As the various oils are fully described in the second vol. of this treatise, it will be expedient, in order to avoid repetition, to discuss briefly in a special chapter the general methods used in their investigation and the most common adulterants met with.
The practice of adulteration of the volatile oils, which is probably as old as the manufacture itself, had in the beginning a certain justification, as with the incomplete technical equipment of the early times the addition of fatty oils, turpentine oil, or alcohol was often necessary in order to extract from the plants their odorous principle. Later, when the preparation of the pure oils was already known, the practice of making these additions was still retained.
Even forty years ago, it was customary to distill coriander with the addition of orange oil and to put the distillate on the market as coriander oil. Since pure coriander oil can now be prepared without difficulty, the product obtained by using orange oil, as it is found now and then even at the present time, must be considered as adulterated and if the foreign ingredient is not made known, its sale is a fraud.
The adulteration need not always be by the addition of a less valuable body, it sometimes consists in that the more valuable constituent of the oil has been partially removed. The effect is the same, whether from a caraway oil of the specific gravity 0,910 so much carvone be removed that an oil of the specific gravity 0,890 remains behind, or whether the same result is attained by the addition of limonene to the same oil.
Although the adulterations themselves mostly find a sufficient explanation in the profitableness and the pecuniary advantage to the adulterator, it cannot, however, be denied, that often the ignorance of the consumer, and above all the desire to buy as cheaply as possible, is the cause of the spurious composition of many an oil. More than once the producer may have been induced to adulterate, because he found no buyers for his pure products at a reasonable price, while his adulterating competitor was able to do a lucrative business at lower prices.
The main reason for the extensive adulteration to which volatile oils have been subjected at times, is to be sought in the fact that the detection of adulterants was very difficult and often entirely impossible.
Owing to the development of the chemistry of the terpenes and their derivatives, great progress has been made during the last twenty five years in the detection of adulterations. Knowing the composition of not a small number of volatile oils, it has become possible not only to distinguish between a pure and an adulterated oil, but also to judge the quality of these oils. This is effected by estimating the amount of the principal, or the most important constituent. In lavender oil, bergamot oil, petit-grain oil and others, the amount of esters present are therefore determined; in thyme oil, clove oil, pimenta oil, bay oil, and Cretian origanum oil the amount of phenols are estimated; in cassia oil and lemongrass oil the amount of aldehyde; in caraway oil the amount of carvone. The assay of santalwood oil shows how much santalol, that of palmarosa oil, how much geraniol is contained in the oil. The quality of the oils named finds numerical expression in the percentage strength of the active constituents such as esters, phenols, aldehydes, ketones and alcohols.
In a second class of oils, whose composition is likewise sufficiently known, an assay is not yet possible. The reason for this is twofold: first the value of the oil depends not upon a single constituent but upon the blending of the properties of several; and secondly the chemical methods of investigation are not sufficiently developed.
With these oils, the examination is restricted as a rule to the determination of the normal composition of the oil and the absence of commonly used adulterants. Such oils are lemon oil, orange oil, rosemary oil and spike oil, which should be tested particularly for turpentine oil.
The incomplete knowledge of the composition and the defectiveness of the methods of testing most of the oils, do not at present allow of an investigation resting on a rational chemical basis. With this class of oils the entire examination consists in determining the physical constants. As the average and limit values of specific gravity, optical rotation, solubility, etc, of the more common oils are well known through observations extending over many years, variations from these values call the attention of the investigator to adulterants.
Indeed, the physical behavior of an oil is in general very well suited to indicate rapidly the addition of foreign substances; the investigation of volatile oils should therefore begin with the determination of the physical properties, no matter whether the investigation be for practical or for scientific purposes. After this, the special methods are to be used, such as saponification, acetylation, aldehyde and phenol determinations, and finally, if it appears necessary, the tests for turpentine oil, fatty oil, alcohol or petroleum should be applied.
It is of course evident that when the practical value of an oil is to be considered, the examination as to its odor1) and taste must accompany the chemical and physical investigation, for these are the properties on account of which the volatile oils are used in the perfume and soap industries, in the manufacture of candies and liquors, and partly at least in medicine.
1) Attention may here be called to the interesting publications by H. Zwaardemaker: Die Physiologie des Geruchs (Leipzig 1895); Die piechkraft von Lbsungen differenter Konzentration (Arch. f. Anatomie u. Physiologie, Physiol. Abtlg. 1900, 415); Die Kompensation von Geruchsempfindungen (Ibidem 1900, 423); Geruch (Ergebnisse der Physiologie 1 (1902), 896); Riechen und schmecken (Arch. f. Anatomie u. Physiologie, Physiol. Abtlg. 1903, 120; Report of Schimmel & Co. October 1904, 107); Prazisions-Olfaktometrie (Arch. f. Laryngologie Bd. 15, Heft 2; Report of Schimmel & Co. October 1904, 105); Geschmack (Ergebnisse der Psysiologie 2, 2 (1903), 699); Eine bis jetzt un-bekannt gebliebene Eigenschaft des Geruchssinnes (Arch. f. Anatomie u. Physiologie, Physiol. Abtlg. 1904, 43; Report of Schimmel & Co. October 1904, 109); Die Empfindung der Geruchlosigkeit (Untersuchungen aus dem phy-siologischen Laboratorium 5. Reihe IV, II, 376; Report of Schimmel & Co. October 1904, 107); Odorimetrie von prozentischen Lbsungen und von Systemen im heterogenen Gieichgewicht (Untersuchungen aus dem phy-siologischen Laboratorium 5. Reihe IV, II, 387; Report of Schimmel & Co. October 1904, 105); Uber die Proportionen der Geruchskompensation (Arch, f. Anatomie u. Physiologie, Physiol. Abtlg. 1907, 59; Report of Schimmel & Co. April 1908, 173); Die vektorielle Darstellung eines Systems von Geruchs-kompensationen (Arch. f. Anatomie u. Physiologie 1908, 51; Report of Schimmel & Co. October 1908, 160). Comp. also E. Erdmann, Ober den Ge-ruchssinn und die wichtigsten piechstoffe (Zeitschr. f. angew. Chem. 1900, 103) and J. van der Hoeven Leonhard, Piechscharfen und Farbsinnabweichungen (Die Umschau 12 (1908), 367; Report of Schimmel & Co. October 1908, 160).
It is necessary, or at least greatly to be desired, to have for comparison a sample of a genuine, faultlessly distilled oil. A few drops each, of the genuine oil and of the oil to be tested, are put on a strip of filter paper, and compared by smelling alternately of both. This odor test is repeated after the larger part of the oil has volatilized, and in this manner readily volatile as well as more difficultly volatile foreign substances may be recognized.
It is possible, however, to give only a very imperfect expression in language of odor and taste perceptions; moreover, odor and taste are purely subjective and are also quite differently developed faculties in each individual. The perceptions made with the sense of odor and of taste do not admit of expression and comparison by means of figures like other observations. An adulteration may, therefore, be subjectively recognized, but cannot be objectively proven. A good sense of smell is in spite of this limitation of great value in the examination, as it often directs the investigation into the proper channel in the shortest time.
Poorly distilled oils, possessing an empyreumatic odor or "Blasengeruch", or oils carelessly kept but otherwise unadulterated are almost altogether recognized by the sense of smell, rarely by any other means of investigation.