Species of herbaceous perennial plant with fleshy, sour edible stalks
Rhubarb
is the fleshy, edible stalks (
petioles
) of species and hybrids (culinary rhubarb) of
Rheum
in the family
Polygonaceae
, which are cooked and used for food.
[2]
The plant is a herbaceous
perennial
that grows from short, thick
rhizomes
. Historically, different plants have been called "rhubarb" in English. The large, triangular
leaves
contain high levels of
oxalic acid
and
anthrone
glycosides
, making them inedible. The small
flowers
are grouped in large compound leafy greenish-white to rose-red
inflorescences
.
The precise origin of culinary rhubarb is unknown. The species
Rheum rhabarbarum
(syn.
R. undulatum
) and
R. rhaponticum
were grown in Europe before the 18th century and used for medicinal purposes. By the early 18th century, these two species and a possible
hybrid
of unknown origin,
R.
×
hybridum
, were grown as vegetable crops in England and Scandinavia. They readily hybridize, and
culinary rhubarb
was developed by selecting open-pollinated seed, so its precise origin is almost impossible to determine.
[3]
In appearance, samples of culinary rhubarb vary on a continuum between
R. rhaponticum
and
R. rhabarbarum
. However, modern rhubarb cultivars are
tetraploids
with 2
n
= 44, in contrast to 2
n
= 22 for the wild species.
[4]
Although rhubarb is a
vegetable
, it is often put to the same culinary uses as fruits.
[5]
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
. Many
cultivars
have been developed for human consumption, most of which are recognised as
Rheum
×
hybridum
by the
Royal Horticultural Society
.
[6]
Etymology
[
edit
]
The word
rhubarb
is likely to have derived in the 14th century from the
Old French
rubarbe
, which came from the Latin
rheubarbarum
and
Greek
rha barbaron
, meaning 'foreign rhubarb'.
[7]
The Greek physician
Dioscorides
used the Greek word
??
(
rha
), whereas
Galen
later used
??ον
(
rh?on
), Latin
rheum
. These in turn derive from a
Persian
name for species of
Rheum
.
[8]
The specific epithet
rhaponticum
, applying to one of the presumed parents of the cultivated plant, means 'rha from the
region of the Black Sea
'
[8]
or the river
Volga
, Rha being its ancient name.
[9]
Cultivation
[
edit
]
Young rhubarb flowers
Rhubarb is grown widely, and with
greenhouse
production 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.
[10]
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.
[11]
After forcing for commercial production, the crowns are usually discarded.
[10]
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 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.
[12]
These sheds are dotted around the "
Rhubarb Triangle
" in
Yorkshire
between
Wakefield
,
Leeds
, and
Morley
.
[13]
In the United States rhubarb is primarily produced in the states of
Oregon
,
Washington
, and
Wisconsin
with approximately 1,200 acres in production, of which 175 are covered in hothouses.
[14]
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;
[15]
half of all US commercial production is in
Pierce County, Washington
.
[16]
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 in
oxalic acid
, which migrates from the leaves and can cause illness.
[17]
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.
[18]
Historical cultivation
[
edit
]
A bundle of rhubarb
The Chinese call rhubarb "the great yellow" (
da huang
大黃
), and have used rhubarb root for medicinal purposes.
[19]
It appears in
The Divine Farmer's Herb-Root Classic
, which is thought to have been compiled about 1,800 years ago.
[20]
Though
Dioscurides
' description of
ρηον
or
ρ?
indicates that a medicinal root brought to Greece from beyond the
Bosphorus
may have been rhubarb, commerce in the plant 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".
[21]
Later,
[
when?
]
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.
[22]
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.
[20]
The value of rhubarb can be seen in
Ruy Gonzales de Clavijo
's report of his embassy in 1403?1405 to
Timur
in
Samarkand
: "The best of all merchandise coming to Samarkand was from China: especially silks, satins, musk,
rubies
,
diamonds
,
pearls
, and rhubarb...."
[23]
The high price, as well as the increasing demand from apothecaries, stimulated efforts to cultivate the different species of rhubarb on European soil.
[22]
Certain species came to be grown in England to produce the roots.
[24]
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.
[22]
Grieve claims a date of 1820 in England.
[24]
Rhubarb was harvested in
Scotland
from at least 1786, having been introduced to the Botanical Garden in
Edinburgh
by the traveller
Bruce of Kinnaird
in 1774. He brought the seeds from
Abyssinia
and they produced 3,000 plants.
[25]
Though it is often asserted that rhubarb first came to the United States in the 1820s,
[26]
John Bartram
was growing medicinal and culinary rhubarbs in
Philadelphia
from the 1730s, planting seeds sent to him by
Peter Collinson
.
[27]
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."
[28]
Cultivars
[
edit
]
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.
[29]
A homemade
rhubarb pie
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 garden
RHS Bridgewater
where winter cold and rainfall are better suited for rhubarb.
[30]
The following cultivars have gained the
Royal Horticultural Society
's
Award of Garden Merit
:
[31]
- 'Grandad's Favourite'
[32]
- 'Reed's Early Superb'
[33]
- 'Stein's Champagne'
[34]
- 'Timperley Early'
[35]
Uses
[
edit
]
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 18th- to 19th-century England after affordable
sugar
became more widely available.
[22]
[24]
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 popular
strawberry rhubarb pie
.
Food
[
edit
]
The species
Rheum ribes
has been eaten in the
Islamic world
since the 10th century.
[38]
In Northern Europe and North America, the stalks are commonly cut into pieces and stewed with added sugar until soft.
[39]
The resulting
compote
, sometimes thickened with corn starch, can then be used in pies, tarts and crumbles. Alternatively, greater quantities of sugar can be added with
pectin
to make
jams
. A paired spice used is
ginger
, although
cinnamon
and
nutmeg
are 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 rhubarb
fool
. 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.
[40]
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".
[40]
Rhubarb can also be used to make alcoholic drinks, such as
fruit wines
or Finnish rhubarb
sima (mead)
. It is also used to make
Kompot
.
[41]
Nutrition
[
edit
]
Raw rhubarb is 94% water, 5%
carbohydrates
, 1%
protein
, and contains negligible fat (table). In a
100-gram (
3
+
1
⁄
2
-ounce) reference amount, raw rhubarb supplies 88 kilojoules (21 kilocalories) of
food energy
, and is a rich source of
vitamin K
(28% of the
Daily Value
, DV), a moderate source of
vitamin C
(10% DV), and contains no other
micronutrients
in significant amounts (table).
Traditional Chinese medicine
[
edit
]
In
traditional Chinese medicine
, rhubarb roots of several species were used as a
laxative
for several millennia,
[42]
although there is no
clinical evidence
to indicate such use is effective.
[43]
Phytochemistry and potential toxicity
[
edit
]
The roots and stems contain
anthraquinones
, such as
emodin
and
rhein
.
[19]
Emodin
"represents a
genotoxic
risk for humans" while
rhein
is "a compound devoid of genotoxic capabilities."
[44]
The anthraquinones have been separated from powdered rhubarb root for purposes in
traditional medicine
,
[42]
although long-term consumption of
anthraquinones
has been associated with
acute kidney failure
.
[43]
The rhizomes contain
stilbenoid
compounds (including
rhaponticin
), and the
flavanol
glucosides
(+)-
catechin-5-
O
-glucoside
and (?)-
catechin-7-
O
-glucoside
.
[45]
Oxalic acid
[
edit
]
Rhubarb leaves contain
poisonous
substances, including
oxalic acid
, a
nephrotoxin
.
[43]
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 during
World War I
when the leaves were mistakenly recommended as a food source in Britain.
[46]
[47]
[48]
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
).
The
LD
50
(median lethal dose) for pure oxalic acid in rats is about 375 mg/kg
body weight
,
[49]
or about 25 grams for a 65-kilogram (143 lb) human. Other sources give a much higher oral
LD
Lo
(lowest published lethal dose) of 600 mg/kg.
[50]
While the oxalic acid content of rhubarb leaves can vary, a typical value is about 0.5%,
[51]
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
.
[52]
The leaves are believed to also contain an additional, unidentified toxin,
[53]
which might be an anthraquinone
glycoside
(also known as
senna glycosides
).
[54]
In the
petioles
(leaf stalks), the proportion of oxalic acid is about 10% of the total 2?2.5% acidity, which derives mainly from
malic acid
.
[12]
Serious cases of rhubarb poisoning are not well documented.
[55]
Both fatal and non-fatal cases of rhubarb poisoning may be caused not by oxalates, but rather by toxic
anthraquinone
glycosides.
[43]
[55]
[56]
Pests
[
edit
]
Rhubarb is a host to the rhubarb curculio,
Lixus concavus
, which is a
weevil
. Damage is mainly visible on leaves and stalks, with
gummosis
and oval or circular feeding and egg-laying sites.
[57]
Hungry wildlife may dig up and eat rhubarb roots in the spring, as stored starches are turned to sugars for new foliage growth.
Cookbook
[
edit
]
Wikibooks
Cookbook
has a recipe/module on
Gallery
[
edit
]
-
Rhubarb displayed for sale at a market
-
A 19th-century
apothecary
jar for rhubarb
-
Dried strawberry-flavoured rhubarb
-
1804 illustration of a rhubarb seller in London
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[
edit
]
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Further reading
[
edit
]
External links
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