The Chemistry of Cookery

by W. Mattieu Williams

The fat used in frying requires occasional purification. I may illustrate the principle on which it should be conducted by describing the method adopted in the refining of mineral oils, such as petroleum or the paraffin distillates of bituminous shales. These are dark, tarry liquids of treacle-like consistency, with a strong and offensive odour. Nevertheless they are, at but little cost, converted into the ‘crystal oil’ used for lamps, and that beautiful pearly substance, the solid, translucent paraffin now so largely used in the manufacture of candles. Besides these, we obtain from the same dirty source an intermediate substance, the well-known ‘Vaseline,’ now becoming the basis of most of the ointments of the pharmacopœia. This purification is effected by agitation with sulphuric acid, which partly carbonises and partly combines with the impurities, and separates them in the form of a foul and acrid black mess, known technically as ‘acid tar.’ When I was engaged in the distillation of cannel and shale in Flintshire, this acid tar was a terrible bugbear. It found its way mysteriously into the Alyn river and poisoned the trout; but now, if I am correctly informed, the Scotch manufacturers have turned it to profitable account.

Animal fat and vegetable oils are similarly purified. Very objectionable refuse fat of various kinds is thus made into tallow, or material for the soap-maker, and grease for lubricating machinery. Unsavoury stories have been told about the manufacture of butter from Thames mud or the nodules of fat that are gathered therefrom by the mudlarks, but they are all false (see paper on ‘The Oleaginous Product of Thames Mud’ in ‘Science in Short Chapters’). It may be possible to purify fatty matter from the foulest of admixtures, and do this so completely as to produce a soft, tasteless fat, i.e. a butter substitute, but such a curiosity would cost more than half a crown per pound, and therefore the market is safe, especially as the degree of purification required for soap-making and machinery grease costs but little and the demand for such fat is very great.

These methods of purification are not available in the kitchen, as oil of vitriol is a vicious compound. During the siege of Paris some of the Academicians devoted themselves very earnestly to the subject of the purification of fat in order to produce what they termed ‘siege butter’ from the refuse of slaughter-houses, &c., and edible salad oils from crude colza oil, from the rancid fish oils used by the leather-dresser, &c. Those who are specially interested in the subject may find some curious papers in the ‘Comptes Rendus’ of that period. In vol. lxxi., page 36, M. Boillot describes his method of mixing kitchen-stuff and other refuse fat with lime-water, agitating the mixture when heated, and then neutralising with an acid. The product thus obtained is described as admirably adapted for culinary operations, and the method is applicable to the purpose here under consideration.

Further on in the same volume is a ‘Note on Suets and Alimentary Fats’ by M. Dubrunfaut, who tells us that the most tainted of alimentary fats and rancid oils may be deprived of their bad odours by ‘appropriate frying.’ His method is to raise the temperature of the fat to 140° to 150° Cent. (284° to 302° Fahr.) in a frying-pan; then cautiously sprinkle upon it small quantities of water. The steam carries off the volatile fatty acids which produce the rancidity in such as fish oils, and also removes the neutral offensive fatty matters that are decomposable by heat. In another paper by M. Fua this method is applied to the removal of cellular tissue of crude fats from slaughter-houses. It is really nothing more than the old farmhouse proceeding of ‘rendering’ lard, by frying the membranous fat until the membranous matter is browned and aggregated into small nodules, which constitute the ‘scratchings’—a delicacy greatly relished by our British ploughboys at pig-killing time, but rather too rich in pork-fat to supply a suitable meal for people of sedentary vocations.

The action of heat thus applied and long-continued is similar to that of the strong sulphuric acid. The impurities of the fat are organic matters more easily decomposable than the fat itself, or otherwise stated, they are dissociated into carbon and water at about 300° Fahr., which is a lower temperature than that required for the dissociation of the pure oil or fat. By maintaining this temperature, these compounds become first caramelised, then carbonised nearly to blackness, when all their powers of offensiveness vanish.

In the more violent factory process of purification by sulphuric acid the similar action which occurs is due to the powerful affinity of this acid for water: this may be strikingly shown by adding to thick syrup or pounded sugar about its own bulk of oil of vitriol, when a marvellous commotion occurs, and a magnified black cinder is produced by the separation of the water from the sugar.

The following simple practical formula may be reduced from these data. When a considerable quantity of much-used frying-fat is accumulated, heat it to about 300° Fahr., as indicated by the crackling of water when sprinkled on it, or, better still, by a properly-constructed thermometer. Then pour the melted fat on hot water. This must be done carefully, as a large quantity of fat at 300° poured upon a small quantity of boiling water will illustrate the fact that water when suddenly heated is an explosive compound. The quantity of water should exceed that of the fat, and the pouring be done gradually. Then agitate the fat and water together, and, if the operator is sufficiently skilful and intelligent, the purification may be carried further by carefully boiling the water under the fat and allowing its steam to pass through; but this is a little dangerous, on account of the possibility of what the practical chemist calls ‘bumping,’ or the sudden formation of a big bubble of steam that would kick a good deal of the superabundant fat into the fire.

Whether this supplementary boiling is carried out or not, the fat and the water should be left together to cool gradually, when a dark layer of carbonised impurities will be found resting on the surface of the water, and adhering to the bottom of the cake of fat. This may be peeled off and put into the waste grease-pot to be further refined with the next operation. Ultimately the worst of it will sink to the bottom of the water.

A careful cook may keep the supply of frying fat continually good, by simply pouring it into a basin (a deep pudding-basin with small area at bottom is best), letting it solidify there, and then paring away the bottom sediment. Even this dirty-looking sediment need not be altogether wasted. When a considerable quantity has accumulated it may be purified by the method of Dubrunfaut and Fua above described.