How Almonds Went From Deadly To Delicious

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St. Basil's Hexaemeron, a Christian text from around the fourth century, contains a curious botanical instruction: Pierce an almond tree in the trunk near its roots, stick a "fat plug of pine" into its center — and its almond seeds will undergo a remarkable change.

"Thus the ... bitter almonds ... lose the acidity of their juice, and become delicious fruits," the text reads. "Let not the sinner then despair of himself. ... If agriculture can change the juices of plants, the efforts of the soul to arrive at virtue, can certainly triumph over all infirmities." The cause of this change, scientists later theorized, was stress: Jamming pine wood into the almond tree's core may have halted production of the toxins.

We don't need pine wood to turn almonds sweet anymore. Most almonds produced today are naturally tasty and safe to eat. Back then, though, many were bitter and poisonous. Even today, consuming 50 — or fewer — wild, bitter almonds could potentially kill an adult, and just a handful contain enough cyanide to be lethal to a child.

Over time, farmers have bred domesticated almond trees to produce mostly sweet seeds. But wild almonds helped us out — and now we know just how they went from deadly to delicious. A study published this week in the journal Science sequenced the almond genome and shows that a single genetic mutation "turned off" the ability to make the toxic compound thousands of years ago — a key step before humans could domesticate almonds.

The bitterness and toxicity of wild almonds come from a compound called amygdalin. When ingested, this compound breaks down into several chemicals, including benzaldehyde, which tastes bitter, and cyanide, a deadly poison. Wild, bitter almond seeds serve as amygdalin storehouses, keeping predators away with their nasty taste and poisonous effect.

But at some point thousands of years ago, a mutation occurred in a wild almond. This mutation inhibits the production of amygdalin almost completely. Sweet almonds still have trace amounts of amygdalin but not enough, by any reasonable measure, to produce dangerous amounts of cyanide.

"Wild almonds are bitter and lethal, even in tiny amounts, because [they have] this amygdalin," says study co-author Stefano Pavan, a professor in agricultural genetics and plant breeding at the University of Bari in Italy. (Pavan's primary co-author was Raquel Sánchez-Pérez, a senior biochemistry researcher at CEBAS-CSIC, an agricultural research center in Spain.) "This mutation is very important because it's the mutation that allowed almond domestication."

Sometime after the almond mutation occurred, according to the researchers, humans discovered this sweet variant. When exactly this happened, though, is still unknown. Almond trees are widely believed to be among the world's first domesticated trees. Archaeological evidence of cultivated almonds dates back to 3,000 B.C. But some geneticists think that humans probably started cultivating sweet mutated almonds much earlier than that, around 12,000 years ago.

What we do know: Once humans started encountering these new, tasty almonds, we embraced them with gusto. From Greece to California, we planted almond trees in droves and picked our trees carefully for the "sweet" allele — which is dominant over the "bitter" allele anyway. Over time, domesticated almonds lost almost all of their amygdalin.

Today, many people have never even heard of poisonous almonds, much less come across one in the wild — though some folks still eat bitter almonds in small doses. In Tunisia, for instance, people still make orgeat syrup with bitter almonds.

Dianne Velasco, a postdoctoral researcher in plant genetics at the University of California, Davis, whose work focuses on almonds and peaches, says that the research could potentially be put to use "very quickly" in helping plant breeders raise almonds more efficiently.

She says that right now, the earliest that almond breeders can assess the bitterness of their almond varieties is when their trees ripen and produce almonds, at three to five years of age. Knowing what mutation causes bitterness, she says, could potentially allow breeders to select the sweet varieties before they plant them. "This cuts into how much land usage [breeders] need, as well as cost," she says.