Couldn't you just steam the leaves?
June 3, 2020 10:57 AM   Subscribe

Rhubarb leaves are poisonous. This seems to be a fact. When I look up what makes rhubarb leaves poisonous, the answer seems to be "high levels of oxalic acid" But soluble oxalic acid can be steamed out of greens, and insoluble oxalic acid isn't a problem I thought. So - is there another reason?

I'm actually not desperate to eat my Rhubarb leaves or anything (though I do have tons!) it's just a thing I've always been curious about. My dad, who was big hippie vegan type for a lot of years, always has a lot to say about the ways you have to cook certain vegetables and foraged greens to reduce the oxalic acid which can build up if you are eating mostly that type of food for your diet.

Like parsley for instance, has VERY high oxalic acid levels (I believe higher than rhubarb leaves), but we don't warn people against eating it or anything. Also soluble oxalic acid can be reduced a lot by blanching or steaming greens and tossing the water.

Anyways, a bit rambling, but my question is: I'm having trouble verifying on google WHY rhubarb leaves are considered poisonous, because the common line is oxalic acid which doesn't seem to make sense, and my googling is failing to find any further information for me. I'm not very experienced in trying to look up science and nutrition information, so I suspect I'm hitting a blind spot by not knowing how to google this. Does anyone have any information or can help to point me in the right direction of where to look?
posted by euphoria066 to Home & Garden (7 answers total) 2 users marked this as a favorite
 
I have eaten cooked rhubarb leaves, in my grandmother's strawberry rhubarb pie, and was told that they must be sufficiently cooked so that they would not be poisonous.
posted by amtho at 11:23 AM on June 3, 2020 [1 favorite]


To be fair, most people dont eat that much parsley at a time.

Rhubarb leaves have a LOT of oxalic acid by weight, around 1%. A fatal dose would probably be eating 5-10 lbs at a time, depending on how they're prepared. This is obviously a ridiculous amount for one serving but if you eat them frequently it could lead to serious kidney problems, and both mild poisoning and deaths have been reported at lower doses.

It has has also been suggested that both fatal and nonfatal rhubarb leaf poisoning may be caused by another substance known as anthraquinone glycosides — not the oxalic acid.
posted by ananci at 11:26 AM on June 3, 2020


For a long time, I was told that we don't eat potato leaves because they're poisonous.

Then, I spent some time in a region where they do eat potato leaves — cooking them carefully in several changes of water to make them safe to eat. And there, I learned the other reason we don't eat potato leaves, which is that their flavor is so completely mediocre that unless you're very poor subsistence farmers in a climate where few things grow, all that cooking is not worth the effort.

I'm not saying that's the deal with rhubarb too. But I wouldn't be surprised.
posted by nebulawindphone at 1:40 PM on June 3, 2020 [11 favorites]


Anecdote: my dad, who loves rhubarb more than anything, tried preparing the leaves once. He took one taste, said nothing, but spat it out and tipped the rest of the boiled leaves down the sink.
posted by scruss at 2:29 PM on June 3, 2020 [2 favorites]


I've seen a case presentation of kidney failure due to rhubarb leaves, although it was a lot of rhubarb - part of a meal most days. This case lists a half gram (?) of rhubarb a day as a contributing factor in kidney failure leading to dialysis, with pictures of the oxalic acid deposits in the kidney. They mention a ~half reduction in oxalic acid levels in rhubarb with cooking - raw 805 mg/100 g, stewed 460 mg/100 g. Overall, not worth risking.
posted by quercus23 at 2:30 PM on June 3, 2020 [3 favorites]


Apparently you’d have to eat about ten pounds of rhubarb leaves to show symptoms.
posted by Segundus at 3:24 PM on June 3, 2020


The article linked by quercus23 says
She reported an extensive consumption of rhubarb, i.e. 500 mg fresh weight of plant per day for the least 4 weeks. This resulted in an uptake of approximately 2.3 g oxalate per day [7].
The "500 mg" is obviously a typo (you can't get 2.3 g of oxalate from 0.5 g of rhubarb) and it should clearly read 500 g, i.e., about a pound of fresh rhubarb per day for a month. (I am disappointed that the peer reviewer didn't catch this obvious and important error.) This was in a patient with preexisting mild diabetic nephropathy. And the article just says "rhubarb", so I'm going to assume that means stems. That works out to about 0.5% by weight of uptake-able oxalate.
posted by heatherlogan at 6:04 PM on June 3, 2020


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