Essential Oils-Treatment of the Plant Material

(a) Treatment of the Plant Material.


Comminution of the Plant Material. 

The chief application of distillation is in the initial isolation of essential oils from the aromatic plant material. This process involves the handling of predominantly solid products, and the preparation of the material must, therefore, be carried through carefully if the most efficient and complete recovery of the valuable essential oils is to be assured. The essential oils are enclosed in "oil glands," "veins,” "oil sacks," or "glandular hairs" of the aromatic plants. If the plant material were left intact, the oils could be removed (vaporized) by the steam only after they had passed through the plant tissues to an exposed surface. This can be accomplished only by hydrodiffusion, a mechanism which will later be shown to play a very important part in plant distillation. Diffusion is always a slow process, and if the plants or parts of plants were left intact, the rate of recovery of oil would be determined entirely by the rate of diffusion. Consequently, before distillation, the plant material must be disintegrated to some extent. This disintegration process, commonly termed comminution, results in exposing directly as many oil glands as is practically possible. It always reduces the thickness of material through which diffusion must occur, greatly increasing the rate or speed of vaporizatio and distillation of the essential oils. Even in comminuted plant material only a portion of the oil is freed, the balance remaining enclosed or being tightly covered by comminuted plant particles. All actually exposed volatile oil will soon be entrained by passing steam and carried away from the plants. 
Pair high roller mill
 FIG. 3.3. 3-Pair high roller mill.
The extent of comminution required varies with the nature of the plant material. Flowers, leaves and other thin and nonfibrous parts of the plant can be distilled without comminution. The cell walls in these parts are in most cases sufficiently thin and permeable to permit rapid removal of the oil. Seeds (fruits), on the other hand, must be thoroughly crushed in order to rupture as many of the cell walls as possible, to render the oil easily accessible to the passing steam. Roots, stalks and all woody material should be cut into short lengths in order to expose a great number of oil glands.
Seeds can best be crushed by passing them through smooth rolls. These rolls should be arranged so that the distance between them can be varied. The width of this space will determine the extent of crushing obtained. A similar effect can be achieved by regulating the flow of the material upon the distributor above the rolls. If the rolls operate at different speeds, the crushing action is supplemented by a usually advantageous shearing action. Each roll should also be equipped with a scraping device, called a "doctor blade," which serves to keep it free of adhering crushed material. A typical piece of equipment for handling seeds and fruit is shown in Fig. 3.3.
Roots and stalks can best be handled in a hay or ensilage cutter, or similar device. This action simply reduces the long natural parts of the plant to short lengths which are more readily handled in the distillation proper and, above all, assures a more uniform and compact charge in the still. Otherwise the live steam would find ready passages through the wide interspaces of uncut roots or stalk material and escape without coming in close contact with all plant particles. The result, especially in the case of steam distillation, would be a very inferior yield of oil. Woody parts may be sawed into small pieces or chipped mechanically. Typical machines for handling these raw materials are shown in Fig. 3.4.
The principal purpose of comminution being to render the essential oils more readily removable by the passing steam, it is evident that once the plant material has been crushed or reduced in size it must be distilled immediately. Otherwise, the essential oils, being somewhat volatile, will partly evaporate, with two adverse effects: first, the total yield of oil will be reduced by an amount equal to the extent to which evaporation has occurred ; second, the composition of the oil will change, thereby affecting its odor. This second effect results from the fact that the essential oils are mixtures of several, often numerous, compounds, the more volatile components evaporating to a greater extent than the higher boiling and less volatile ones. In the case of crushed caraway seed, for example, the evaporation loss consists mainly of limonene, which is lower boiling than carvone; the oil distilled from crushed seed which has been left in contact with open air for some time will, therefore, possess also a somewhat higher specific gravity. The extent of these oil losses by evaporation can be demonstrated easily by crushing a small quantity of caraway seed, weighing it on an analytical balance, airing it for a few minutes and checking the weight. Von Rechenberg6 reported a loss of 0.5 per cent which he attributed entirely to evaporation of oil, not of moisture, because air-dried seed was used in the experiment. It is, therefore, imperative that comminution be carried out immediately before the product is charged into the still if highest yields and best quality oils are to be obtained.
After the plant materials have been properly prepared for distillation, they are packed into the still and distillation can be started. Methods of charging and the construction of the still itself will be discussed after the general distillation methods have been presented.
Stainless, non-corrosive comminuting machine

FiQ. 3.4. Stainless, non-corrosive comminuting machine.
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6 "Theorie der Gewinnung und Trennung der atherischen Ole," Leipzig (1910), 391.

Storage of the Plant Material. 

The storage of plant material before comminution also offers some hazard in the way of ultimate loss of volatile oil. The situation here is not quite so serious as in the case of comminuted material and, therefore, if a delay in the distilling of the plant material cannot be avoided, it should be stored in its natural condition. Gradual evaporation results in some loss under these circumstances, the major sources of loss being represented by oxidation and resinification of the essential oils. If the plant material must be stored before processing, it should be kept in a dry atmosphere at a low temperature, and in a room free from air circulation if possible in an air-conditioned storehouse. All such losses are obviously avoided if the plants are processed immediately.

Loss of Essential Oil in the Plant Material Prior to Distillation. 

The volatile oil enclosed in the plant tissue is usually in one way or another affected by the drying of the plant material after the harvest. This effect has been studied and described by von Rechenberg7 whose findings are so interesting that the author feels justified in quoting a few passages of that work almost verbatim.
Some fresh plants, or parts, with a high water content (e.g., roses, tansy, calamus root) lose much of their essential oil by air drying; others very little. This loss is caused by evaporation, oxidation, resinification and other chemical actions. Contrary to expectation, evaporation here seems to play a subordinate role to oxidation and resinification. Indeed, actual evaporation of the volatile oil through the walls of the plant tissue cannot take place readily because the oil must first be brought to the surface through hydrodiffusion, with water or plant moisture acting as a carrying medium. Thin-walled flowers and leaves present no obstacle to the forces of diffusion, and in most cases evaporation will affect the more water-soluble constituents of a volatile oil rather than the low boiling terpenes. Arriilaga, Colon, Rivera and Jones8 showed that by field drying and stacking of citronclla grass or lemongrass prior to distillation the total acetylizable constituents of the oil decreased considerably with time after cutting. Since losses of acetylizable constituents were sufficient to account for most of the decrease in yield of oil, these authors concluded that the major factor leading to a loss of oil was oxidation. Evaporation of whole oil accounted for the additional loss. With both grasses it can be concluded that, for the best results, field drying, with or without subsequent stacking, should not be practiced.
According to von Rechenberg, distillation experiments seldom give reliable data on the loss of volatile oil by evaporation during plant drying.
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7 "Theorie der Gewinnung und Trennung der atherischen Ole," Leipzig (1910), 279.
8 Am. Perfumer 46 (May 1944), 49.

The reason is simply that distillation of plant material with a high water content usually leaves doubt as to its completeness. Peppermint offers a classical example in this respect. Formerly it was assumed that its oil content increases during the drying of the cut herb, but systematic distillation experiments proved the fallacy of this assumption. Fresh peppermint herb, like most plants or plant parts with a high moisture content, simply cannot be exhausted completely by distillation, or only with great difficulty, and after long hours of distillation. By distilling one portion of peppeimint herb in the fresh state right after the harvest, and the other portion in wilted, almost dry ("clover dry") condition, and by calculating the yields upon 100 kg. of fresh herb, it has been shown that the fresh herb contains a little more, possibly much more, oil than the dried herb, but it is very difficult to exhaust the fresh herb completely by distillation.
The loss of oil during the period of wilting and drying of the plant material is much greater than the loss occurring during storage of the plant material after it has been dried. This may be explained by the fact that, during the first stages of wilting and drying, the plant retains a large amount of moisture in the cells, which by diffusion carries the oil to the surface, and aids in its vaporization. Once the moisture has disappeared, and the plant has become air dried, hydrodiffusion can no longer take place. Any loss of oil during storage of the air dried plant material depends upon several factors condition of the material, method and length of storing, and the chemical composition of the oil. As a rule, but with many exceptions, flowers, leaves and herbs do not endure prolonged storing, whereas seeds, bark, roots and wood, by their very nature, retain their volatile oils much longer. Method of storing (packing tightly in sacks or bales, or spreading on the floor and heaping loosely) plays an important role in this respect. Air currents and extreme variations in moisture content of the atmosphere favor oil evaporation, resinification and, particularly, oxidation. It is possible to keep many types of plant materials for a long period, provided they are stored at sufficiently low temperature and in an air-conditioned room. Under such conditions, caraway seed docs not lose volatile oil even over a period of six months. In isolated cases, plant materials guaiac wood and sandalwood, for example retain their essential oil for many years, even though exposed to considerable variations of weather.
Von Rechenberg claims that the greatest loss of volatile oil by evaporation and oxidation occurs in comminuting the plant material prior to distillation, especially if this is done in rapidly rotating grinders and mills. The extent of loss depends upon the speed of air circulation in the system, the degree of heat development in the material, and the composition of the volatile oil (its boiling range and resistance to oxidation).

Change in the Physicochemical Properties of Essential Oils During Plant Drying. 

Essential oils distilled either from fresh or from dried plant parts show wide variations in physicochemical properties and chemical composition. With many oils it seems advisable, therefore, to state whether they were distilled from fresh, wilted, or air-dried plant material. This is especially true of flowers, leaves, herbs and roots, which in the fresh state contain much moisture.
Peppermint oil, for example, displays marked variations in its properties. Oil from fresh herb, according to von Rechenberg,9 had a specific gravity of 0.908, that from clover dried herb a gravity of 0.912. (The term "clover dried" means that the stalks are still flexible but the leaves dry.)
A series of interesting distillation experiments were carried out by Schimmel & Company :16
Angelica Root Oil
From fresh angelica roots: d1515 0.857 to 0.860
From dried angelica roots: d1515 0.870 to 0.902
The specific gravity of angelica root oil increases in proportion to the length of time the roots have been stored.
Lovage Root Oil
From fresh lovage roots: d1515 1.002 to 1.035
From dried lovage roots: d1515 1.039 to 1.040
Fresh and dried lovage roots exhibit a difference in behavior during distillation. During the distillation of dried lovage root, a yellow, gluey, resinous mass appears together with the oil, especially toward the end of distillation. This mass is largely dissolved in the oil ; part of it separates in the condenser pipes, and in the Florentine flask. Fresh lovage roots do not yield this resin, and wilted roots in only a small amount. Oil of lovage from fresh roots, when rectified, is entirely volatile ; the oil from dried roots upon rectification leaves in the still considerable quantities of a high boiling residue, which cannot be redistilled with water or steam.
Calamus Root Oil
From fresh calamus roots: d1515 0.962 to 0.968; αD +20o to +31o
From dried calamus roots: d1515 0.903 to 0.978; αD +15o to +20o
The oil from the fresh roots is more soluble in 70 per cent alcohol than is the oil from the dried roots. The solubility of the oil decreases with aging (storing) of the root.
9 "Theorie dcr Gewimumg und Trennung dcr atherischen Ole," Leipzig (1910), 270.
10 Ber. Schimmel & Co., April (1895), 9.

Estragon Oil
From fresh estragon herb: d1515 0.918 to 0.934; αD +2o to +4o
From dried estragon herb: d1515 0.890 to 0.923; αD +5o to +8o

Tschirch11 reported interesting observations on the resinifioation of volatile oils in spice plants. Whether the formation of these so-called resins is caused by the polymerization of homogeneous compounds or the addition reactions of heterogeneous compounds, by oxidation, or other forms cf conversion of volatile compounds, is not entirely clear.
Natural (not rectified) peppermint oil distilled from a fresh herb is more soluble in 70 per cent alcohol than is the oil distilled from dried herb, but the solubility decreases after a few months. If oil from fresh herb is rectified, it resinifies again, whereas oil from dried herb, when rectified, retains its original solubility. Certain constituents of peppermint oil, including possibly menthofurane, seem to resinify during the drying of the herb.
During wilting and drying, the cell membranes gradually break down, and the liquids are free to penetrate from cell to cell, giving rise to the formation of new volatile compounds e.g., by glycoside splitting. A typical example is found in bitter almond oil, which develops in the course of brief storing of crushed and moistened almond or apricot kernels. In the live fruit, the enzyme (emulsin) cannot contact the glucoside (amygdalin) in aqueous solution ; but it can readily do so in the crushed and wetted kernels. Analogous reactions and cleavages undoubtedly take place in many other cases. Fresh orris roots, for example, possess a rather disagreeable "green" and "herby" odor; whereas the dried roots, upon aging, develop a faint violet odor. Freshly harvested patchouli leaves are almost odorless; the well-known typical patchouli odor develops only on drying and curing. Vanilla beans constitute another example, the fresh pods resembling to some degree our common garden beans. The odor of grass is very different from that of hay, which develops its typical coumarin note only during the drying process. A phenomenon not yet explained is the disappearance of geraniol in dried roses, while the content of phenylethyl alcohol seems to increase.
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11 "Harze und Harzbehalter," 2nd Ed. (1906), Vols. I and II.

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