The Evaluation of Natural Flower Oils and Resinoids

(c) The Evaluation of Natural Flower Oils and Resinoids.


The assay of distilled volatile oils. has made remarkable progress during the last fifty years, probably because such oils are employed in much larger quantities than natural flower oils obtained by extraction with volatile solvents or by enjleurage. Moreover, the pharmaceutical profession, which uses many volatile oils, has always endeavored to assay carefully any products employed as medicine. Yet, it seems strange that so little attention should have been paid to the assay of such highly priced products as extracted flower oils, especially since it is common knowledge that sophistication of concretes and absolutes has become quite frequent, causing considerable loss to the often too credulous buyers. The reason for this neglect may be sought hi the unfamiliarity of many users with these highly priced yet somewhat ambiguous products, the quality of which may depend upon many factors methods of manufacturing, solvents used, degree of concentration, care in purification, etc. No wonder then that definite norms of quality do not yet exist and that the manufacturer may offer various explanations for deviations in his products. Indeed, some manufacturers market their natural flower oils in several grades, according to different degrees of dilution, in order to suit the usage and the purse of the users. Too, they claim that such "standardization" will guarantee a uniformity every year which nature alone does not achieve. "Les arts perfectionnent la nature," to quote an inscription on an old fountain in Grasse.
Natural flower oils, therefore, have remained strictly articles of confidence, and the examination of them is usually carried out by simple olfactory tests. Even such tests, however, require an intimate knowledge of the subject, a well-trained sense of smell, and familiarity with manufacturing methods and possible variations in quality, which very few buyers or even perfumers possess. Furthermore, any olfactory test should be based upon the comparison of an offered sample with standard samples of unquestioned purity and, if possible, of the same age. (Such samples, unfortunately, are seldom on hand.) It is surprising how the odor character of a natural flower oil may change during the first six months or year after its manufacture. Some odors improve for a certain period, and then slowly deteriorate, assuming a somewhat sour or rancid note. Something of a parallel may be drawn with wines of young and older vintage. In view of these facts, it seems highly desirable and timely to establish definite and universal standard methods for the physicochemical assay of natural flower oils and resinoids from gums, balsams, and similar plant material.
The adaptation of routine methods as applied to distilled volatile oils cannot per se be extended to extracted flower oils or resinoids, as these products contain large proportions of natural substances which, although olfactorily inert, possess a variety of chemical functions which would make the interpretation of the analysis most difficult, if not outright impossible.
Logically, any physicochemical assay should, therefore, be applied mainly to the odoriferous portions of the extracted flower oils, which are usually identical with the volatile fraction. In other words, the extracted floral oil is steam distilled and the separated volatile portion examined by the usual tests for specific gravity, optical rotation, refractive index, acid number, ester number, ester number after acetylation, content of aldehydes, ketones, phenols, etc.
Separation of the two portions by dry distillation at reduced pressure is inadvisable, because of the tendency toward pronounced and often destructive pyrolysis of the higher boiling constituents. Consequently, the method of distillation with steam suggests itself for the separation of volatile and nonvolatile portions.
The first attempts toward establishing such a standard method were made by Walbaum and Rosenthal40 who described an apparatus for the determination of the content in products distillable with steam from concrete and absolute flower oils. However, the separation of the volatile constituents from the waxes in this apparatus remains incomplete, even after 5 hr. of distillation, and gives much trouble, such as frothing, etc.
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40 Ber. Schimmel & Co., Jubilaums Ausgabe (1929), 189.

Furthermore, live steam at atmospheric pressure causes hydrolysis and other chemical reactions. The method, therefore, can at best give only comparative results as far as the yield of volatile constituents is concerned, and only if carried out under absolutely unvarying and most carefully controlled conditions.
Several years later, Naves41 suggested a more reliable, accurate and practical method, using distillation with superheated steam under reduced pressure. When superheated, dry steam behaves like a gas, follows the gas laws, and in the condensate yields a higher ratio of volatile aromatic constituents to carrier steam. Acting solely by its volumetric effect, dry, superheated steam is chemically less active than wet steam. Thus, with superheated steam it becomes possible to distill delicate esters and other compounds which would undergo hydrolysis with wet steam at the temperature of boiling water. For details of the method and a description of a cleverly constructed apparatus, the reader is referred to an interesting paper by Naves, Sabetay and Palfray,42 who also examined a number of flower oils for their content of volatile constituents, and recorded the physicochemical properties of the distillable portions. The data given, however, are not yet complete enough to establish reliable standards universally adaptable by the trade. Much work along these lines must yet be done, and many more samples of unquestionable purity will still have to be examined before the essential oil industry can agree on definite norms.
Further progress in the perfection of the assay of natural flower oils was achieved by Sabetay,43 who suggested that concretes and absolutes should be examined for their content of volatile constituents by codistilling these floral products or resinoids, etc., with ethylene glycol in a partial vacuum. (The same author41 also suggested applying this method to the determination of volatile oils in drugs and spices.) Ethylene glycol is a more efficient carrier than steam, and the waxes or residues remaining in the distilling flask will be practically devoid of any odoriferous compounds. The application of a partial vacuum reduces the distillation temperature to a degree not harmful to the delicate constituents of the floral oils. Sabetay's method possesses the added advantage of simplicity:
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41 Documentation scientifique No. 50, December (1936), 303. Chem. Abstracts 31 (1937), 4772.
42 Perfumery Essential Oil Record 28 (1937), 331.
43 Ann. chim. anal. chim. appl. 21 (1939), 173. Chem. Abstracts 34 (1940), 3018.
44 Ibid. 22 (1940), 217. Chem. Abstracts 35 (1941), 4547.

If a concrete or absolute is mixed with glycol and distilled under 8-15 mm. pressure at a temperature of 90o-100o, all of the volatile oil contained in the concrete or absolute can be driven over, separated and measured. For instance, weigh 1-10 g. of the concrete or absolute, add 25 cc. of ethylene glycol and distill at 90o-100o, from a 50-100 cc. Claisen flask with Vigreux points, fitted with a thermometer, capillary tube and receiver, and with a metallic or oil bath as a source of heat. As a rule, the residue in the flask will have little odor, but, if necessary, 20 cc. more of ethylene glycol can be added, and the distillation repeated a second and, possibly, a third time, until the distillate no longer becomes turbid upon addition of water. The combined distillate is diluted with water (or brine), treated with sodium chloride (if brine is not used) and extracted with three 20 cc. portions of a mixture of equal parts of pentane and ether. Dry the combined ether-pentane extracts over anhydrous sodium sulfate, remove most of the solvent by distillation, rinse the residue with pentane into a small Claisen flask with Vigreux points fitted with a capillary tube, and heat gently under 50-100mm. pressure to constant weight. The volatile oil thus obtained may then be subjected to the usual physicochemical tests.
By comparing the figures (yield of volatile oil from the absolute or concrete or resinoid, specific gravity, optical rotation, refractive index, acid and saponification number of the volatile oil) thus obtained with those of absolutely genuine products, conclusions can be drawn as to the purity of the flower oil sample investigated.
The method of Sabetay may have to be modified in certain respects, and will have to be applied to numerous lots of unquestionably pure natural flower oils before definite standards can be agreed upon by the trade. Naves45 prefers the use of superheated steam for the isolation of the volatile constituents from natural flower oils rather than codistillation with glycol, as certain constituents are relatively soluble in glycol-water solution.
Before concluding this chapter it might be well to discuss briefly the interpretation of analytical results, as well as the deterioration and possible adulteration of natural flower oils.
If absolutely pure, and manufactured according to unvarying methods, and from flowers grown in the same geographical location, natural flower oils should be of similar character and show little variation, especially in regard to their content of volatile (distillable) portions, and to the physicochemical properties of the volatile constituents. This, however, is not always the case, particularly with enfleurage products. The care exercised in the manufacturing process, and especially in the final purification of the product, exerts considerable influence upon its quality. The latter depends primarily upon the ratio between the weight of flowers treated during the entire enfleurage season and the weight of fatty vehicle (corps) employed.
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Chim. Ada 27 (1944), 1103, 1108. Soap, Perfumery, Cosmetics 29, No. 1 (1946), 38.

Thus a jasmine pomade will contain more volatile constituents and possess a much stronger odor if 1 kg. of natural corps has been treated during the flowering season with 2.5 or 3.0 kg. of jasmine flowers rather than only 1.5 kg. of flowers. Concretes obtained by extracting the flowers three times with solvent, instead of only twice, will contain more waxes. An absolute obtained by extracting the concrete four or five times with alcohol, instead of only three times, will contain more alcohol soluble waxes and other inert material, and correspondingly less volatile, odoriferous material.
Concretes and absolutes usually acquire a reddish color upon aging. This color alteration, noticeable particularly in jasmine and orange flower extracts, may be attributed mainly to the presence of indole. The odor improves, usually, for a fewr months, and assumes a harmonious fullness and depth lacking in the freshly extracted product. After a year or two of stability, depending of course upon proper storage, the product deteriorates gradually, finally acquiring a somewhat acid, rancid note, which is caused by the formation of acetic acid and ethyl acetate. This holds true especially if the product originally contained a small percentage of ethyl alcohol which was not removed during the final concentration. Hence, it is advisable to examine the aqueous phase of the analytical distillate, after extraction with ether, for its acid and ester number.
Pomades and absolutes of enfleurage are particularly susceptible to rancidity and development of acidity. In fact, even the freshly prepared absolutes of enfleurage show a relatively high acid number (which should not exceed 80), but this is caused by the presence of alcohol soluble free fatty acids extracted from the fat corps.
Adulteration of natural flower oils can be carried out in different ways, viz., by substitution with natural flower oils from lower priced geographical sources, by addition of volatile oils, or fractions therefrom, aromatic isolates or synthetic aromatics, or by dilution with inert materials. Thus, an absolute of jasmine marketed under the label of the Grasse region may contain the Egyptian product, a misrepresentation which, at present, can be detected only by olfactory tests, as we do not yet possess sufficient analytical data to differentiate between the products from these two geographical sources. The addition, to concretes, of exhausted natural flower waxes, obtained as alcohol insoluble residues in the preparation of alcohol soluble absolutes, results in a correspondingly lowered content of distillable volatile portions of the concrete. This can be proved by the above described distillation tests. Determination of the congealing point of the concrete may also give valuable hints in this respect.
A dangerous form of adulteration consists in the addition, to concretes or absolutes, of both odorless matters such as exhausted waxes, fats or fatty oils, and volatile, odoriferous compounds which occur also in the genuine flower oil, but which can be obtained synthetically or by isolation from lower priced essential oils. Thus benzyl acetate, benzyl alcohol, indole, etc., may be added to jasmine absolute; phenylethyl alcohol, rhodinol, etc., to rose absolute; linalool, linalool acetate, methyl anthranilute, etc., to orange flower absolute. If cleverly carried out by properly balancing the ratio between odorless nondistillable and odoriferous distillable compounds, such sophistication may give considerable trouble to the analyst, who will have to depend upon olfactory tests and that, as pointed out, requires a highly trained and experienced sense of smell. Occasionally, natural flower oils are adulterated with odorless solvents such as diethyl phthalate, specific tests for which will be found in the chapter on "Examination and Analysis of Essential Oils, Synthetics, and Isolates."
When evaluating any natural flower oil, it is always advisable to test first for solvents and for alcohol. Traces of alcohol should not be objectionable as they are difficult to remove in the final purification during the manufacturing process without impairing the quality of the product.
However, a flower oil should never contain any solvents like petroleum ether, benzene, or, particularly, kerosene, because their presence indicates incomplete purification ; they impart to the product an off-note most detrimental to the delicate odor of natural flower oils. Special tests for alcohol and petroleum ether are described in Chapter 4 on "Examination and Analysis of Essential Oils, Synthetics, and Isolates."

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