Amygdalin may be obtained crystallized when almonds deprived of their oil are boiled with alcohol of 84 to 94 per cent. The product amounts at most to 2½ or 3 per cent. Amygdalin per se dissolves in 15 parts of water at 8-12° C., forming a neutral, bitter, inodorous liquid, quite destitute of poisonous properties.
It would appear from the investigations of Portes (1877) that in young almonds, amygdalin is formed before the emulsin.
When bitter almonds have been freed from amygdalin and fixed oil, cold water extracts from the residue chiefly emulsin and another albuminoid matter separable by acetic acid. The emulsin upon addition of alcohol falls down in thick flocks, which, after draining, form with cold water a slightly opalescent solution. This liquid added to an aqueous solution of amygdalin, renders it turbid, and developes in it bitter almond oil. The reaction takes place in the same manner, if the emulsin has not been previously purified by acetic acid and alcohol, or if an emulsion of sweet almonds used. But after boiling, an emulsion of almonds is no longer capable of decomposing amygdalin.
What alteration the emulsin itself undergoes in this reaction, or whether it suffers any alteration at all, has not been clearly made out. The reaction does not appear to take place necessarily in atomic proportions; it does not cease until the emulsin has decomposed about three times its own weight of amygdalin, provided always that sufficient water is present to hold all the products in solution.
The leaves of Prunus Lauro-cerasus L., the bark of P. Padus L., and the organs of many allied plants, also contain emulsin or a substance analogous to it, not yet isolated. In the seeds of various plants belonging to natural orders not botanically allied to the almond, as for example in those of mustard, hemp, and poppy, and even in yolk of egg, albuminous substances occur which are capable of acting upon amygdalin in the same manner. Boiling dilute hydrochloric acid induces the same decomposition, with the simultaneous production of formic acid.
The distillation of bitter almonds is known to offer some difficulties on account of the large quantity present of albuminous substances, which give rise to bumping and frothing. Michael Pettenkofer (1861) has found that these inconveniences may be avoided by immersing 12 parts of powdered almonds in boiling water, whereby the albuminous matters are coagulated, whereas the amygdalin is dissolved. On then adding an emulsion of only 1 part of almonds (sweet or bitter), the emulsin contained in it will suffice to effect the required decomposition at a temperature not exceeding 40° C. In this manner, Pettenkofer obtained in some experiments performed with small quantities of almonds, as much as 0·9 per cent. of essential oil. In the case alluded to on the opposite page, in which 28 cwt. of almonds were treated, the yield of essential oil amounted to 0·87 per cent. From data obligingly furnished to us by Messrs. Herrings and Co. of London, who distill large quantities of almond cake, it appears that the yield of essential oil is very variable. The yearly averages as taken from the books of this firm, show that it may be as low as 0·74, or as high as 1·67 per cent., which, assuming 57 pounds of cake as equivalent to 100 pounds of almonds, would represent a percentage from the latter of 0·42 and 0·95 per cent. respectively. Mr. Umney explains this enormous variation as due in part to natural variableness in the different kinds of bitter almond, and in part to their admixture with sweet almonds. He also states that the action of the emulsin on the amygdalin when in contact with water, is extremely rapid, and that 200 pounds of almond marc are thoroughly exhausted by a distillation of only three hours.
In the distillation, the hydrocyanic acid and bitter almond oil unite into an unstable compound. From this, the acid is gradually set free, and partly converted into cyanide of ammonium and formic acid. Supposing bitter almonds to contain 3·3 per cent. of Amygdalin, they must yield 0·2 per cent. of hydrocyanic acid. Pettenkofer obtained by experiment as much as 0·25 per cent., Feldhaus (1863) 0·17 per cent.
Some manufacturers apply bitter almond oil deprived of hydrocyanic acid, but such purified oil is very prone to oxidation, unless carefully deprived of water by being shaken with fused chloride of calcium. The sp. gr. of the original oil is 1·061-1·065; that of the purified oil (according to Umney) 1·049. The purification by the action of ferrous sulphate and lime, and re-distillation, as recommended by Maclagan (1853), occasions, we are informed, a loss of about 10 per cent.
Bitter almond oil, C₆H₅(COH), being the aldehyde of benzoic acid, C₆H₅(COOH), is easily converted in that acid by spontaneous or artificial oxidation. The oil boils at 180°C. and is a little soluble in water; 300 parts of water dissolve one part of the oil.
There are a great number of plants which if crushed, moistened with water, and submitted to distillation, yield both bitter almond oil and hydrocyanic acid. In many instances the amount of hydrocyanic acid is so extremely small, that its presence can only be revealed by the most delicate test,—that of Schönbein.[956]