2. Rhubarb
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 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 on imported rhubarb tariffs, as tariffs were higher for vegetables than fruits. Rhubarb also contains glycosides especially rhein, glucorhein and emodin which impart cathartic and laxative activities to it. It is hence useful as a cathartic in case of constipation.
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, 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 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).
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 the raw stalks may not be hazardous, although the tart taste of raw stalks is so strong as to be unpalatable to many.
Rhubarb is a relative of buckwheat and has an earthy, sour flavor. Rhubarb thrives in cold climates and originated in Western China, Tibet, Mongolia, Siberia and neighboring areas. The traditional role was medicinal-the dried root was a popular remedy for a wide range of illnesses. Its primary function was to induce vomiting, although rhubarb is also a
mild astringent. This medicinal role caused the price of the dried root to rise. In 1542, rhubarb sold for ten times the price of cinnamon in France and in 1657 rhubarb sold for over twice the price of opium in England (Schneider, 2001). Beginning in the eighteenth century, rhubarb began to be consumed in foods, primarily drinks and meat stews.
Botanically speaking, rhubarb is considered a vegetable, but it’s most often treated as a fruit — though it’s rarely eaten raw. Just like fresh cranberries, rhubarb is almost unbearably tart on its own and needs the sweetness of sugar, honey, or fruit juice added to it to balance out the acidity. Rhubarb’s nickname is the “pie plant” because that is the
primary use for this vegetable.
Rhubarb was introduced to the United States at the end of the eighteenth century. Today most rhubarb is frozen for commercial and institutional use; only about a quarter of the crop is sold fresh.
Rhubarb has been used in Chinese traditional medicine for a long time. its side effects should be minimal for usual doses and dosage design. However, the side effects could be serious if you have cancer or vascular issues. It is unclear if rhubarb is anti-cancer or carcinogenic. There are reports that it may cause cancer; there are also reports that its ingredients have anti-cancer activities. Its effect on vascular system is also unclear. A few reports indicated its vasodilutory effects, while some showed its muscle contraction effects. It may also enhance blod clotting.
The side effects for eating its leave can be fatal. The leaves are poisonous because they contain oxalate. This toxin, plus another unknown toxin also found in the leaves, has been reported to cause poisoning when large quantities of raw or cooked leaves are ingested. Some ingredients of rhubarb were also found to be phototoxic.
Oxalates are contained in all parts of rhubarb plants, especially in the green leaves. There is some evidence that anthraquinone glycosides are also present and may be partly responsible. It is not clear as to the exact source of poisoning from rhubarb, possibly a result of both compounds. The stalks contain low levels of oxalates, so this does not cause problems.
The biodynamic (toxicity) mechanism by which oxalic acid works is somewhat different from organic poisons and is more analogous to heavy metal poisoning. Organic poisons often work through at the biochemical level, e.g. cyanide by interfering with respiration at the cellular level, strychnine by screwing up inter-synaptic transmission. There are many molecular substances in foods which offer no nutritional benefit, and must be processed and excreted. Oxalic acid, for example, is excreted in the urine, and its crystals are commonly found in microscopic urinalysis. Too much oxalic acid in the urine will result in kidney or bladder stones. Calcium combines with oxalic acid to form the less soluble salt, calcium oxalate, which is also found in kidney stones. Plant leaves, especially rhubarb, cabbage, spinach, and beet tops, contain oxalic acid. Oxalic acid is also found in potatoes and peas. Vitamin C is metabolized to oxalic acid; it contributes to over-saturation of the urine with crystals and possibly to stone formation.
Oxalic acid is a strong acid of the composition HOOC-COOH, which crystallizes as the ortho-acid (HO)3 CC (OH)3 . It is sometimes also called “ethane diacid”. It occurs naturally in some vegetables (like rhubarb). The can also be produced by heating sodium formate and treating the resulting oxides with sulfuric acid. It can also be obtained by the action of nitric acid on sugar, or of strong alkali’s on sawdust. The product is normally traded as colorless crystals with a melting point of 101.5?C, and can be dissolved in water or alcohol. Oxalic acid reduces iron compounds, and is therefore used in metal polishes, stain removers, and writing inks. When it absorbs oxygen, it is converted to the volatile carbon dioxide and to water, and it is used as a bleaching agent, in detergents, and as a mordant in dyeing processes.
rom an MSDS (Material Safety Data Sheet) for Oxalic acid, LD50 (LD50 is the Median Lethal Dose, which is the dose of a drug or chemical predicted to produce a lethal effect in 50 percent of the subjects to whom the dose is given) in rats is 375 mg/kg. So for a person about 145 pounds (65.7 kg) that’s about 25 grams of pure oxalic acid required to cause death. Rhubarb leaves are probably around 0.5% oxalic acid, so that you would need to eat quite a large serving of leaves, like 5 kg (11 lbs), to get that 24 grams of oxalic acid. Note that it will only require a fraction of that to cause sickness.
Symptoms of Oxalic Acid Poisoning
On the body body as a whole one might experience weakness, burning in the mouth, death from cardiovascular collapse; on the respiratory system – difficulty breathing; on the eyes, ears, nose, and throat – burning in the throat; one the gastrointestinal system – abdominal pain, nausea, vomiting, diarrhea; and on the nervous system – Convulsions, coma.
Precautions for rhubarb gardening
- Trim leaves from stalk immediately.
- Don’t use stalks from frost bitten plants.
- Wash the stalks well.
- Children should be taught to eat only the rhubarb stalks, preferably under supervision.
Special Precautions & Warnings:
Pregnancy and breast-feeding: Rhubarb is possibly unsafe when used in amounts greater than those found in foods.
Diarrhea or constipation: Rhubarb can make diarrhea or constipation worse, depending on the preparation used.
Gastrointestinal (GI) conditions: Don’t take rhubarb if you have a bowel obstruction; appendicitis; unexplained stomach pain; or inflammatory conditions of the intestines including Crohn’s disease, colitis, and irritable bowel syndrome (IBS).
Kidney disease: There is a chemical in rhubarb that might harm the kidneys. In fact, a supplement that contained rhubarb has been linked to one report of kidney failure. If you already have kidney disease, don’t risk making it worse by taking rhubarb.
Kidney stones: Rhubarb contains a chemical that the body can convert into kidney stones. If you have ever had kidney stones, don’t take rhubarb.