Source, Etc

The bitter almond tree (Prunus Amygdalus, Stokes, var. amara, Baillon) is indistinguishable from the sweet by any permanent botanical character, and enjoys the same geographical distribution, although it is not cultivated to so large an extent. It is remarkable, therefore, that the seeds should invariably contain a constituent - amygdalin - that is never present in the seed of the sweet almond tree. The poisonous properties of the water distilled from the bitter almond (and the cherry-laurel leaf), due to the presence and decomposition of a cyanogenetic glucoside, have long been known; it was the discovery of hydrocyanic acid in it that led to the recognition of the poisonous nature of this acid, which, curiously enough, had up to then (1802) escaped observation.

Bitter almonds are imported chiefly from northern Africa, from Sicily, and from the south of France.

Description

In form and appearance bitter almonds closely resemble Valencia almonds, but they are usually smaller and less regular. They have, however, a bitter taste, and yield with water an emulsion easily distinguished from that of the sweet almond by its characteristic odour.

Constituents

Bitter almonds resemble the sweet in containing both a bland fixed oil and proteids, the former of which is obtained by crushing the almonds between horizontal grooved rollers and pressing in powerful hydraulic presses. But they differ essentially from the sweet in containing a colourless, crystalline glucoside, amygdalin (2.5 to 4 per cent.), which is not present in the sweet. This substance is left in the cake obtained after the oil has been expressed, and can be extracted from it by digestion with alcohol. It has a bitter taste and is odourless, but when an aqueous solution is mixed with an emulsion of sweet almonds the amygdalin is decomposed with production of benzaldehyde, hydrocyanic acid and dextrose.

C20H17NO11

+

2H1O

=

2C6H12O6

+

C6H5COH

4

HCN

Amygdalin

Dextrose

Benzaldehyde

Hydrocyanic acid

This change is effected by the emulsin contained in the sweet almond. Emulsin is also contained in the bitter almond itself, but, being localised in particular cells of the seed, is unable to act upon the amygdalin until the seeds are crushed and water added. When therefore bitter almonds (or cake) are crushed and mixed with water, the characteristic odours of benzaldehyde and hydrocyanic acid are developed. If, after standing a few hours, the mixture is subjected to distillation, an oily liquid of strong bitter almond odour is obtained, together with a quantity of watery distillate; the oil is volatile or essential oil of bitter almonds, and consists of benzal-dehyde and hydrocyanic acid, partly in the free state and partly combined as benzaldehyde-cyanhydrin. It can be freed from hydrocyanic acid by shaking it with milk of lime; all the hydrocyanic acid, both free and combined, forms calcium cyanide. The addition of ferrous sulphate then converts the cyanide into calcium ferro-cyanide, and the oil can be obtained free from hydrocyanic acid by redistilling in a current of steam. No benzaldehyde or hydrocyanic acid is developed by the sweet almond, because it contains no amygdalin.

Bitter almonds yield from 0.5 to 0.8 per cent, of volatile oil and about 0.25 per cent, of hydrocyanic acid.

Many other Rosaceous plants contain amygdalin, such as the peach, apricot, plum, etc, not only in the seed, but also in the young shoots and flower-buds.

Amygdalin is closely allied to, but not identical with, prulaurasin, and is also distinct from a similar glucoside that occurs in the bark of Prunus serotina, Ehrhart (compare also p. 252). Its hydrolysis is said to occur in three stages, each effected by a particular enzyme contained in the emulsin.

Benzaldehyde is also prepared from benzyl chloride, C6H5.CH1C1, or benzal chloride, C6H5.CHC12, and sold as oil of bitter almonds; it has a less pleasant odour than that obtained from bitter almonds.

Uses

Bitter almonds are sedative, but as the poisonous hydrocyanic acid yielded by them varies in quantity they are unreliable. They are also employed for flavouring, but they should for a similar reason be used with caution.

Substitutes

Apricot kernels contain constituents similar to those of the bitter almond. They are imported in large quantities from Syria and California and are often used by confectioners in the place of bitter almonds. The fixed oil expressed from them is commonly sold as ' Oleum Amygdalae Persicum' or ' peach kernel oil.' From the cake an essential oil (0.6 to 1.0 per cent.) is distilled as from bitter almond cake.