Chapter VIII: , and the quantity of caustic soda calculated which will

neutralise the given bulk of oil to be treated. The following is a more detailed account of the process:—

The calculated quantity of alkali is dissolved in water, the solution diluted to 12 or 15° Tw. (8° or 10° B.), and one-third of it added, either in a fine stream, or through a sprinkler, to the oil contained in a steam-jacketed tank. The mixture is now heated first to 100° F., and then gradually to 120° F., the whole being well agitated mechanically, or by blowing a current of air through a pipe inserted to the bottom of the tank.

After about fifteen minutes the agitation is stopped, and the oil allowed to rest for some time, preferably overnight, to allow the soap and impurities to settle down to the bottom, whence they may be drawn off. This treatment is then repeated a second and third time, with the same quantity of caustic soda solution, but usually of weaker strength, and in exactly the same manner as just described, after which a clear, yellow oil should be obtained.

The agitation with air must not be unduly prolonged, as this tends to oxidise the oil, raising its specific gravity and refractive index, and also injuring its flavour.

Treatment with caustic soda solution is also frequently employed for refining other vegetable oils, notably cocoanut oil. In all cases the principle is the same, viz., combination of the alkali with the free fatty acids to form soap, which on settling carries down with it colouring matter and other impurities. Only a weak solution, of say 12° Tw. (8° B.), should be used, and the quantity added should not be more than sufficient to neutralise the free acid, since otherwise some of the neutral oil may be saponified.

_Sodium Carbonate._—This may be employed instead of caustic soda to neutralise the free fatty acids of an oil or fat. In practice, however, it is less frequently used by itself for refining purposes, though there is less risk of saponification of the neutral oil if a slight excess is added with this reagent than with caustic soda.

A process has been patented in France by G. Muller (Fr. Pat. 334,366, 1903) for the treatment of cacao butter with sodium bicarbonate. The fat is heated with sodium bicarbonate and water, then cooled with constant agitation until it congeals, allowed to stand for twenty-four hours, and finally subjected to a process of pressing and kneading. The fat thus treated is claimed to be softer and less brittle.

_Sodium Silicate._—This is an alkaline salt, and its action is very similar to that of sodium carbonate. Its use for bleaching oils and fats has been patented by Godard (Eng. Pat. 22,085, 1903), who mixes the oil with sodium silicate, separates the soap formed, and then deodorises the neutral oil by means of steam in a fine state of division.

_Alkaline Earths._—Lime and magnesia are sometimes used for removing the free fatty acids from oils and fats, insoluble calcium and magnesium soaps being formed. In Rocca’s patent, to which reference has already been made (vide _supra_), the oil is first neutralised with caustic soda or sodium carbonate, decanted from the resulting soap, a small quantity of strong acid added to decompose any soap remaining, and the oil finally neutralised with lime, magnesia, or baryta.

Fresenius (Eng. Pat. 19,171, 1902) neutralises the oil with caustic soda, lime, or magnesia, under a pressure of 2 or 3 atmospheres, either in the presence of carbon to prevent oxidation, or according to a later process in an atmosphere of an inert gas. The increased pressure is claimed to facilitate the separation of the soap emulsion.

=Bleaching of Oils.=—The colouring matter of crude oils consists of chlorophyll, which gives them a greenish tinge, or of substances frequently of a resinous nature, which impart a brown colour.

In the case of some oils, such as olive oil, the natural greenish tint is allowed to remain, but the dark colour of certain other crude oils has to be reduced before the product is saleable.

As was mentioned above, treatment with alkali removes from cotton-seed oil a large proportion of the dark colouring matter at the same time as the constituents that cause turbidity.

The methods in which fuller’s earth or milk is used to refine oils have also some effect in producing a filtrate of lighter colour, while a treatment with freshly prepared animal charcoal is effective as a decolorising process in some cases.

_Charcoal._—Bleaching with charcoal may be effected by mixing the oil with 1 to 5 per cent. of _animal_ charcoal, in a granular form, warming for a short time, and filtering through a filter press. The bleaching action of animal charcoal, attributed by Knecht to the presence of nitrogen compounds, is greater in the presence of acid.

Crude charcoal requires preliminary treatment before use for bleaching purposes. It should be well boiled, first with pure water, and then after the addition of sufficient sodium carbonate or hydroxide to render it alkaline. It is next washed free from alkali, and boiled for twelve hours with four times its weight of a mixture of equal parts of commercial hydrochloric acid and water, after which it is washed free from acid, dried, and burned in closed vessels. A good bleaching charcoal is thus obtained.

_Fuller’s Earth._—Fuller’s earth (aluminium magnesium hydrosilicate) should be dehydrated by roasting prior to use, in order to secure the best results. The quantity required varies from 2-5 per cent. for cocoanut, palm-kernel, and olive oils, to upwards of 10 per cent. for arachis and cotton-seed oils, and the oil should be thoroughly mixed with the reagent and maintained at a temperature of say 100° F. for about fifteen minutes, and then filtered through a filter press. The fuller’s earth retains about 80 per cent. of the oil, which may be extracted by means of a solvent, the latter distilled off, and the recovered oil treated with a fresh quantity of fuller’s earth. The spent fuller’s earth may be “regenerated” by heating it up to 400-500°. Fuller’s earth is frequently used in America for the filtration of “premier jus.”

According to Hirzel (_Chem. Rev. Fett- u. Harz-Ind._, 1904, 116-118; 145-146), the earthy flavour sometimes remaining after contact with fuller’s earth may be removed by washing with 10 per cent. of a 10 per cent. solution of brine, and by adding 1 to 1½ per cent. of powdered, dry sodium bicarbonate.

Godard (Eng. Pat. 22,086, 1903) carries out the agitation with reagents _in vacuo_ in order to prevent oxidation of the oil.

No very definite details as to the quantity of charcoal or fuller’s earth to be used, the temperature to which the oil should be raised in contact with them, and the time it should be maintained thereat, can be given, as these vary considerably for different oils, and can only be determined by actual experiments with the particular oil it is required to bleach.

The use of finely divided alumina, bauxite, or magnesite previously ignited at a low temperature, has also been patented for decolorising oils, and a process for the recovery of spent decolorising materials (which consists in mixing them with salt water, heating the mixture to about 85° C. with sulphuric acid, then treating it with sodium carbonate and mechanically agitating it for a short time) has been patented by the Soc. Anon. Huilerie et Savonnerie de Lurian (Fr. Pat. 499,915, 1909). The carbon dioxide liberated by this treatment carries the oil to the surface.

Methods in which chlorine or bleaching powder are employed are only applicable for bleaching oils intended for the making of soap or other technical purposes, owing to the reagent attacking the glycerides and imparting a flavour. For similar reasons Watts’ bichromate process of bleaching palm oil is also unsuitable for the treatment of fat intended for food.

Methods of oxidation, either by means of hot air passed through the heated oil or fat, or by means of ozone or ozonised air, under controlled conditions, are employed both to obtain paler oils and to remove substances of unpleasant odour.

The bleaching of beeswax by exposing it in thin strips to the action of air and sunlight has long been practised, and Japan wax is also rendered nearly white by similar means.

The use of artificial light in place of sunlight is claimed in several patents for bleaching oils, such as cotton-seed oil. The oil is made to pass across a transparent surface through which are transmitted the rays from a powerful arc light.

Ultra-violet rays, such as those emitted by the mercury vapour lamp, are employed as the bleaching agent in similar processes.

_Ozone._—The use of ozone as a bleaching agent has long been known, but its application to the treatment of oils and fats has hitherto met with little success. It has been the subject of many patents, among which may be mentioned that of Andreoli (Eng. Pat. 14,570, 1898), who makes use of the joint action of ozone and a hypochlorite solution, and that of J. Harris (Eng. Pat. 22,430, 1906). Harris first ozonises the oil at a temperature of 100° to 180° F. for fifteen to thirty minutes, until there is an appreciable rise—say 0·5 per cent.—in the free fatty acids. The treatment with ozone is then stopped, and the oil neutralised with alkali, preferably an aqueous solution of caustic soda at about 15° Tw. (10° B.). The impurities are next separated, and the oil dried by means of a current of air while warm, after which it may be still further refined by heating it with 5 per cent. of aluminous earth, followed by filtration.

This latter process seems to be rather a mistaken one, as from experiments made by one of the authors (S.), the best results are obtained when the ozonisation is only carried to such a point that there is no appreciable increase in the free acidity of the oil.

Recent experiments by him on the bleaching and deodorising action of pure ozone _free from nitrogenous compounds_, as obtained with the Ozonair apparatus, have given results which are most promising. Palm oil, even of the crudest description, has been most effectively and cheaply bleached by merely passing a strong current of ozonised air through it, and the colour of certain other vegetable oils has been distinctly improved by this treatment; while in the case of a very acid sample of cocoanut oil, the fat was not only made whiter in colour, but had also almost entirely lost its characteristic odour. Hence there seems to be a likelihood of the successful employment of the ozone process for this purpose in the future.

_Hydrosulphites_ or “hyposulphites.”—Sodium hydrosulphite, obtained by digesting a concentrated solution of sodium bisulphite with zinc dust or turnings, is a powerful reducing agent, and has been patented as a means of bleaching oils and fats by Metz and Clarkson (Eng. Pat. 11,983, 1906). It is particularly applicable to maize oil, which may be bleached by agitating 200 parts of the oil with 600 parts of water and 15 parts of sodium hydrosulphite for ten hours in a closed vessel, allowing the mixture to stand for thirty-two hours, and then separating the oily layer.

Sodium hydrosulphite formaldehyde, which was shown by Baumann, Thesmar, and Frossard in 1904 to be commercially a mixture in almost equal proportions of sodium sulphoxylate formaldehyde and sodium bisulphite formaldehyde, is also recommended for the purpose, the oil being heated to 70° C. with this reagent in a closed vessel, and then allowed to stand.

The formaldehyde-sulphoxylates are now sold under a variety of fancy names, the sodium compound being termed _Rongalite C._ and _Hydraldite C._, and the zinc salt _Decroline_.

_Sodium Bisulphite._—This salt, which possesses the property of combining with aldehydes, has been utilised in some cases for deodorising oils and fats, with good results.

_Organic Peroxides._—The Vereingte Chem. Werke have recently patented the use of various organic peroxides (such as those of benzoyl, acetyl, and acetone, together with the oxidation products obtained by the action of nitrogen tetroxide on organic compounds) for the bleaching of oils and fats. The oil is heated to 100° C. with about 0·2 per cent. of the peroxide, and allowed to stand in a warm place for a short time, until bleaching has taken place.

There is now a large number of per-salts available, _e.g._, persulphates, percarbonates, perborates, which are being utilised for various bleaching purposes, but so far they do not appear to have been employed to any appreciable extent in the treatment of oils and fats, though ammonium persulphate (palidol) is now being used for bleaching soap in the pan.

=Deodorisation of Fats.=—The odours of oils and fats are due to the presence of small quantities of volatile substances, either derived from the vegetable substance as in the case of maize and wheat oils, or formed by slight decomposition of the oil itself.

It is chiefly in connection with cocoanut oil, which forms the basis of so many vegetable lards and butters, that the problem of deodorisation of a fresh fat has to be faced.

According to the recent research of Haller and Lassieur (_Comptes Rend._, 1910, 150, 1013), the unpleasant odour of commercial cocoanut oil must be attributed, partly to a process of decomposition of the glycerides, with the liberation of fatty acids (caproic, caprylic, capric acids, etc.) with a pronounced odour; and partly to the presence of certain substances, which include methyl-heptyl ketone and methyl-nonyl ketone. Traces of these may also be detected in the refined fat, and account for its odour when heated. The substances to which the odour is due may be obtained in the form of an essence (échappés) by distilling the cocoanut oil in a current of superheated steam.

Speaking broadly, two methods only have so far been discovered, or, at any rate, published, for the deodorisation of cocoanut oil—(1) washing out of the odoriferous bodies with alcohol, and (2) their volatilisation by treatment with steam. Other unpublished processes are doubtless used by some manufacturers, but their secrecy is very jealously guarded.

The washing of cocoanut oil with alcohol of course removes free fatty acids, which are soluble therein. This method of treatment was first introduced by Chevreul, and has been also utilised by Schlinck, who employed a joint process of washing with alcohol and treatment with charcoal. A special apparatus for its application was patented by Urbain and Feige (Fr. Pat. 361,966, 1905), consisting of a series of vessels so arranged that the oil descends from vessel to vessel, and meets hot alcohol circulating in the opposite direction, so that the more impure oil comes in contact with the more impure alcohol. The temperature is maintained at 70° C. during the treatment, and means are provided for evaporating and condensing the alcohol after extraction is complete.

An improvement upon this patent has been since protected by the patentees, in which sufficient alkali is added to combine with the free fatty acids prior to the extraction with alcohol.

With so volatile a substance as alcohol, there is necessarily a considerable loss by evaporation during working, and the cost of alcohol in this country is too high for the process to be economical.

The treatment of cocoanut oil with steam in order to deodorise it dates back to 1882, when a process was patented in Germany by Jeserich and Meinert, in which the volatile fatty acids were first driven off by passing high pressure steam at 6 to 8 atmospheres into the fluid oil, with constant stirring for about two or three hours, after which steam was shut off, and the non-volatile fatty acids separated by addition of 0·25 per cent. of calcined magnesia, the magnesium soap formed rising to the surface, whence it could be skimmed off.

Several modifications of this process have since been patented. Klimont (Eng. Pat. 3164, 1902) neutralises the oil with alkali prior to treatment with superheated steam, and then removes non-volatile impurities by either raising the temperature of the oil to above 100° C., treating it with calcium or magnesium hydroxide, or extracting it with some solvent such as acetone, which dissolves the oil and precipitates the impurities.

The Fabrique de Prod. Chim. de Thann et de Mulhouse treat the oil with steam in the absence of air, either _in vacuo_, or in an atmosphere of an indifferent gas.

In another method of deodorisation cocoanut oil is freed from its more fluid constituents (as in the preparation of chocolate fats, _q.v._) and the fatty acids eliminated in the form of calcium salts by treatment with lime; sodium silicate is employed in another process as the means of removing the fatty acids. Yet however carefully the odoriferous substances may have been removed, the readiness with which cocoanut oil undergoes hydrolysis, and the nature of the fatty acids contained in its glycerides, renders this fat particularly liable to acquire an odour again, after having been exposed for a short time to the air.

=Treatment of Rancid Fats.=—Although the practice is to be condemned, it is not uncommon for fats that have become slightly rancid to be subjected to treatment to eliminate the compounds to which rancidity is due, and thus enable the fat to be blended with fresh fats.

In the development of the changes understood by the term “rancidity,” the glycerides undergo hydrolysis with the liberation of fatty acids, which are in turn decomposed or oxidised, with the formation of various compounds such as fatty anhydrides, aldehydes, and hydroxy acids.

These changes are brought about under the influence of light and atmospheric oxidation, and are probably promoted by enzymic action or by the influence of micro-organisms when albuminous substances have been left in the fat.

The removal of free fatty acids is frequently sufficient to make the oil appear fresh. This may be effected by mechanically agitating the fat with a suitable proportion of milk of lime or magnesia, which combine with the free fatty acids to form insoluble calcium or magnesium soap, which can readily be separated by filtration.

Neutralisation with caustic alkali (as in the refining of cotton-seed oil) or with a solution of sodium silicate, also effects the removal of free fatty acids, but the latter reagent is liable to produce an obstinate emulsion when employed on a large scale.

In another process the rancid oil is treated with a suitable proportion of precipitated chalk, and is subsequently filtered while hot through a layer of animal charcoal.

Reference has already been made to the method of dissolving out the free fatty acids by means of alcohol. Oils in which a process of hydrolysis has once started are liable, after removal of the free fatty acids, not to keep so well as freshly expressed products.

Rancidity may be present in an oil without the liberation of free fatty acids, though, as a rule, the acid value of a fat will afford some index of the degree of rancidity, since the development of acidity and of rancidity are often simultaneous.

For the removal of the aldehydic compounds formed in the changes that occur, a treatment with a strong solution of sodium bisulphite is used in a process described by Nagel, while volatile products are expelled by heating the oil in a current of steam at gradually increasing temperatures.

Other reagents used for sweetening rancid oils are a solution of ordinary salt, dilute (1 per cent.) sulphuric acid, a solution of myrrh in methyl alcohol, etc.

As a rule, the different processes are employed successively, and followed by filtration and drying of the oil.

The tendency of an oil to become rancid depends largely upon the proportion of volatile fatty acids and unsaturated fatty acids it contains. Thus butter and cocoanut oil readily turn rancid, whereas beef stearin and cacao butter will keep for a long time unaltered.

Any oil or fat is best protected from rancidity by keeping it in the dark in a vessel from which all air is excluded. Rancidity will take place in the dark, but much less rapidly than when the fat is exposed to light. This is due to the accelerating effect of light upon the action of oxygen. Heat has also a pronounced influence in promoting chemical changes in oils, and cold has a retarding influence, although it does not stop the oxidation.

The odour of rancid fats that do not (like cocoanut oil) contain volatile fatty acids is due to the formation of aldehydes or of esters, the latter being probably produced, in some cases at all events, by the action of certain micro-organisms upon the liberated glycerin.

Rancidity is accompanied by a decrease in the iodine value of the fat, which is due to the absorption of oxygen by the unsaturated bonds of the liquid fatty acids. A determination of this value may therefore, in some cases, give indications of the freshness of a fat.

A more promising method, however, is to distil the fat in a current of steam, and to estimate the amount of aldehydes in the distillate.