Almond is the leading tree nut species worldwide.
The seeds of the almond tree (Prunus amygdalus dulcis) are predominantly sweet, but individual trees produce bitter kernels (Prunus amygdalus amara.) The bitter flavor in almonds is imparted by a recessive (non-dominant) genetic trait, kept at bay by human domestication for safety concerns since the almond bitterness was associated with unfortunate poisoning events.
Kernels of wild almond species are highly toxic to humans and herbivores because the cyanogenic glycoside amygdalin accumulates in the cotyledon (the living seed).
Amygdalin is produced from the cyanogenic monoglycoside prunasin, which is biosynthesized in the seed coat (tegument). It was recently shown that two genes in the prunasin biosynthetic pathway are poorly expressed in the tegument of sweet genotypes relative to bitter genotypes.
When hydrolyzed (decomposed) by a specific enzyme, amygdalin releases cyanohydric acid (HCN), known as prussic acid. Cyanohydric acid is extremely toxic to a broad spectrum of organisms, due to its ability of bonding minerals (iron, manganese, and copper) that are functional to a correct metabolic activity, inhibiting processes like the oxygenation of tissues and bringing oxidative stress to the cells.
Depending on its concentration, the biochemical damage conferred by cyanide in the organism may lead to acute neurological intoxication in the most severe cases, being fatal at a threshold of 0.5 ‒ 3.5 mg/kg (0.2 ‒ 1.6 mg/lb) of body weight (approximately 50 bitter almonds.)
Bitter almonds, which appear slightly broader and shorter than the sweet variants, contain 42 times higher cyanide levels than the trace levels found in sweet almonds.
Processing methods involving fermentation and heat—such as roasting and drying—can inactivate cyanogenic glycosides and minimize cyanide poisoning risk.
In any case, better not to chance direct consumption with bitter or ‘raw’ and ‘unroasted’ sweet almonds, especially when we cannot be sure of the ‘natural’ content of potentially toxic substances versus the nutritional benefit of the food product.