Hydrogen cyanide (aka hydrocyanic acid) has a faint bitter-almond scent – although not everyone can smell it, due to a recessive gene. It is released in plants by compounds called cyanogenic glycosides. One is a cyanohydrin called amygdalin. Amygdalin will slowly release hydrogen cyanide when it comes into contact with the plant enzymes that are activated by tissue damage, caused by chewing or crushing. Fruits in the Rosaceae family, such as apples, apricots, bitter almonds, cherries, crab apples, damsons, hawthorn berries, pears, peaches, plums and sloes, have stones that contain small amounts of amygdalin, and other cyanohydrins like mandelonitrile. One hundred grams of crushed apple seeds can yield about 70 mg of hydrogen cyanide.
The lowest reported oral lethal dose of hydrogen cyanide for humans is 0.54 mg/kg body weight (WHO report), and the average absorbed dose at the time of death has been estimated at 1.4 mg hydrogen cyanide per kilo of body weight. So, if you weigh 70 kg, your lowest lethal dose is 37.8 mg – about 54 grams of crushed apple seeds! You need to crush them – ideally into a powder – for all the enzymes to come into contact with all the amygdalin. If you’re worried about this, as symptoms of toxicity can occur from hydrogen cyanide at a concentration of 0.5 mg/kg of body weight, and some apples contain up to 4 mg amygdalin per gram, assuming an apple pip weighs about 0.7 g, avoid powdering and eating 66 apple seeds in one go. Incidentally, the amygdalin content of commercially-available pressed apple juice is low, ranging from 0.01 to 0.04 mg per ml.
Other popular farmed crops that are highly cyanogenic include lima beans, butter beans, almond, sorghum, macadamia nut, flax, white clover and cassava. (The “bitter” roots of the cassava plant may yield up to 1 gram of hydrogen cyanide per kilogram.) Seeds from cucumber, courgette, marrow, melon and squash also contain low levels of amygdalin – under a quarter of a gram per gram of seed. Processing, such as toasting pumpkin seeds and UHT processing of apple juice, also lowers the levels still further.
Incidentally, it’s not just plants that do this. Your own neurons can also release hydrocyanic acid when your opioid receptors are activated. This is necessary for opioids to provide pain relief. Your blood cells (leukocytes) also generate hydrocyanic acid when they are engulfing other cells (phagocytosis). They can kill bacteria, fungi, and other pathogens by generating several different toxic chemicals, one of which is hydrogen cyanide. You’re most likely to come into contact with hydrogen cyanide from vehicle exhaust fumes, cigarette smoke and the smoke from burning plastics.
Amygdalin decomposes into three parts. Hydrogen cyanide (as mentioned), glucose and benzaldehyde. Benzaldehyde, which tastes strongly of almonds, amaretto or marzipan is a common chemical in many young plants. Rowan buds for example, have a very intense marzipan flavour. Blackthorn (sloe) leaves are used to make a drink called épine which tastes like an amaretto-flavoured port. The flowers of hawthorn, sloe and cherry all smell of benzaldehyde.
Hydrogen cyanide is released to put mammals and insects off munching on the plant’s seeds. Cyanide-free benzaldehyde is an attractive flower scent for insect pollinators.
Footnote on cherry brandy
Having been asked about cherry pit brandy, most commercial brandies like Bols are made from pitted cherries with just a few stones added for taste. People who have made cherry brandy keeping the stones in the cherries, for a year or two, have reported headaches and high blood pressure. However, there is a lot of debate about this. Cracking the stones and kernels releases more cyanide as it triggers the enzyme reaction, as does crushing the kernels and people’s individual tolerance for eating apple pips and cherry pips varies widely. Some react to a single pit, some have eaten them for years! A calculation done by D S O’Neil here, works out that 500mg pits in 1L brandy yields about 1mg HCN per ml. So 50ml – a double shot – could be very unpleasant and even kill a weak or sick person. O’Neil doesn’t state if this is cracked kernels but I assume it is cracked and ground, as in the apple pip illustration above, with all the amygdalin activated and converted. But to cut a long story short… keep pits a LOT lower than this ratio to be on the safe side and just leave them in for a few weeks and not years!