Extraction.—In the extraction of the oil, there are two distinct processes—(1) crushing, and (2) pressing.
In the first process, the fruit is by some completely crushed ; by others, the pericarp only is first crushed, and when the oil from that part of the fruit has been separately expressed, the more complete crushing is applied for obtaining the remainder of the oil. This difference of system arises from the fact that opinions differ as to the quality of the oil from the several parts of the fruit. There is no doubt that much of the delicacy of flavour which characterises the oils of highest repute is due to the pressing and storing, rather than to the crushing ; while it is also influenced to no slight extent by the variety of the olive, and the degree of maturity and the condition of the fruit when crushed.
The time for gathering the fruit is the eve of maturity. It is overripe for the finer quality of oil, if allowed to fall. This condition being complied with, much still depends upon the length of time allowed to elapse between the gathering and crushing, and the treatment to which the fruit is subjected in the interval. There is no doubt that fermentation in the fruit should be carefully watched, as anything like excess impairs the quality of the oil produced. On the other hand, no amount of fermentation affects the quantity of oil ; and where this is the main object of the maker, the olives are often allowed to ferment in heaps for months, till it is convenient to crush them, when they have to be dug out of the bins to put through the mill. But a slight degree of fermentation, if unaccompanied by any material heating, does not appear to affect injuriously the quality of the oil, while it facilitates the separation of the oil from the mucilage. The extent, however, to which fermentation is allowed to proceed should be jealously regulated, as there is no doubt that, beyond a certain point, the oil suffers in quality, and becomes unfit for the more delicate uses of food and cookery. The safest plan is to gather the olives at the right time, and crush them as soon as there are enough together. Meanwhile, they should he stored in moderate layers on shelves, the most complete arrangement being one which will admit of free currents of air above and below the layers.
The fruit is first reduced to a pulp, either with or without crushing the stones, according to the views of the miller as to the effect which this has upon the quality of the first droppings from the press, which are always regarded as the best. The crushing process should be conducted by a slow and regular movement, without jerking, in order that all the oil-cells shall be broken, and the press not he called upon to do any of the work which is supposed to have been previously done by the mill. The pulp or paste is then shovelled into bags, which are placed one on the other to a convenient depth in the press. In this process, as in that of the crushing, the power should he
applied steadily, slowly, and regularly, to afford time for the oil, as it exudes, to escape from the press through the proper channels. The pressing should be conducted in a warm temperature, and with as little exposure to the air as possible.
" Virgin" oil is that obtained by the first pressing, before the application of water or heat to the pulp. This is run into water, where it is allowed time to deposit its mucilage, and, after being skimmed off, is kept separate. In the district of Montpellier, however, the term is applied to the oil which spontaneously separates from the paste of crushed olives. This oil is not met with in commerce, the quantity being obviously too small ; it appears to be used by watchmakers, and for other purposes requiring extreme purity. So soon as all exudation of oil from the first pressing ceases, the screw is reversed, and the bags are removed and emptied. The pressed pulp being put carefully aside, and the bags refilled, pressure is again applied, and the process is repeated until the whole crushing has gone through the mill.
The mare, which has thus been once pressed, is then thoroughly separated, and stirred up with boiliug water, and the process of pressing is renewed, this time the pressure being increased, though still gradual and steady. This second oil is nearly as good as the first, but apt to become rancid in time. The bulk of the oil, after this second process, is skimmed off the water in the receivers; hut entire separation takes a long time, and, when it is complete, the process is reversed by the water being drawn off from below. Once more is the mare subjected to treatment with boiling water ; and it is at this stage that, when the stones were not crushed in the first milling, that process is now gone through, and the last of the oil is obtained. This pressing is, however, regarded as of inferior quality, and is kept carefully separate from the results of numbers one and two, being commonly termed " pyrene-oil." The water which has been used in the several processes, and which still contains an admixture of oil, is conducted into large reservoirs, generally constructed underground. Here it is left for a considerable period, during which, the mucilage, water, and oil thoroughly separate—the first falling to the bottom, while the last rises to the top, whence it is ultimately skimmed off, and applied to local uses of an inferior character, such as burning in lamps. This oil, taking its name from the French designation of the reservoirs used in its extraction, is termed " oil of the infernal regions.' The process of the second extraction by the aid of heat is in large mills sometimes effected by an arrangement for the thorough separation of the pulp, and freeing of tho oil, as illustrated in Fig. 1020.