Field of Science

Cyanide in food plants, naturally!

Your taste buds are a nice simple chemical testing kit. You can measure pH, how acid (sour) to how basic (bitter) something tastes; you can detect salts, soluble carbohydrates (sugars), and to some extent, metallic tastes too. In particular humans have a strong aversion to very bitter tastes, and this is an excellent instinct for avoiding two very common classes of toxic compounds, alkaloids and glycosides. One of the more famous glycosides (a molecule plus a glucose sugar) produces cyanide by combining with water and dissociating after the sugar is removed, and many plants use this compound as a chemical defense mechanism against herbivores. If you have ever read any murder mysteries, a good detective always notices the odor of "bitter almonds", and then declares, "cyanide." Actually it always smells more like rank almonds to me, but it's so hard to describe smells. We eat "sweet" almonds, seed of a close relative of the apricot, peach, and plum, and they are not actually sweet, but here sweet is used as meaning "not bitter", so they don't have any appreciable amount of the cyanogenic glycoside. The seeds of these other species tucked safely away inside the inner stony fruit wall (pit) are quite bitter and inedible, as are apple and pear seeds. Don't worry if you happen to consume an apple seed or two. First, you seldom chew them, and when unchewed their impervious seed coat allows the seed to transit your GI tract intact (a typical small seed gambit). Second, very small doses of cyanide are not particularly harmful. Whew! Larger pitted fruits operate on the eat the flesh and discard the pit model, something we primates are quite good at. Here's a recent news article about a colleague who studies cyanide producing plants, and while this subject is covered in my economic botany class, even the Phactor was surprised to learn that Lima (lee-ma, as in Peru, their native home) beans were cyanogenic. The good news is that cyanogenic compounds are both water soluble and heat volatile, so people have developed methods of food processing that reduce the bitter taste, thus reducing their toxicity. However, when we jigger with the genes and turn off the glycoside production, the crop suddenly becomes very tasty to herbivores. So you can't win even when you rig the game.
HT to the BSA.

1 comment:

Pat said...

Somewhere in West Africa they detoxify cassava leaf by mixing it with palm oil. Washing mashed leaf would wash away most of the nutrients. The radio programme (earlier this year) that reported this was rather dismissive and was promoting low-cyanogenic varieties. As far as I could find out the process should work as carotenoids absorb cyanide. I couldn't find any reports of chemists actually trying the resulting compounds, unfortunately.

Holm oak acorns can be delicious. When I was in Spain I had roasted holm acorns a few times and they were as sweet as a peeled sweet chestnut. In another part of Andalucia I was told that the farm I was working on had a range of holm oaks and only some were sweet, many were too tanniny. My informant then demonstrated their method of improving the holm oak. He tasted the acorns of a nearby tree, professed them good and filled his pockets for scattering elsewhere. He found a moist place and dug the acorn in with his toe. "For the grandchildren".