Description—The fruits, which in structure correspond to those of other plants of the order, are laterally compressed and ovate. The mericarps which hang loosely suspended from the arms of the carpophore, are in the English drug about ⅙ of an inch in length and ¹/₂₀ in diameter, subcylindrical, slightly arched, and tipped with the conical, shrivelled stylopodium. They are marked with five pale ridges, nearly half as broad as the shining, dark brown furrows, each of which is furnished with a conspicuous vitta; a pair of vittæ separated from each other by a comparatively thin fibro-vascular bundle, occurs on the commissure.
Caraways are somewhat horny and translucent; when bruised, they evolve an agreeable fragrance resembling that of dill, and they have a pleasant spicy taste. In the London market, they are distinguished as English, Dutch, German, and Mogador, the first sort fetching the highest price. The fruit varies in size, tint and flavour; the English is shorter and plumper than the others; the Mogador is paler, stalky, and elongated—often ³/₁₀ of an inch in length.
Microscopic Structure—Caraways are especially distinguished by their enormous vittæ, which in transverse section display a triangular outline, the largest diameter, i.e. the base of the triangle, often attaining as much as 300 mkm. Even those of the commissure are usually not smaller.
Chemical Composition—Caraways contain a volatile oil, which the Dutch drug affords to the extent of 5·5 per cent., that grown in Germany to the amount of 7 per cent.[1170]; in Norway 5·8 per cent. have also been obtained from indigenous caraways.[1171] The position and size of the vittæ account for the fact that comminution of the fruits previous to distillation, does not increase the yield of oil.
Völckel (1840) showed that the oil is a mixture of a hydrocarbon C₁₀H₁₆, and an oxygenated oil, C₁₀H₁₄O. Berzelius subsequently termed the former Carvene and the latter Carvol.
Carvene, constituting about one-third of the crude oil, boils at 173° C., and forms with dry hydrochloric gas crystals of C₁₀H₁₆ + 2HCl. It has been ascertained by us that carvene, as well as carvol, has a dextrogyrate power, that of carvene being considerably the stronger; there are probably not many liquids exhibiting a stronger dextrogyrate rotation. Carvene is of a weaker odour than carvol, from which it has not yet been absolutely deprived; perfectly pure carvene would no doubt prove no longer to possess the specific odour of the drug. By distilling it over sodium it acquires a rather pleasant odour; its spec. gr. at 15° C. is equal to 0·861.
Carvol at 20° C. has a sp. gr. of 0·953; it boils at 224° C.; the same oil appears to occur in dill (see Fructus Anethi), and an oil of the same percental constitution is yielded by the spearmint. The latter however deviates the plane of polarization to the left. If 4 parts of carvol, either from caraways, dill, or spearmint, are mixed with 1 part of alcohol, sp. gr. 0·830, and saturated with sulphuretted hydrogen, crystals of (C₁₀H₁₄C)₂SH₂ are at once formed as soon as a little ammonia is added.[1172]
Oil of caraway of inferior quality is obtained from the refuse of the fruit; we find it less dextrogyrate than the oil from the fruits alone; this is due to the admixture of oil of turpentine before distilling.
If the carvol is distilled there remains in the still a thickish residue, from which a substance of the phenol class may be extracted by caustic lye.
Oil of caraway distilled in England from home-grown caraways is preferred in this country. On the Continent, that extracted from the caraways of Halle and Holland is considered to be of finer flavour than the oil obtained from those of Southern Germany.