Fats and Oils.
It is very undesirable that any of these materials should be allowed to enter the soap pan without an analysis having first been made, as the oil may not only have become partially hydrolysed, involving a loss of glycerine, or contain albuminous matter rendering the soap liable to develop rancidity, but actual sophistication may have taken place. Thus a sample of tallow recently examined by the authors contained as much as 40 per cent. of an unsaponifiable wax, which would have led to disaster in the soap pan, had the bulk been used without examination. After observing the appearance, colour, and odour of the sample, noting any characteristic feature, the following physical and chemical data should be determined.
Specific Gravity at 15° C. This may be taken by means of a Westphal balance, or by using a picnometer of either the ordinary gravity bottle shape, with perforated stopper, or the Sprengel U-tube. The picnometer should be calibrated with distilled water at 15° C. The specific gravity of solid fats may be taken at an elevated temperature, preferably that of a boiling water bath.
Free acidity is estimated by weighing out from 2 to 5 grammes of the fat or oil, dissolving in neutral alcohol (purified methylated spirit) with gentle heat, and titrating with a standard aqueous or alcoholic solution of caustic soda or potash, using phenol-phthalein as indicator.
The contents of the flask are well shaken after each addition of alkali, and the reaction is complete when the slight excess of alkali causes a permanent pink coloration with the indicator. The standard alkali may be N/2, N/5, or N/10.
It is usual to calculate the result in terms of oleic acid (1 c.c. N/10 alkali = 0.0282 gramme oleic acid), and express in percentage on the fat or oil.
Example.—1.8976 grammes were taken, and required 5.2 c.c. of N/10 KOH solution for neutralisation.
| 5.2 × 0.0282 × 100 | ||
| ————————— | = | 7.72 per cent. free fatty acids, expressed as oleic acid. |
| 1.8976 |
The free acidity is sometimes expressed as acid value, which is the amount of KOH in milligrammes necessary to neutralise the free acid in 1 gramme of fat or oil.
In the above example:—
| 5.2 × 5.61 | ||
| ————— | = | 15.3 acid value. |
| 1.8976 |
The saponification equivalent is determined by weighing 2-4 grammes of fat or oil into a wide-necked flask (about 250 c.c. capacity), adding 30 c.c. neutral alcohol, and warming under a reflux condenser on a steam or water-bath. When boiling, the flask is disconnected, 50 c.c. of an approximately semi-normal alcoholic potash solution carefully added from a burette, together with a few drops of phenol-phthalein solution, and the boiling under a reflux condenser continued, with frequent agitation, until saponification is complete (usually from 30-60 minutes) which is indicated by the absence of fatty globules. The excess of alkali is titrated with N/1 hydrochloric or sulphuric acid.
The value of the approximately N/2 alkali solution is ascertained by taking 50 c.c. together with 30 c.c. neutral alcohol in a similar flask, boiling for the same length of time as the fat, and titrating with N/1 hydrochloric or sulphuric acid. The "saponification equivalent" is the amount of fat or oil in grammes saponified by 1 equivalent or 56.1 grammes of caustic potash.
Example.—1.8976 grammes fat required 18.95 c.c. N/1 acid to neutralise the unabsorbed alkali.
Fifty c.c. approximately N/2 alcoholic potash solution required 25.6 c.c. N/ acid..
25.6 - 18.95 = 6.65 c.c. N/1 KOH required by fat.
1.8976 × 1000 / 6.65 = 285.3 Saponification Equivalent.
The result of this test is often expressed as the "Saponification Value," which is the number of milligrammes of KOH required for the saponification of 1 gramme of fat. This may be found by dividing 56,100 by the saponification equivalent or by multiplying the number of c.c. of N/1 alkali absorbed, by 56.1 and dividing by the quantity of fat taken. Thus, in the above example:—
6.65 × 56.1 / 1.8976 = 196.6 Saponification Value.
The ester or ether value, or number of milligrammes of KOH required for the saponification of the neutral esters or glycerides in 1 gramme of fat, is represented by the difference between the saponification and acid values. In the example given, the ester value would be 196.6 - 15.3 = 181.3.
Unsaponifiable Matter.—The usual method adopted is to saponify about 5 grammes of the fat or oil with 50 c.c. of approximately N/2 alcoholic potash solution by boiling under a reflux condenser with frequent agitation for about 1 hour. The solution is then evaporated to dryness in a porcelain basin over a steam or water-bath, and the resultant soap dissolved in about 200 c.c. hot water. When sufficiently cool, the soap solution is transferred to a separating funnel, 50 c.c. of ether added, the whole well shaken, and allowed to rest. The ethereal layer is removed to another separator, more ether being added to the aqueous soap solution, and again separated. The two ethereal extracts are then washed with water to deprive them of any soap, separated, transferred to a flask, and the ether distilled off upon a water-bath. The residue, dried in the oven at 100° C. until constant, is the "unsaponifiable matter," which is calculated to per cent. on the oil.
In this method, it is very frequently most difficult to obtain a distinct separation of ether and aqueous soap solution—an intermediate layer of emulsion remaining even after prolonged standing, and various expedients have been recommended to overcome this, such as addition of alcohol (when petroleum ether is used), glycerine, more ether, water, or caustic potash solution, or by rotatory agitation.
A better plan is to proceed as in the method above described as far as dissolving the resulting soap in 200 c.c. water, and then boil for twenty or thirty minutes. Slightly cool and acidify with dilute sulphuric acid (1 to 3), boil until the fatty acids are clear, wash with hot water free from mineral acid, and dry by filtering through a hot water funnel.
Two grammes of the fatty acids are now dissolved in neutral alcohol saturated with some solvent, preferably a light fraction of benzoline, a quantity of the solvent added to take up the unsaponifiable matter, and the whole boiled under a reflux condenser. After cooling, the liquid is titrated with N/2 aqueous KOH solution, using phenol-phthalein as indicator, this figure giving the amount of the total fatty acids present. The whole is then poured into a separating funnel, when separation immediately takes place. The alcoholic layer is withdrawn, the benzoline washed with warm water (about 32° C.) followed by neutral alcohol (previously saturated with the solvent), and transferred to a tared flask, which is attached to a condenser, and the benzoline distilled off. The last traces of solvent remaining in the flask are removed by gently warming in the water-oven, and the flask cooled and weighed, thus giving the amount of unsaponifiable matter.
Constitution of the Unsaponifiable Matter.—Unsaponifiable matter may consist of cholesterol, phytosterol, solid alcohols (cetyl and ceryl alcohols), or hydrocarbons (mineral oil). Cholesterol is frequently found in animal fats, and phytosterol is a very similar substance present in vegetable fats. Solid alcohols occur naturally in sperm oil, but hydrocarbons, which may be generally recognised by the fluorescence or bloom they give to the oil, are not natural constituents of animal or vegetable oils and fats.
The presence of cholesterol and phytosterol may be detected by dissolving a small portion of the unsaponifiable matter in acetic anhydride, and adding a drop of the solution to one drop of 50 per cent. sulphuric acid on a spot plate, when a characteristic blood red to violet coloration is produced. It has been proposed to differentiate between cholesterol and phytosterol by their melting points, but it is more reliable to compare the crystalline forms, the former crystallising in laminæ, while the latter forms groups of needle-shaped tufts. Another method is to convert the substance into acetate, and take its melting point, cholesterol acetate melting at 114.3-114.8° C., and phytosterol acetate at 125.6°-137° C.
Additional tests for cholesterol have been recently proposed by Lifschütz (Ber. Deut. Chem. Ges., 1908, 252-255), and Golodetz (Chem. Zeit., 1908, 160). In that due to the former, which depends on the oxidation of cholesterol to oxycholesterol ester and oxycholesterol, a few milligrammes of the substance are dissolved in 2-3 c.c. glacial acetic acid, a little benzoyl peroxide added, and the solution boiled, after which four drops of strong sulphuric acid are added, when a violet-blue or green colour is produced, if cholesterol is present, the violet colour being due to oxycholesterol ester, the green to oxycholesterol. Two tests are suggested by Golodetz (1) the addition of one or two drops of a reagent consisting of five parts of concentrated sulphuric acid and three parts of formaldehyde solution, which colours cholesterol a blackish-brown, and (2) the addition of one drop of 30 per cent. formaldehyde solution to a solution of the substance in trichloracetic acid, when with cholesterol an intense blue coloration is produced.
Water.—From 5 to 20 grammes of the fat or oil are weighed into a tared porcelain or platinum dish, and stirred with a thermometer, whilst being heated over a gas flame at 100° C. until bubbling or cracking has ceased, and reweighed, the loss in weight representing the water. In cases of spurting a little added alcohol will carry the water off quietly.
To prevent loss by spurting, Davis (J. Amer. Chem. Soc., 23, 487) has suggested that the fat or oil should be added to a previously dried and tared coil of filter paper contained in a stoppered weighing bottle, which is then placed in the oven and dried at 100° C. until constant in weight. Of course, this method is not applicable to oils or fats liable to oxidation on heating.
Dregs, Dirt, Adipose Tissue, Fibre, etc.—From 10 to 15 grammes of the fat are dissolved in petroleum ether with frequent stirring, and passed through a tared filter paper. The residue retained by the filter paper is washed with petroleum ether until free from fat, dried in the water-oven at 100° C. and weighed.
If the amount of residue is large, it may be ignited, and the proportion and nature of the ash determined.
The amount of impurities may also be estimated by Tate's method, which is performed by weighing 5 grammes of fat into a separating funnel, dissolving in ether, and allowing the whole to stand to enable the water to deposit. After six hours' rest the water is withdrawn, the tube of the separator carefully dried, and the ethereal solution filtered through a dried tared filter paper into a tared flask. Well wash the filter with ether, and carefully dry at 100° C. The ether in the flask is recovered, and the flask dried until all ether is expelled, and its weight is constant. The amount of fat in the flask gives the quantity of actual fat in the sample taken; the loss represents the water and other impurities, and these latter may be obtained from the increase of weight of the filter paper.
Starch may be detected by the blue coloration it gives with iodine solution, and confirmed by microscopical examination, or it may be converted into glucose by inversion, and the glucose estimated by means of Fehling's solution.
Iodine Absorption.—This determination shows the amount of iodine absorbed by a fat or oil, and was devised by Hübl, the reagents required being as follows:—
(1) Solution of 25 grammes iodine in 500 c.c. absolute alcohol; (2) solution of 30 grammes mercuric chloride in 500 c.c. absolute alcohol, these two solutions being mixed together and allowed to stand at least twelve hours before use; (3) a freshly prepared 10 per cent. aqueous solution of potassium iodide; and (4) a N/10 solution of sodium thiosulphate, standardised just prior to use by titrating a weighed quantity of resublimed iodine dissolved in potassium iodide solution.
In the actual determination, 0.2 to 0.5 gramme of fat or fatty acids is carefully weighed into a well-fitting stoppered 250 c.c. bottle, dissolved in 10 c.c. chloroform, and 25 c.c. of the Hübl reagent added, the stopper being then moistened with potassium iodide solution and placed firmly in the bottle, which is allowed to stand at rest in a dark place for four hours. A blank experiment is also performed, using the same quantities of chloroform and Hübl reagent, and allowing to stand for the same length of time.
After the expiration of four hours 20 c.c. of 10 per cent. solution of potassium iodide and 150 c.c. water are added to the contents of the bottle, and the excess of iodine titrated with N/10 sodium thiosulphate solution, the whole being well agitated during the titration, which is finished with starch paste as indicator. The blank experiment is titrated in the same manner, and from the amount of thiosulphate required in the blank experiment is deducted the number of c.c. required by the unabsorbed iodine in the other bottle; this figure multiplied by the iodine equivalent of 1 c.c. of the thiosulphate solution and by 100, dividing the product by the weight of fat taken, gives the "Iodine Number".
Example.—1 c.c. of the N/10 sodium thiosulphate solution is found equal to 0.0126 gramme iodine.
0.3187 gramme of fat taken. Blank requires 48.5 c.c. thiosulphate.
Bottle containing oil requires 40.0 c.c. thiosulphate.
48.5 - 40.0 = 8.5, and the iodine absorption of the fat is—
| 8.5 × 0.0126 × 100 | ||
| ————————— | = | 33.6. |
| 0.3187 |
Wijs showed that by the employment of a solution of iodine monochloride in glacial acetic acid reliable iodine figures are obtained in a much shorter time, thirty minutes being sufficient, and this method is now in much more general use than the Hübl. Wijs' iodine reagent is made by dissolving 13 grammes iodine in 1 litre of glacial acetic acid and passing chlorine into the solution until the iodine is all converted into iodine monochloride. The process is carried out in exactly the same way as with the Hübl solution except that the fat is preferably dissolved in carbon tetrachloride instead of in chloroform.
Bromine absorption has now been almost entirely superseded by the iodine absorption, although there are several good methods. The gravimetric method of Hehner (Analyst, 1895, 49) was employed by one of us for many years with very good results, whilst the bromine-thermal test of Hehner and Mitchell (Analyst, 1895, 146) gives rapid and satisfactory results. More recently MacIlhiney (Jour. Amer. Chem. Soc., 1899, 1084-1089) drew attention to bromine absorption methods and tried to rewaken interest in them.
The Refractive index is sometimes useful for discriminating between various oils and fats, and, in conjunction with other physical and chemical data, affords another means of detecting adulteration.
Where a great number of samples have to be tested expeditiously, the Abbé refractometer or the Zeiss butyro-refractometer may be recommended on account of the ease with which they are manipulated. The most usual temperature of observations is 60° C.
The Titre or setting point of the fatty acids was devised by Dalican, and is generally accepted in the commercial valuation of solid fats as a gauge of firmness, and in the case of tallow has a considerable bearing on the market value.
One ounce of the fat is melted in a shallow porcelain dish, and 30 c.c. of a 25 per cent. caustic soda solution added, together with 50 c.c. of redistilled methylated spirit. The whole is stirred down on the water bath until a pasty soap is obtained, when another 50 c.c. of methylated spirit is added, which redissolves the soap, and the whole again stirred down to a solid soap. This is then dissolved in distilled water, a slight excess of dilute sulphuric acid added to liberate the fatty acids, and the whole warmed until the fatty acids form a clear liquid on the surface. The water beneath the fatty acids is then syphoned off, more distilled water added to wash out any trace of mineral acid remaining, and again syphoned off, this process being repeated until the washings are no longer acid to litmus paper, when the fatty acids are poured on to a dry filter paper, which is inserted in a funnel resting on a beaker, and the latter placed on the water-bath, where it is left until the clear fatty acids have filtered through.
About 10-15 grammes of the pure fatty acids are now transferred to a test tube, 6" × 1", warmed until molten, and the tube introduced through a hole in the cork into a flask or wide-mouthed bottle. A very accurate thermometer, graduated into fifths of a degree Centigrade (previously standardised), is immersed in the fatty acids, so that the bulb is as near the centre as possible, and when the fatty acids just begin to solidify at the bottom of the tube, the thermometer is stirred round slowly. The mercury will descend, and stirring is continued until it ceases to fall further, at which point the thermometer is very carefully observed. It will be found that the temperature will rise rapidly and finally remain stationary for a short time, after which it will again begin to drop until the temperature of the room is reached. The maximum point to which the temperature rises is known as the "titre" of the sample.