Filter and thoroughly wash the precipitate with ether. Save the filtrate for the determination of the iodin number of the liquid fatty acids by the Muter method. The soaps on the filter are washed back into the flask by means of a stream of hot water acidified with hydrochloric acid. Add an excess of dilute hydrochloric acid, partially fill the flask with hot water, and heat until fatty acids form a clear, oily layer. Fill the flask with hot water, allow the fatty acids to harden and separate from the precipitated lead chlorid; wash, drain, repeat washing with hot water, and dissolve the fatty acids in 100 c.c. of boiling 90 percent (by volume) alcohol. Cool down to 15° C., shaking thoroughly to aid crystallization. From 5 to 10 percent of peanut oil can be detected by this method, as it effects a complete separation of the soluble acid from the insoluble, which interferes with the crystallization of the arachidic acid. Filter, wash the precipitate twice with 10 c.c. of 90 percent (by volume) alcohol, and then with alcohol of 70 percent (by volume). Dissolve off the filter with boiling absolute alcohol, evaporate to dryness in a weighed dish, dry and weigh. Add to this weight 0.0025 gram for each 10 c.c. of 90 percent alcohol used in the crystallization and washing if done at 15° C.; if done at 20°, 0.0045 gram for each 10 c.c. The melting point of arachidic acid obtained in this way is between 71° and 72° C. Twenty times the weight of arachidic acid will give the approximate amount of peanut oil present. No examination for adulterants in olive oil is complete without making the test for peanut oil.
The above process to be of any particular value can only be carried out by an experienced chemist, but the presence of peanut oil may be readily determined by any one who is experienced by following out the above process.
Where only small quantities of peanut oil are concerned, namely, not to exceed five percent, even in the hands of an experienced chemist, the above process may not lead to certain results.
Peanut oil is obtained from the peanut by the ordinary method of hydraulic pressure. The first cold pressing furnishes the oil of finest character for edible purposes. Subsequent pressures or pressure with heat furnish an additional supply or a great quantity of oil but not of the same palatability. Peanut oil is highly prized as a salad oil either alone or mixed with other oil, notably olive oil and sesamé. The oil is purified by a large settle and by filtration and by the processes usually practiced with other oils of vegetable origin. The oil is easily and completely digested and furnishes an abundant source of heat and energy to the system. The number of calories produced by the combustion of one gram of oil, either by ordinary burning or by oxidation in the body is about 9,300.
The cake which is left after the pressing out of the oil is very highly nutritious, containing still considerable quantities of oil, the whole of the protein matter, and other digestible solids of the nut.
As before stated, it is extensively used as cattle food and as fertilizer. It may also be ground to a meal and used as human food, but furnishes an unbalanced ration in which the protein is far in excess.
Rape Oil (Colza Oil) (Brassica campestris L.).
—There are different kinds of oil which belong to the general class which is known as rape oil or rapeseed oil. The different kinds are derived from different varieties of Brassica campestris. The English names of the three most important varieties are—(1) colza oil, derived from the seeds of Brassica campestris; (2) rape oil, derived from the seeds of Brassica napus L.; (3) rübsen oil, derived from the seeds of Brassica rapa L. The character of the oil also varies according to the manner of its extraction. The first pressings from the cold powdered seeds is of a finer quality for salad purposes than the heavier later pressings from the hot seeds. The oil is also sometimes chilled and the crystallized stearin separated in order to keep it in a liquid state during the winter time, so that the winter and summer varieties are sometimes recognized in trade. There is, however, no difference in the other characteristics of the oil.
The specific gravity of rape oil at 15.5 degrees C., compared with water at the same temperature, is about .916. The variations from this mean number are not very great. Rapeseed oil absorbs almost its exact weight of iodin,—the average iodin number being not far from 99.