‘Tis the season for rhubarb.  I know this because my Dad, who is a professional grower of green stuff, brought me some last week.  Sadly I didn’t inherit his green thumb (and fingers, and probably toes) but I can knock up a half decent rhubarb crumble*, and I can make custard from scratch.  Yes really, no packet or anything.  Impressive eh?

But anyway, Dad suggested we might grow rhubarb in our garden, since we have a shady, slightly soggy patch of ground near the house where, apparently, it might do well even with my appalling track record of plant abuse and neglect.  (If there were a Royal Society for the Protection of Plants, trust me I’d be top of their most wanted list.)  This led me to to some musings about safety, since we have both a cat and a small person in the house.  I vaguely recalled that the leaves were poisonous, and Dad confirmed that this particular piece of information wasn’t merely a product of my fevered brain, but was in fact correct.

“So what’s in rhubarb leaves then?” Asked Dad.

“Erm, oxalic acid I think…” I said.  And it occurred to me, before the inevitable “what’s that then?” question, that I didn’t actually know very much about oxalic acid.

So being a motivated learner, I went and looked it up.  The first thing that drew my eye was its fire diamond.  That description conjures up images of impressive gemstones, but sadly it’s not quite that cool.  Fire diamonds are those red, blue, yellow and white diamonds that you see on the side of big chemical containers.  They’re useful because they summarise the key safety and hazard information at a glance, without anyone needing to mess about reading text on a warning label or having to find a knowledgable, if slightly hysterical, scientist to explain the dangers in the middle of a disaster.

The first thing you need to know about these diamonds is that 0 is pretty safe, and 4 is very bad.  As for the colours: blue is for health and basically covers toxicity, yellow is for reactivity (is it likely to explode or generally do something unpleasant and probably loud), red is predictably for flammability and white is for ‘special’.  Special in this case meaning, ‘does something else nasty’.

The 3 in the blue bit means oxalic acid is pretty toxic. Chlorine gas has a 3, as does pure ammonia.  The official description is “short exposure could cause temporary incapacitation or possible residual injury”.  Eeek.

So is it time to put child-safe locks on your rhubarb patch?  It’s probably not necessary.  Rhubarb leaves have roughly 0.5% oxalic acid by weight.  There are various lethal doses mentioned out there, but even the more conservative is about 0.6 g per kg of body weight.  A small child, weighing say 10 kg, would need to consume about 6 g of oxalic acid, or about 1.2 kg of rhubarb leaves to receive a lethal dose.  There are other harmful things in rhubarb leaves, of which more later, so they might be quite a bit more dangerous than this implies, but even a determined toddler probably isn’t going to much their way through enough of the pretty horrible-tasting leaves to give them more than a tummy ache.  Still, I don’t recommend you leave them to experiment.

There is, in fact, also a bit of oxalic acid in the petioles (the stalks), but not very much.  Most of the acidity in the stalks is due to malic acid, which turns up all over the place (it’s one of those mysterious ‘fruit acids’ beloved of skincare product manufacturers) and is fairly harmless.  Cooking helps to break it down what tiny bit of oxalic acid is present, but it’s such a small quantity that even if you ate nothing but raw rhubarb stalks it probably wouldn’t be dangerous.  The occasional crumble certainly isn’t going to be a worry.  And, I checked, a little bit of stewed rhubarb is safe for babies.

Why is oxalic acid (in largish quantities) poisonous?  It’s not because it’s an acid; as I’ve mentioned before, lots of things are acids and are pretty harmless.  It turns out that oxalic acid is nephrotoxic, which means it damages your kidneys.  Oxalic acid reacts with iron and calcium ions in the body to form crystals which are then excreted in urine.  This is bad for two reasons, firstly it’s removing calcium and iron from the body, and they’re both quite important.  Secondly calcium oxalate is the main component of kidney stones, which are at best agonisingly painful and at worst can cause a nasty infection if they block the urinary tract.  It can also cause joint pain because similar precipitates form in the joints.  Interestingly, ethylene glycol (used in antifreeze) will produce oxalic acid in the body if you drink it (not recommended), so can generate the same kinds of problems.

The salt of oxalic acid is oxalate, remember that
acid + base –> salt + water
which is why the word oxalate keeps cropping up.  Oxalates turn up in lots of other foods as well as rhubarb, including parsley, spinach, and tea.  They’re not harmful in the quantities usually eaten, and the plants invariably contain all kinds of other beneficial nutrients that more than make up for the presence of a bit of oxalate.  Still, best not to go all Popeye and try to live off spinach.

During World War I rhubarb was briefly recommended as a vegetable, and this led to some deaths.  This mistake might have been caused when the gardener of the Earl of Shrewsbury at Alton Towers (now home of the theme park) had a letter published in Gardeners Chronicle some decades before which said that rhubarb leaves had been used as a food there for many years.  He apparently later wrote again to say he meant the stalks, and the correction was published, but by then the information was out there in print and perpetuated.  These days it would be so much easier wouldn’t it?  Someone could just write one of those “FORWARD THIS TO ALL YOUR FRIENDS NOW!!!!!!” emails and the information would be round the world in a flash.

As I mentioned earlier it’s not just oxalic acid in rhubarb leaves that’s dangerous, although exactly why the leaves are so toxic doesn’t seem to be well understood.  It might be anthraquinones, ring-shaped molecules used to make dyes and in paper-making.  There is definitely more to the story than oxalic acid, because post mortems of some of deaths supposedly due to rhubarb leaves didn’t find lethal quantities of oxalic acid.

It’s possible that some of these cases weren’t  entirely due to the leaves themselves, but something on them such as a pesticide (bearing in mind that some quite scary things have been chucked around as pesticides in times gone by, with little if any controls).  Rhubarb leaves have a large surface area and are slightly cone-shaped, and it’s not beyond the realms of possibility that something harmful could accumulate in them and then be eaten.  As far as I can establish, no one has carried out a recent study on the toxicity of rhubarb leaves (if you know differently, please let me know!) so it’s difficult to be sure.

So there we have it.  Rhubarb leaves contain oxalic acid, and it’s toxic, so don’t eat them.  Your pets are likely to be safe since, as most gardeners will tell you, few animals will willingly eat rhubarb leaves.  Children are also unlikely to try munching the huge green leaves, but it might be wise to keep an eye on them.  The stalks, on the other hand, make delicious crumble.  And look out for fire diamonds.

And what do you know, I posted this and then caught up with today’s episode of Pointless, and oxalic acid was a pointless answer in the first round.  What are the chances of that?  I love Pointless.

* Handy rhubarb hint: if you don’t want to use extra sugar, or should I say sucrose, add a chopped and peeled apple and banana to your rhubarb – it sweetens it up nicely.  I’m not giving up my custard recipe.  Sorry.