Panisse Fruit. New York, NY: Harper Collins, 2002. p 278
  7. ^ Rombauer, Irma S. Joy of Cooking Indianapolis/New York:1975 Bobbs-Merrill Co., Inc. Page 142
  8. ^ Rhubarb Varieties
  9. ^ Rombauer, Irma S. Joy of Cooking Indianapolis/New York:1975 Bobbs-Merrill Co., Inc. Page 142
  10. ^ Wakefield Metropolitan District Council. "Rhubarb". http://www.wakefield.gov.uk/CultureAndLeisure/HistoricWakefield/Rhubarb/default.htm. Retrieved on 2006-03-12. 
  11. ^ McGee, Harold. On Food and Cooking: The Science and Lore of the Kitchen. New York, NY: Scribner, 2004. p 367
  12. ^ Mrs M Grieve. "botanical.com - A Modern Herbal". http://www.botanical.com/botanical/mgmh/r/rhubar14.html. Retrieved on 2009-07-07. 
  13. ^ "Rhurbarb poisoning on rhurbabinfo.com". http://www.rhubarbinfo.com/rhubarb-poison.html. 
  14. ^ GW Pucher, AJ Wakeman, HB Vickery. THE ORGANIC ACIDS OF RHUBARB (RHEUM HYBRIDUM). III. THE BEHAVIOR OF THE ORGANIC ACIDS DURING CULTURE OF EXCISED LEAVES. Journal of Biological Chemistry, 1938
  15. ^ Everist, Selwyn L., Poisonous Plants of Australia. Angus and Robertson, Melbourne, 1974, p. 583
  16. ^ "Rhubarb leaves poisoning". Medline Plus Medical Encyclopedia. http://www.nlm.nih.gov/medlineplus/ency/article/002876.htm. 
  17. ^ http://www.cbif.gc.ca/pls/pp/ppack.info?p_psn=171&p_type=all&p_sci=sci
  18. ^ McGee, Harold. On Food and Cooking: The Science and Lore of the Kitchen. New York, NY: Scribner, 2004. p 367
  19. ^ Foster, Steven, and Chongxi Yue. 1992. Herbal emissaries bringing Chinese herbs to the West : a guide to gardening, herbal wisdom, and well-being. Rochester, Vt: Healing Arts Press. Pages 134-38.

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