This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
The tonka bean is a South American product, which plays an important part in the adulteration of vanilla, which it closely resembles in odor. Unlike the latter, which is a climbing plant, the tonka bean tree grows sixty to ninety feet high, with a trunk sometimes three feet in diameter. The pods, which do not open spontaneously at maturity, like the vanilla bean, are about two inches long, are almond-shaped and very thick. The single seed is over an inch long, shaped somewhat like a large kidney bean, has a wrinkled skin and is shiny black. The odor, which is remarkably strong, resembles that of sweet clove, due to a concrete essence or principle termed coumarin, which is a crystallizable, volatile, neutral substance, obtained by distilling the tonka beans with water, when coumarin separates in crystals, the yield being about one and one-half per cent. It is very soluble in alcohol and ether, with difficulty in cold, but easily in boiling water, from which it crystallizes on cooling. The beans are usually covered with a crystalline efflorescence of coumarin. Formerly the beans were much used to scent snuff, and they are often called "snuff beans". The odor of the tonka beans bears some resemblance to the true vanilla, and much of the latter extract is adulterated with it. In some of the cheaper extracts it is entirely substituted for that costly material; but any one with a nice sense of smell can readily detect the presence of the tonka adulterant.
The effect of coumarin is that it will produce great and even fatal depression, and in the dose of thirty to sixty grains it occasioned nausea, giddiness, depression, vomiting, and drowsiness.
The product of coumarin is also artificially prepared by various methods. After one method it is prepared from salicyl aldehyd (spirae oil or artificially prepared salicyl aldehyd) with acetic acid and acetate of soda; after another from phenol and malic acid Various other methods are proposed. The artificial coumarin has the same properties and effects as the natural one obtained from tonka beans.
As the yield of coumarin from the tonka beans is about one and one-half per cent., one pound of tonka beans will contain approximately 115 grains of coumarin; therefore, in employing solutions of coumarin, 115 grains in solution should be equal in aroma to the tincture of one pound of the beans. If of inferior quality apply the same comparison method as explained under "Vanilla," substituting tonka for vanilla beans.
Prepare as follows: Coumarin, one ounce; alcohol 95°, two pints; dissolve the coumarin crystals in the alcohol, then dilute, by adding six pints of water to make one United States gallon.
If the coumarin is properly prepared, and of proper strength, this tincture should be about four times the strength of that of Formula I. prepared from the beans. This Formula is improved by adding some tincture of vanilla or vanillin, cinnamon or cassia (see Tincture of Vanil-lin).