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Rhubarb! Rhurbarb! Rhubarb!

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Coldicote | 16:53 Fri 22nd Aug 2008 | Food
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I've always understood that rhubarb leaves are poisonous, not that I've ever tried eating them. Because of that, I'm also wary of eating rhubarb stems as used in cookery. Are there any chemistry wizards out there that can explain why rhubarb as it is normally used can be safe, if its leaves are poisonous? Thanks.

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The stems are fine to eat ... it's the leaves and the roots that are not good for eating as they contain oxalic acid.... This can be destroyed (like a lot of legumes) by furious boiling in water for at least 10 minutes but the results are so pathetic, it's not worth bothering trying to do this to the leaves and roots. The stems don't contain this acid so they're ok to eat - either raw or cooked but the raw stems might be a bit too tart on their own if they aren't really young (they probably need a bit of sugar).
Additionally, according to rhubarbinfo.com, From 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.
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Thank you both. I envy your knowledge of Oxalic acid and certainly won't be experimenting with rhubarb leaves. It still seems odd to me though that stems can be safe when leaves are poisonous. Must be something to do with the natural growing process, but I'm still inclined to shun rhubard stems as food and wonder if they contain any worthwhile nourishment.
They're actually fairly nutritions, coldicote... see here:

http://www.moondragon.org/nutrition/foodguide/ rhubarb.html

You probably have no qualms about eating, potatoe, tomatoes, Bell pepper. maybe sweet potatoes and other related vegetables, yet the leaves of all contain Nightshade...a toxin as well. Many edible plants have parts that have toxicity... just have to know which is which. As you probably already know, tomatoes were thought to be poisonous for many years...(Also once thought to be an aphrodisiac)...
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Thanks Clanad. It just shows how much we take for granted about food - and remarkable that so many of us manage to survive, let alone keep fit. As for tomatoes once being thought an aphrodisiac, I'm glad that's in the past. Even a diet of 'love and fresh air' is dubious!
Someone once told me your not allowed (illegal) to boil the leaves on an allotment!! Is this true?
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I haven't hear that before Cassa, but burning (not boiling) might be something to do with a local byelaw.

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