Chapter III: Refining, Bleaching, and Deodorising Fats and Oils

Physical Methods—Washing, freezing, filtration,
treatment with charcoal and fuller’s earth,
steaming. Chemical methods—Caustic soda, sodium
carbonate and silicate, alkaline earths, ozone,
hydrosulphites, sodium bisulphite, organic
peroxides.

Although, as was pointed out in Chapter II., too much stress cannot be laid upon the importance of using only the freshest materials of the best possible quality in the manufacture of edible fats, yet even these frequently require a certain amount of preliminary treatment in order to bleach, deodorise, or refine them and render them palatable for human food. Numerous processes, both physical and chemical, have been devised, and in many cases patented, for these different purposes, and the following is a summary of the more important ones.

=Physical Methods.=—Among the physical methods employed may be mentioned washing with hot water, the removal of suspended matter by settling or filtration, and of excess of stearin by subjection to low temperatures, bleaching by filtration through animal charcoal or fuller’s earth, and deodorising by injecting steam either at atmospheric pressure, in vacuo, or in presence of an indifferent gas.

The impurities in freshly expressed oil which are partly in suspension and partly in solution, consist chiefly of dirt, fragments of vegetable fibre, and mucilaginous and albuminous substances. A portion of them rapidly deposits when the oil is allowed to stand, and the upper liquid may then be drawn off from the sediment and subjected to filtration or to further refining processes.

In some cases a simple filtration, after standing for a short time, is sufficient to render the oil brilliant, but special treatment is necessary when a large proportion of albuminous matter is present, since such oils either pass through the filter without becoming bright, or if a closer filtering medium is used, the pores of the filter speedily become clogged.

Various methods are employed to coagulate or precipitate albuminous matters before filtration, such as dry heat, or the introduction of fine jets of steam, or the addition of a small quantity of insoluble powder (_e.g._ fuller’s earth or kieselguhr), which as it subsides attracts and carries down simultaneously the particles of the gum-like mucilage.

The formation of an insoluble precipitate within the oil answers the same purpose. Thus, in Linde’s process a small quantity of milk is introduced and the mixture heated so as to coagulate the casein, the subsidence of which removes at the same time the substances that cause turbidity in the oil.

Other substances, such as solutions of tannin, are used in the same way in refining oils for technical purposes, but are inadmissible in the case of edible oils.

In the removal of dissolved impurities, alkali solutions, milk of lime, or magnesia are reagents in common use, while dilute sulphuric acid is employed to clarify linseed and certain fish oils for industrial uses.

To facilitate the purification of oils by washing with water, Dubovitz has recently recommended the addition to the water of aluminium sulphate, in the proportion of about ¾ oz. to 220 gallons per degree of hardness. This forms with the lime in the water a bulky, colloidal precipitate, which serves to bleach and clarify the oil.

=Removal of Stearin.=—When certain oils are exposed to a low temperature they become turbid and in some cases give a white deposit. This consists of glycerides of the more solid fatty acids, and is known as “stearin,” and its formation is frequently regarded as objectionable, notwithstanding the fact that when the oil is gently heated the stearin is redissolved.

In the preparation of the best edible oils, therefore, a process of chilling followed by filtration is often employed in order to remove part of the stearin, and the oils thus treated may then be exposed to a low temperature without giving a further deposit.

Oils treated in this way are commonly known as “winter oils,” and those which will only keep brilliant at the ordinary summer temperature are termed “summer oils.”

The solid fat separated in this way from cotton-seed oil is known as cotton oil stearin, although it is quite free from stearic acid. A similar process is employed for the separation of cocoanut and palm-nut oils into their respective stearins and oleins (see _Chocolate Fats_).

=Methods of Filtration.=—The types of filter press used in the filtration of oils are very varied. A common form consists of a hydraulic press containing a series of communicating plates with rims raised so as to form a space into which filter cloths may be fitted.

In other forms of apparatus the oil is introduced from below into a chamber, and rises upwards through the filtering medium into a compartment in which a partial vacuum has been created. Or methods of centrifugal filtration may be employed, as in apparatus in which the oil is introduced into a revolving chamber with a perforated wall round which is wrapped a filtering cloth. The oil is flung against the wall of the revolving chamber in a fine state of division, and passes through the filtering medium into an outer chamber, whence it can be drawn off.

The materials used as filtering media include sand, kieselguhr, Spanish clay, fuller’s earth, animal charcoal, paper pulp, and a mixture of wool and vegetable fibres disintegrated into a pulp.

=Chemical Methods.=—The number of chemicals employed in refining oils and fats for edible purposes is necessarily very limited. It is of course very objectionable to employ any reagent which is poisonous, though the use of barium oxide has been patented by Rocca (Fr. Pat. 325,381, 1902), and processes involving the use of mineral acids are, in general, inadmissible, as they spoil the flavour of the oil. The chief reagents employed are caustic soda (sodium hydroxide), sodium carbonate, sodium silicate, calcium or magnesium oxide, ozone, hydrosulphites, formaldehyde-sulphoxylates, and organic peroxides.

_Caustic Soda._—Of the alkaline refining process the treatment of cotton-seed oil with a solution of sodium hydroxide or potassium hydroxide is the best example. The oil is mechanically agitated with the alkali solution, which usually has a specific gravity of about 1·10, either with or without the aid of heat, and the mixture then allowed to stand until it separates into two layers, the lower of which contains a sediment of impurities. This is drawn off, and the treatment repeated, but this time with a more dilute solution of alkali, and finally the oil thus clarified is washed with water to remove the excess of alkali.

In this process of refining not only are the albuminous and resinous matters precipitated, but the free fatty acids in the oil are neutralised, and accordingly freshly prepared cotton-seed oil is almost neutral in its reaction, and has a bland taste. The treatment also removes a large proportion of the colouring matter separated from the seed in the expression of the oil, and changes the dark-brown colour of the crude product to a light-golden tint.

The residue left from the refining of cotton-seed and other oils is a thick deposit containing the impurities in a concentrated form, together with a considerable proportion of oil; such residues are known as “foots,” and are utilised in the manufacture of soap.

The amount of caustic soda required depends on the degree of acidity of the oil, which it should be just sufficient to neutralise. The acidity of the oil is therefore first determined, as described in