In this chapter the endeavour has been made to describe the general methods and apparatus employed in "winning " essential oils from their raw materials. The description of botanical and chemical characteristics is avoided, as these data are better obtained from books which treat this aspect specially.1

Where the method of treatment is unique or worthy of particular attention by reason of its wide use or technical interest, that method has been described in some detail; these considerations also apply to the apparatus employed in the industry, but, in general, engineering details have been avoided and merely the type of machine or apparatus indicated.

So far we have only dealt with the steam distillation of the essential oil which has already been freed from the non-volatile organic matter with which it occurs in the natural state. The presence of this nonvolatile matter introduces technical problems of some importance, and it is necessary so to prepare the raw material that the influence of the non-volatile matter is diminished as far as possible.

In any given distillation the rate will depend not only on the vapour pressure of the essential oil and the quantity of heat supplied, but also on the rate at which the volatile oil is liberated from the accompanying non-volatile matter. This last factor produces a lag which under unfavourable conditions may be of serious moment, and the object of this preliminary treatment of the raw material is to reduce this lag to a minimum.

It is possible roughly to estimate the value of the lag and thus to determine the best condition of the raw material for distilling. If the lag be nil, then the rate of distillation of the oil will be the same as that in the absence of the non-volatile matter, other conditions being equal; any slower rate will be a measure of the lag.

In order to reduce the lag to a minimum the raw material must be reduced to such a state that the steam, introduced for the purpose of volatilising the essential oil, can readily penetrate the mass and yet can come into contact with every particle of essential oil in as short a time as possible.

1 Cf. Parry, The Chemistry of Essential Oils, Scott Greenwood ; Schimmel's Die Aetherische Oele, Leipzig.

At first sight the obvious treatment would be to reduce the raw material to an exceedingly fine powder; this, however, is seldom feasible and introduces subsequent difficulties through lack of porosity.

Frequently the raw material is best distilled without any previous treatment, as for instance in the case of leaves, twigs, petals, flowers, buds, grasses, and similar material, but occasionally the problem is one of considerable difficulty and a compromise between the fineness of division and the porosity of the mass is the best that can be attained. If the material were of a coarse texture, such as blocks of wood, the time necessary in order to distil out the oil would be excessive and a heavy consumption of fuel would result; but if the other extreme had to be dealt with, such as a fine powder, the mass would become more or less impervious to the steam, which, in consequence, would find its way through the mass by means of fissures, leaving the greater part untouched. This latter difficulty can generally be overcome by diluting the charge with water in a quantity sufficient to render the resulting paste mobile.

Various methods for the treatment of large pieces of wood and branches present themselves. It is generally sufficient to chop branches up into short lengths, relying on expansion and capillary action to cause the oil to extrude during the distillation. The American practice in the case of the various pine oils is to chop the branches into pieces of one-half to one inch long and of less than half an inch in diameter; pieces of this size yield their oil readily and the lag is comparatively small. Wood in the form of large blocks is one of the most difficult to deal with, and a large amount of power may have to be utilised to reduce it to a state suitable for distillation. The procedure generally is to saw it into suitable lengths and then to rasp it to a coarse powder ; alternative methods are to split it or tear it apart along the grain, or to shave it into chips, or to splinter or crush it by means of heavy crushing jaws similar to those used for crushing rocks. Rosewood and sandalwood are generally reduced to small chips by means of a "raboteuse " chipping or planing machine.

Seeds and berries can frequently be distilled without preliminary treatment in spite of the enclosing epidermis, the heat of the steam and the increase of internal pressure due to the expansion of the contents and to osmosis being sufficient to rupture the epidermis and to set free the contents, but if the epidermis be too tough it is necessary to rupture it mechanically before distillation.

Schimmel found that uncrushed Ajowan seeds yielded 20 per cent less oil than crushed seeds, although the former were distilled twice as long as the latter. It is desirable that seeds similar to Ajowan, such as aniseed, caraway, and thyme, should be crushed.

Essential oils being volatile, care must be taken to ensure that the materials which bear them are not unduly exposed to conditions that would cause loss on this account.1 Schimmel found that caraway seeds which under ideal conditions yielded 6.7 per cent of volatile oil gave only

1 Cf; Perfumery and Essential Oil Record, 1921, p. 290.

5.45 per cent when they had been crushed and exposed to the air to dry, although the quality of the seeds was the same in both cases ; this difference represents a loss of 18.7 per cent of the oil. There is one compensating fact, namely, that the most volatile constituents of essential oils are usually the comparatively worthless terpenes, and the loss is, in consequence, one of quantity rather than of quality and is therefore not so serious as would appear at first sight. Exposure to air may, however, occasion loss other than by evaporation, since the conditions may be such that oxidation or resinification can ensue, and the loss may therefore not be confined to the terpene constituents of the oil but may extend to the constituents on account of which the oil is chiefly of value. The possibility of destructive bacterial fermentation must also not be overlooked, although the powerful antiseptic properties of most essential oils render loss on this account unlikely.

It is obvious therefore that during the reduction of the raw material to a state requisite for the subsequent distillation, care must be exercised in order to prevent undue exposure of the material to the air or to heat. This excludes the use of machines that rely on rubbing surfaces for their crushing action or on very swiftly revolving knives or centrifugal disintegrators, since the first generate heat and the latter pass large currents of air over the material. Centrifugal disintegrators can, however, be arranged so that the same air circulates again and again through the machine and the loss of volatile oil is thereby limited.

The best type of mil] for seeds of the caraway type is that employing enclosed crushing rolls which run slowly at the same or at very nearly the same speed.

In the case of peach and apricot kernels and bitter almonds these conditions do not apply, the essential oil not being present as such at the time of grinding, being subsequently developed by spontaneous fermentation; excessive heat should, however, be avoided, as this tends to kill the ferment.

Roots such as vertivert, ginger, and orris do not contain any serious quantity of very volatile substance and are best treated by drying and grinding in an ordinary drug mill. Schimmel found that whereas a certain batch of cut vertivert root yielded 1.09 per cent of essential oil on distillation, a similar batch of uncut root yielded only 0.3 per cent.

In the case of orris and patchouli it is necessary to dry the material in order to develop the odour.

It is the practice in some countries to dry the raw material before distilling, especially if it is necessary to delay the distillation, since if the material be kept too long in a moist condition bacterial action sometimes tends to take place with consequent deterioration. As has been shown above, drying results in a distinct loss of oil in the case of crushed caraway seeds, but it is stated that the loss occasioned by drying peppermint in the United States is negligible ; the drying in this case is necessary since violent fermentation may ensue. With lavender, however, the loss is quite serious, as the following results show :11 Messrs. Schimmel & Co.