So there’s piles of rhubarb in the shops at the moment, and the central character in my crime novels is both a foodie and a PI, and rhubarb leaves are high in oxalic acid, which is poisonous if taken in large quantities, and you can probably guess where all this is leading…
How poisonous? And why don’t you don’t come across many oxalic acid poisonings in crime fiction?
More likely it’ll be prussic acid (hydrocyanic acid). A decent toxic punch of cyanide, and your murderer won’t need a chemistry degree either. A good pestle and mortar more like, because prussic acid is to be found in cherry stones and apricot kernels (and is supposed to have that telltale smell of bitter almonds).
Hold on. So how come you come across significant quantities of sweet apricot kernels in recipes for, say, amaretti biscuits?
I tried the Food Safety Authority of Ireland’s website to sort out this question, but couldn’t find anything remotely related – apart from the usual nutty stuff on allergens.
The site’s extensive range of FAQs had frequently asked questions about the safety of clingfilm or how to store salami, but zilch on how to do away with a nasty character in your latest opus using, say, a bag of apricot kernels from Fallon & Byrne. They did have a section called “Novel Food Applications“, but it wasn’t remotely about food applications in novels.
So I ended up on the other side of the planet, as you do, with the FSANZ. These food safety experts for Australia and New Zealand say:
…it is unsafe for adults to eat more than three raw apricot kernels per day. Children should not eat any.
“Consuming processed foods derived from apricots such as apricot nectar or products made with apricot kernels as an ingredient (e.g. amaretti biscuits, almond finger biscuits, apricot jams) doesn’t pose a risk because processing or cooking these foods reduces cyanide to safe levels.
Oxalic acid murders
So murder by amaretti biscuits is out of the question. Back to our rhubarb leaves, and their oxalic acid. The name comes from the Greek word Oxalis, which means sorrel. Said acid occurs in sorrel, hence the “father of modern chemistry” Lavoisier dubbed it oxalic acid. It also occurs in spinach and cabbage.
Before checking up on the typical fatal dose I needed a little info about the symptoms. My next stop was a website run by some chemists from Nepal.
Make sure you’ve already had your breakfast before reading this bit…
The person poisoned with the oxalic acid runs down the strange streaks from the angles of the mouth. Due to corrosion, there is whitening or yellow-white discoloration of the lips, lining of the mouth and upper surface of the tongue. The lining of the stomach in oxalic acid poisoning is blackened by the production of acid haematin. There may be superficial corrosion. The stomach may contain fresh or altered blood. Furthermore crystals of calcium oxalate can be demonstrated in scrapings of the stomach mucosa. The kidneys of a person dying of oxalic acid poisoning are congested and swollen with oedema (accumulation of excess fluid in body tissues).
Yuck. The acid’s corrosive properties also mean the victim’s throat is quickly damaged. Excellent: this will make it harder to make out their final cryptic message…
“What did he say?”
“I think it was ‘The thirty nice pets’ or something.”
Death in High Heels
The only time I’ve seen oxalic acid being employed thus in crime fiction is in Christianna Brand’s Death in High Heels. Here’s its first mention, at the chemist’s…
“Good morning, young ladies, and what can I do for you to-day?”
“I want some oxalic acid,” said Rachel.
“Oxalic acid crystals? What did you want them for?”
“She wants to murder Miss Gregory,” said Victoria, laughing. “You know Miss Gregory – she’s the one that made all the flap about you giving us tick for the showroom soap; don’t you remember?”
“Oh, yes, I remember Miss Gregory,” said the chemist, a trifle grimly. “I think everyone knows her in the small shops around here. But I can’t be a party to her murder, you know.”
“Don’t listen to Mrs David,” said Rachel, “she reads too many detective stories. I want to clean a straw hat, that’s all it is.”
Oh yeah, forgot to mention: it’s also used in hat-cleaning. Forensic Medicine Canada is in broad agreement with Ms Brand about what constitutes a fatal dose. It says “One drachm is the smallest, but half an ounce is usually fatal.”
How much rhubarb leaf would that be again? A site called Rhubarbinfo.com says:
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.
Five kilos? I’m beginning to go off rhubarb leaves for my next murderer. But that still leaves us with a rake of stalks…
Rhubarb (or rúbarb or biabhóg in Irish) is technically a vegetable, not a fruit, and is packed with zing and vitamin C, perfect after the cold dark winter months.
It’s very versatile and goes with everything from crumbles to mackerel, but the first thing to do with the early shoots is to treat them simply, as the star of the show: stew them gently or – better still – bake them.
Cut the stalks into roughly equal sized lengths, about an inch. Pack the cut stalks into an oven dish. Add the juice of an orange – a blood orange if you have one – and a few shavings of fresh ginger (or a pinch of dried ginger, or some zest from the orange) and either brown sugar or honey to counter the tartness of the rhubarb. It will create a juicy pink syrup.
Bake in a medium oven for about 20 minutes until just cooked (not a mush). Serve warm or chilled, with a couple of drops of vanilla extract well mixed into a dollop of plain yoghurt. And a dash of prussic acid if you fancy.
While you may have discounted rhubarb leaves and almond kernels in your next murder mystery, here’s a rather good round-up of 16 cute killers in the garden worth checking out. I’ve always been a bit of a fan of the foxglove (digitalis), mistletoe berries, and the flower buds of hydrangeas…