Why “Natural” isn’t always better: almond extract and cyanide

Right now the various species of Prunus are in flower all over northern California; the ornamental plums that are so popular as sidewalk decor are shedding petals everywhere, apricot blossoms are peeking out from yards, and the almond trees that crop up as renegades from the big orchards near Davis and in the central valley are covered in popcorn-y pinkish white flowers.  With constant reminders of stone fruit everywhere but none actually in season to eat, I’ve been doing a lot of baking with almonds and almond extract.

A sprinkling of wild plum blossoms (Prunus americana) on my way to lab.

Natural almond extract is not made from the edible almonds that are farmed here (Prunus dulcis), rather, it’s usually made from closely-related bitter almonds (Prunus amygdalus), and sometimes from more surprising sources, like cinnamon bark.  The chemical that we recognize so definitively as almond flavoring is benzaldehyde, and it’s found in several plants both in and out of the Prunus genus.  If you have a bottle of “natural almond extract” at home, chances are that yours, like mine, is a suspension of bitter almond oil in either glycerin or ethanol.

The taste of almonds (or apricot kernels, or sometimes cinnamon or cassava masquerading as almonds): benzaldehyde.

On to the cyanide part…and why that delicious flavor is really the plant trying to scare you off or kill you.

Gabriel Garcia Marquez once wrote poetically about the scent of bitter almonds and the fate of unrequited love as a lead-in to murder by cyanide poisoning.  And in bitter almond oil as in a tragic romance, the sweet and the toxic are inextricably entangled.  Benzaldehyde is made by the decomposition of amygdalin (named for Prunus amygdalus, and in turn responsible for the bitterness that gives bitter almonds their common name).  The other decomposition products are glucose (sweet) and hydrogen cyanide (toxic).

The bitter almond molecule amygdalin, which breaks down (left to right) into glucose, glucose, hydrogen cyanide (aka Prussic acid to the classicists), and benzaldehyde.

As with so many other useful or toxic chemicals that we get from plants, the evolutionary utility of amygdalin to the plant is for defense, specifically as a deterrent to grazers from eating the valuable seed as well as the dispensable fruit.  Inside the cells of the almond kernel, amygdalin is sequestered from the enzyme that breaks it down: amygdalin hydrolase.  Crushing, as happens when the plant is grazed upon, brings the enzyme and amygdalin together, and cyanide is produced as a result–as much as 4-9mg per almond.  If the bitter taste of amygdalin doesn’t deter the grazer, the light-headedness, weakness, and racing heart that follows as the cyanide out-competes oxygen for the grazer’s hemoglobin probably will.  It freaks me out a little to remember that, as children, my brother and I noticed that apricot kernels looked exactly like almonds, and tried a few before deciding that the resemblance stopped at the visual level and they were too nasty to keep eating (spoiler alert: we lived).

But despite the almond trees’ efforts to deter us, humans like me have decided we really love the taste of benzaldehyde, and so commercial producers have developed a range of methods to generate it in a form that won’t also kill us.  Most descriptions of bitter almond oil preparation I’ve seen (here’s an example) involve crushing the kernels, which brings the amygdalin and hydrolase together, drying the powder into a cake, macerating the cake in water, which washes away much of the water-soluble cyanide, leaving the benzaldehyde-rich oil behind, and then akali-washing the oil to remove the rest of the cyanide.  The oil that’s left is then resuspended in ethanol or glycerol.  But this is still imperfect, and some retailers of natural almond extract caution that it can still be toxic in large amounts (one estimate is that 7.5mL of bitter almond oil would be lethal).

Why bother with amygdalin at all?

This is the main issue–there’s really no reason to buy natural almond extract.  Benzaldehyde is easy to make synthetically without ever bringing it near cyanide. And ethanol suspensions of benzaldehyde are sold (more cheaply than natural almond extract) as imitation almond extract.  It’s a more strongly-flavored product, and there’s no risk of poisoning.  But as consumers, most of us assume “natural” must be better than “imitation”, and the hippie grocery store we usually shop at doesn’t even sell imitation almond extract.  But don’t buy into the hype!  This is one instance where imitation might be more cost-effective and marginally safer.

Imitation almond extract--benzaldehyde without the pesky cyanide part.

Extra nitty gritty: How an almond ends up sweet or bitter

Although a weaker (or, if you’d like to be euphemistic about it, “more delicate”) version of almond extract can be made by soaking chopped sweet almonds in vodka, the reality is that sweet almonds just don’t make enough amygdalin to yield a high proportion of benzaldehyde.  A recent article in Plant Physiology may help explain how this came to be.  The precursor of amygdalin is prunasin.  There are similar amounts of prunasin in both almonds, and similar amounts of the enzyme that converts prunasin into amygdalin.  So why isn’t there amygdalin in sweet almonds?

In this article, the authors focused on  the enzyme that breaks prunasin down: the glycosidase prunasin hydrolase (PH).  If most of the prunasin is broken down by PH, it won’t be available to make into amygdalin, with the outcome that the almond will be sweet and not bitter. The authors set out to determine whether differences in PH (not pH, mind you) might account for the differences in sweet and bitter almond fruits.  They found that in both species, there were two PHs: PH691 (identical in both species) and PH692, which contains a polymorphism resulting in an arginine residue in sweet almonds but a cysteine in bitter almonds.  This difference in PH692 might result in a different activity: perhaps the sweet almond version of PH692 is more active.

What’s more, the authors found that the localization of PH was different in the two species: while PH started out in the symplast (inside the cell membrane) and moved to the apoplast (outside the cell membrane) in sweet almonds, in bitter almonds the localization was reversed, moving from apoplast to symplast. Although the authors did not directly investigate the consequence of this differential localization, one could well imagine that if the localization of PH better coincided with prunasin accumulation in sweet almonds than bitter almonds, the result would be little remaining prunasin to turn into amygdalin in the mature sweet almond.   Further characterization of these differences may influence the breeding of sweet almonds, where avoiding amygdalin content is the primary goal.

For the scientists: Reference
Sá:nchez-Perez, R., Saez, F., Borch, J., Dicenta, F., Moller, B., & Jorgensen, K. (2012). Prunasin hydrolases during fruit development in sweet and bitter almonds PLANT PHYSIOLOGY DOI: 10.1104/pp.111.192021

For the bakers: Almond Cheesecake Recipe

Start with Better Homes and Gardens’ Cheesecake Supreme recipe

Omit lemon peel.

Omit walnuts, replace with 1/2 cup of crushed almonds (I take sliced almonds, place them between two paper towels, and crush with a rolling pin).

To the filling, add 1tsp of almond extract .

If desired, garnish with sliced almonds.