Field Distillation of Plant Material

(f) Field Distillation of Plant Material.

In primitive countries, where aromatic plants grow wild, or are cultivated by natives as patch crops, essential oils are obtained by a form of distillation which may most appropriately be termed field distillation. Lack of roads prevents transport of the plant material to centrally located larger distilleries, and the distillation equipment has to follow the plant material into the interior of the growing region. Small portable or movable stills must be used ; but they serve only for a certain time of the year, and remain unused for the remainder of the time. They must, therefore, be low priced, sturdy, simple, easy to transport and to install in the fields, and simple to operate. In many cases this type of distillation is old; it has developed along purely empirical lines as an "art" inherited through generations. One should not summarily condemn this industry as antiquated and too primitive, however, because, in many instances and in view of the circumstances, a change to more modern and more expensive equipment is difficult, if not impossible. Indeed, such a change in some cases, might be for the worse, so far as prices of the oils, particularly, are concerned. On the other hand, this method of operation is frequently faulty, although it could be improved readily by only a few slight modifications.

Distillation may be carried out either by heating the still with direct fire or by steam generated in a separate small steam boiler. The former is simply an example of water distillation, or a water and steam distillation. Direct steam distillation, in this case, represents a stage in the transition to larger distilleries, because steam distillation is economical only if the steam generator is connected with several stills.

Despite the often primitive apparatus, the quality of oil resulting from water distillation in some instances has been good. However, the yield of oil in field distillation is often far below that obtained by water distillation on a large scale in more modern factories. The principal reason is probably that the small distillers do not always observe the fundamental rules of efficient water distillation, i.e., a small plant charge and a quick distillation. Most small operators are inclined to charge their retorts as high as possible in order to utilize them fully; furthermore, the speed of distillation is usually limited by too small a condenser. Also, in primitive operation the plant material is seldom comminuted, although a thorough comminution in the case of water distillation is often of prime importance for a normal yield of oil.

The following cases of actual distillation in the field will prove to what degree the yield, as well as the quality, of an essential oil depends upon the method of distillation. They also show that in many countries production of essential oils remains utterly primitive, and that the introduction of better methods would result in a considerable improvement in the yield and quality of the oils. The data are cited partly from von Rechenberg's "Theorie der Gewinnung und Trennung der atherischen Ole," Leipzig, 1910, but have been confirmed by the author during his own investigations in the interior of China, Mexico, France, Spain and many other countries.

Distillation of Lavender in France.

Years ago lavender oil used to be produced in Southern France in numerous small distillation posts, distributed throughout the growing regions of the De'partements Basses-Alpes, Drome, Vaucluse, Alpes-Maritimes and Var. These posts consisted of old-fashioned direct fire stills, holding about 60 kg. of plants and 60 liters of water. An operation was completed after about 15 liters of distillation water had been collected. The action of the boiling water upon linalyl acetate, the main constituent of lavender oil, resulted in considerable hydrolysis of this ester, and the lavender oils obtained by this method were relatively low in esters. The introduction of water and steam distillation, in which the plant material is packed on a perforated grid above the boiling water, resulted in a marked increase of the ester content. This effect was even more pronounced when Schimmel & Company showed by systematic experiments in their modern distillery in BarrSnie (B. A.) that oils containing 50 per cent and more of esters could be obtained by rapid distillation with direct steam. 

 PLATE 5. An old-fashioned direct fire still as used years ago for the distillation of lavender

in Southern France.

Distillation of Petitgrain Oil in Paraguay.

Similar conditions prevail in regard to the distillation of petitgrain oil in Paraguay. There, too, the leaf material is charged into primitive field stills and, during distillation, is partly submerged in boiling water. As a result, linalyl acetate, the main constituent of petitgrain oil, is partly hydrolyzed. For this reason, principally, the bulk of Paraguay petitgrain oil has an ester content averaging from 43 to 54 per cent only, whereas experiments with direct steam distillation in modern stills have proved that oils containing up to 80 per cent ester can be produced without too much difficulty.

Distillation of Linaloe Wood in Mexico.

The distillation of linaloe wood in Mexico furnishes proof that the yield and quality (physicochemical properties and chemical composition) of an essential oil depend a great deal upon the method of distillation.

The trunks and branches of the felled trees are reduced to chips with axes and machetes. The chips are then charged into galvanized iron retorts 1.10 m. wide and 2 m. high. In past years water was added to the chips and distillation of each batch carried out for about 18 to 20 hr., the heat being supplied by an open fire beneath the still. Distillation of linaloe wood in past years was thus a typical case of water distillation. The yield of oil then varied from only 0.6 to 1.0 per cent, seldom exceeding 2 per cent. This low yield was undoubtedly the result of insufficient reduction of the wood material, and in general of water distillation which in this case should not be applied.

In order to prove this contention, Schimmel & Company²⁵ imported Mexican linaloe logs to Europe and submitted the mechanically and properly comminuted material to direct steam distillation by modern methods. Yields ranging from 6.0 to 11.0 per cent of oil were obtained. The resulting oils differed considerably from the Mexican distilled oil. The latter contained more linalool, the Schimmel oil more of the high boiling constituents. Evidently in Mexico the wood svas not sufficiently comminuted, with the result that little oil was liberated from the oil glands. The old Mexican method of distillation seemed to depend primarily upon the forces of hydrodiffusion, which means that the more water-soluble oil constituents such as linalool were freed from the wood, while the water-insoluble compounds remained, and partly resinified during the long hours of distillation.

During the years of World War II, the author visited the linaioe oil producing regions in Mexico and observed that the method of distillation has been improved considerably. Today the stills are equipped with a perforated tin plate, 60 cm. from the bottom of the still, the perforated plate supporting the chipped wood material. The section below the plate contains water which does not come in contact with the charge. Thus we have here a typical case of water and steam distillation, the water being heated with an open fire beneath the still. As a result of this method of distillation, the yield of oil today ranges from about 2.2 to 2.6 per cent for chips, and from about 3.5 to 4.4 per cent of oil from sawdust.

Each operation now requires 8 to 9 hr. of distillation. Each charge consists of 230 kg. of wood material. In the states of Puebla and Guerrero, it is customary to reduce the linaloe wood into chips, while the producers in the state of Colima reduce the wood into coarse saw dust and thereby obtain a considerably higher yield.

Distillation of Cassia Leaves and Twigs in China.

Large quantities of cassia oil are produced yearly in the south Chinese provinces of Kwangsi and Kwangtung. The stills used by the natives are of antiquated Chinese design. Their principal fault lies in the loose connection of the joining parts and in the insufficient condensers. A charge consists of about 60 kg. of cassia leaves and twigs, and approximately 180 liters of water, which is brought to a boil by an open fire beneath the retort. Distillation of one batch lasts about 2 1/2 hr.

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25 Ber. Schimmel & Co., October (1907), 55.

The conclensate is collected in a series of pots, arranged in the form of cascades. Cassia oil is heavier than water.

The yield of oil from leaves alone averages 0.10 to 0.13 per cent, and that from a mixture of 70 per cent leaves and 30 per cent twigs 0.15 to 0.17 per cent.

Because of insufficient cooling in the condenser, the distillate usually runs quite warm, if not hot, and therefore a part of the oil remains emulsified or suspended in the water. The milky distillation water is added to the next batch of plant material, a procedure which entails a certain loss of oil by evaporation and particularly by resinifi cation. The principal cause of the subnormal yield of oil lies in the use of water distillation in the case of cassia leaves or, more exactly, in the faulty method of carrying it out. Cassia leaves possess a leathery consistency, remaining tough even in boiling water, and, therefore, if not sufficiently comminuted, cannot be completely exhausted by mere water distillation.

In order to study the problem by practical experiments, Schimmel & Company²⁶ imported dried cassia leaves and twigs from China, and submitted them to distillation tests in modern direct steam stills. The leaves yielded 0.7 to 0.8 per cent of oil, the twigs 0.2 per cent. These percentages are much higher than those obtained by the native Chinese distillers. True, the plant material arriving in Europe had lost considerable weight from drying; the Chinese producers use fresher leaves and twigs. Assuming the loss of moisture through drying to be about 50 per cent of the original plant weight, the yields of oil, as calculated upon the fresh plant material would, therefore, be as follows:

Fresh leaves, distilled in Europe                                        0.35 to 0.40%

70% fresh leaves plus 30%, fresh twigs, distilled in Europe  0.31 to 0.34%

Fresh leaves, distilled in China                                          0.10 to 0.13%

70% fresh leaves plus 30% fresh twigs, distilled in China     0.15 to 0.17%

This differential in yield is actually even greater, because as a result of the long transport and desiccation, a part of the cinnamic aldehyde, the main constituent of cassia oil, had been oxidized.

These experiments prove that the native distillation of cassia oil is carried out in such a primitive and faulty way that quantities of oil amounting to about twice the actual production per year are lost in the residual plant material. The use of water distillation is not the only cause of this waste. Another reason for the subnormal oil yield obtained by the Chinese distillers, appears to be this:

Because of insufficient condensation of the steam/oil vapors, distillation must be carried out very slowly. The motion of the plant charge in the boiling water is, therefore, correspondingly slow, and the water between the agglutinating leaves cannot circulate sufficiently. The volatile oil, which diffuses from the leaves into the boiling water partly dissolves therein, and remains, in part, suspended between the agglutinated leaves, without being vaporized by contacting steam bubbles. In other words, presence of the liberated, but not vaporized, oil inhibits further diffusion of oil from the leaves. Evidently, the forces of diffusion can come into play only where there exists a differential in concentration. In other words, the quicker the oil solution is removed from the surface of the leaves, and the quicker the oil is vaporized, the more forcibly diffusion acts. Otherwise an equilibrium in the charge will result, and the distillate will contain very little oil, in spite of the fact that considerable quantities of oil are still retained in the leaves. This also explains the relatively short length of distillation in the native cassia stills; the distillers simply stop the operation when they no longer see oil distilling over.

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26 Ber. Schimmel & Co., October (1896), 11.

It should not be surprising at all that the admixture of twigs to the leaves increases the oil yield, although the actual oil content of twigs amounts to only one-quarter that of the leaves. By the addition of twigs the charge simply becomes looser and the interspaces between the leaves larger. The boiling water, even though moving slowly, can then penetrate the interspaces much better and carry away the oil as it diffuses from the leaves; the oil is thus conducted toward the surface and vaporized.