|Bitter Almond Blossom|
When almonds are used it is the press-cake remaining after extraction of the fixed almond oil that is the starting point which is macerated in warm water prior to extraction. A substance called Amygdalin, present in all the kernels mentioned, is converted by enzymatic action into benzaldehyde and hydrocyanic acid (what is commonly called cyanide when people are talking about the poison, once also called Prussic Acid). There is enough cyanide present in about 10 drops of the crude oil to kill an average person, and it is poisonous by ingestion and by inhalation, so the oil is quite useless at this point as a flavour or perfume agent. The process is nearly identical when it is made from the kernels (stones) of apricots, plums, cherries or peaches.
As an aside, amygdalin is also present in apple pips, so far as I know they have never been used as a commercial source of the oil, but if you’ve ever heard that apple pips are poisonous, now you know why. In practice they don’t contain cyanide unless they are crushed up and fermented and tend to pass through the human gut whole, so if you’re in the habit of eating your apples core-and-all you’re not likely to come to any harm.
The crude oil is cleaned by alkali washing and rectification resulting in the nearly pure benzaldehyde that is sold widely as Bitter Almond Oil. Oddly enough it’s main use in flavouring is as a sweetener - so not only is in not usually made from almonds but it isn’t bitter either! The odour is familiar to most people as marzipan, which was traditionally made with about 1% bitter almond kernels ground up with the sweet almonds into a paste. Now it is more likely to be made with all sweet almonds and some benzaldehyde added to flavour it.
According to Arctander, Hydrocyanic acid smells rather similar to Benzaldehyde - though I don’t recommend testing that assertion.
Arctander also has this to say about the natural vs synthetic origins of the oil:
Benzaldehyde by the way isn’t very stable and tends to turn into Benzoic acid (white crystals that are essentially odourless) on exposure to air and consequently it is often sold with some ethanol added to improve stability.
Benzaldehyde is highly volatile liquid: a top note if you use it in perfumery and isn’t all that widely used. I happen to have some at the moment because one of the things I’m researching is a lilac fragrance, in which it is a component.
A draft of this material appeared on the fragrance discussion forum Basenotes in this post.