CONSTITUTION OF OILS AND FATS, AND THEIR SAPONIFICATION.

Researches of Chevreul and Berthelot—Mixed Glycerides—Modern Theories of Saponification—Hydrolysis accelerated by (1) Heat or Electricity, (2) Ferments; Castor-seed Ferment, Steapsin, Emulsin, and (3) Chemical Reagents; Sulphuric Acid, Twitchell's Reagent, Hydrochloric Acid, Lime, Magnesia, Zinc Oxide, Soda and Potash.

The term oil is of very wide significance, being applied to substances of vastly different natures, both organic and inorganic, but so far as soap-making materials are concerned, it may be restricted almost entirely to the products derived from animal and vegetable sources, though many attempts have been made during the last few years to also utilise mineral oils for the preparation of soap. Fats readily become oils on heating beyond their melting points, and may be regarded as frozen oils.

Although Scheele in 1779 discovered that in the preparation of lead plaster glycerol is liberated, soap at that time was regarded as a mere mechanical mixture, and the constitution of oils and fats was not properly understood. It was Chevreul who showed that the manufacture of soap involved a definite chemical decomposition of the oil or fat into fatty acid and glycerol, the fatty acid combining with soda, potash, or other base, to form the soap, and the glycerol remaining free. The reactions with stearin and palmitin (of which tallow chiefly consists) and with olein (found largely in olive and cotton-seed oils) are as follows:—

CH₂OOC₁₈H₃₅						CH₂OH
\|						\|
CHOOC₁₈H₃₅	+	3NaOH	=	3NaOOC₁₈H₃₅	+	CHOH
\|						\|
CH₂OOC₁₈H₃₅						CH₂OH
stearin		sodium hydroxide		sodium stearate		glycerol

CH₂OOC₁₆H₃₁						CH₂OH
\|						\|
CHOOC₁₆H₃₁	+	3NaOH	=	3NaOOC₁₆H₃₁	+	CHOH
\|						\|
CH₂OOC₁₆H₃₁						CH₂OH
palmitin		sodium hydroxide		sodium palmitate		glycerol

CH₂OOC₁₈H₃₃						CH₂OH
\|						\|
CHOOC₁₈H₃₃	+	3NaOH	=	3NaOOC₁₈H₃₃	+	CHOH
\|						\|
CH₂OOC₁₈H₃₃						CH₂OH
olein		sodium hydroxide		sodium oleate		glycerol

Berthelot subsequently confirmed Chevreul's investigations by directly synthesising the fats from fatty acids and glycerol, the method he adopted consisting in heating the fatty acids with glycerol in sealed tubes. Thus, for example:—

3C₁₈H₃₅O₂H	+	C₃H₅(OH)₃	=	C₃H₅(C₁₈H₃₅O₂)₃
stearic acid		glycerol		tristearin

Since glycerol is a trihydric alcohol, i.e., contains three hydroxyl (OH) groups, the hydrogen atoms of which are displaceable by acid radicles, the above reaction may be supposed to take place in three stages. Thus, we may have:—

(1)	C₁₈H₃₅O₂H	+	C₃H₅(OH)₃	=	C₃H₅(OH)₂C₁₈H₃₅O₂	+	H₂O
					monostearin
(2)	C₁₈H₃₅O₂H	+	C₃H₅(OH)₂C₁₈H₃₅O₂	=	C₃H₅(OH)(C₁₈H₃₅O₂)₂	+	H₂O
					distearin
(3)	C₁₈H₃₅O₂H	+	C₃H₅(OH)(C₁₈H₃₅O₂)₂	=	C₃H₅(C₁₈H₃₅O₂)₃	+	H₂O
					tristearin

There are two possible forms of monoglyceride and diglyceride, according to the relative position of the acid radicle, these being termed alpha and beta respectively, and represented by the following formulæ, where R denotes the acid radicle:—

Monoglyceride:—

	CH₂OR			CH₂OH
	\|				\|
(alpha)	CHOH	and	(beta)	CHOR
	\|			\|
	CH₂OH			CH₂OH

Diglyceride:—

	CH₂OR			CH₂OR
	\|				\|
(alpha)	CHOH	and	(beta)	CHOR
	\|			\|
	CH₂OR			CH₂OH

According to the relative proportions of fatty acid and glycerol used, and the temperature to which they were heated, Berthelot succeeded in preparing mono-, di- and triglycerides of various fatty acids.

Practically all the oils and fats used in soap-making consist of mixtures of these compounds of glycerol with fatty acids, which invariably occur in nature in the form of triglycerides.

It was formerly considered that the three acid radicles in any naturally occurring glyceride were identical, corresponding to the formula—

CH₂OR
|
CHOR
|
CH₂OR

where R denotes the acid radicle. Recent work, however, has shown the existence of several so-called mixed glycerides, in which the hydroxyls of the same molecule of glycerol are displaced by two or sometimes three different acid radicles.

The first mixed glyceride to be discovered was oleodistearin, C₃H₅(OC₁₈H₃₅O)(OC₁₈H₃₅O)₂, obtained by Heise in 1896 from Mkani fat. Hansen has since found that tallow contains oleodipalmitin, C₃H₅(OC₁₈H₃₅O)(OC₁₆H₃₁O), stearodipalmitin, C₃H₅(OC₁₈H₃₅O)(OC₁₆H₃₁O), oleopalmitostearin, C₃H₅(OC₁₈H₃₃O)(OC₁₆H₃₁O) (OC₁₈H₃₅O) and palmitodistearin, CH(OC₁₆H₃₁O)(OC₁₈H₃₅O)₂, the latter of which has also been obtained by Kreis and Hafner from lard, while Holde and Stange have shown that olive oil contains from 1 to 2 per cent. of oleodidaturin, C₃H₅(OC₁₈H₃₃O)(OC₁₇H₃₃O)₂, and Hehner and Mitchell have obtained indications of mixed glycerides in linseed oil (which they consider contains a compound of glycerol with two radicles of linolenic acid and one radicle of oleic acid), also in cod-liver, cod, whale and shark oils.

In some cases the fatty acids are combined with other bases than glycerol. As examples may be cited beeswax, containing myricin or myricyl palmitate, and spermaceti, consisting chiefly of cetin or cetyl palmitate, and herein lies the essential difference between fats and waxes, but as these substances are not soap-making materials, though sometimes admixed with soap to accomplish some special object, they do not require further consideration.

The principal pure triglycerides, with their formulæ and chief constants, are given in the following table:—

Glyceride.	Formula.	Chief Occurrence.	Melting Point, °C.	Refractive Index at 60° C.	Saponification Equivalent.
Butyrin	C₃H₅(O.C₄H₇O)₃	Butter fat	Liquid at -60	1.42015	100.7
Isovalerin	C₃H₅(O.C₅H₉O)₃	Porpoise, dolphin	...	...	114.7
Caproin	C₃H₅(O.C₆H₁₁O)₃	Cocoa-nut and palm-nut oils	-25	1.42715	128.7
Caprylin	C₃H₅(O.C₈H₁₅O)₃	Do. do.	-8.3	1.43316	156.7
Caprin	C₃H₅(O.C₁₀H₁₉O)₃	Do. do.	31.1	1.43697	184.7
Laurin	C₃H₅(O.C₁₂H₂₃O)₃	Do. do.	45	1.44039	212.7
Myristin	C₃H₅(O.C₁₄H₂₇O)₃	Nutmeg butter	56.5	1.44285	240.7
Palmitin	C₃H₅(O.C₁₆H₃₁O)₃	Palm oil, lard	63-64	...	268.7
Stearin	C₃H₅(O.C₁₈H₃₅O)₃	Tallow, lard, cacao butter	71.6	...	296.7
Olein	C₃H₅(O.C₁₈H₃₃O)₃	Olive and almond oils	Solidifies at -6	...	294.7
Ricinolein	C₃H₅(O.C₁₈H₃₃O₂)₃	Castor oil	...	...	310.7

Of the above the most important from a soap-maker's point of view are stearin, palmitin, olein and laurin, as these predominate in the fats and oils generally used in that industry. The presence of stearin and palmitin, which are solid at the ordinary temperature, gives firmness to a fat; the greater the percentage present, the harder the fat and the higher will be the melting point, hence tallows and palm oils are solid, firm fats. Where olein, which is liquid, is the chief constituent, we have softer fats, such as lard, and liquid oils, as almond, olive and cotton-seed.

Stearin (Tristearin) can be prepared from tallow by crystallisation from a solution in ether, forming small crystals which have a bright pearly lustre. The melting point of stearin appears to undergo changes and suggests the existence of distinct modifications. When heated to 55° C. stearin liquefies; with increase of temperature it becomes solid, and again becomes liquid at 71.6° C. If this liquid be further heated to 76° C., and allowed to cool, it will not solidify until 55° C. is reached, but if the liquid at 71.6° C. be allowed to cool, solidification will occur at 70° C.

Palmitin (Tripalmitin) may be obtained by heating together palmitic acid and glycerol, repeatedly boiling the resulting product with strong alcohol, and allowing it to crystallise. Palmitin exists in scales, which have a peculiar pearly appearance, and are greasy to the touch. After melting and solidifying, palmitin shows no crystalline fracture; when heated to 46° C. it melts to a liquid which becomes solid on further heating, again liquefying when 61.7° C. is reached, and becoming cloudy, with separation of crystalline particles. At 63° C. it is quite clear, and this temperature is taken as the true melting point. It has been suggested that the different changes at the temperatures mentioned are due to varying manipulation, such as rate at which the temperature is raised, and the initial temperature of the mass when previously cool.

Olein (Triolein) is an odourless, colourless, tasteless oil, which rapidly absorbs oxygen and becomes rancid. It has been prepared synthetically by heating glycerol and oleic acid together, and may be obtained by submitting olive oil to a low temperature for several days, when the liquid portion may be further deprived of any traces of stearin and palmitin by dissolving in alcohol. Olein may be distilled in vacuo without decomposition taking place.

Laurin (Trilaurin) may be prepared synthetically from glycerol and lauric acid. It crystallises in needles, melting at 45°-46° C., which are readily soluble in ether, but only slightly so in cold absolute alcohol. Scheij gives its specific gravity, d60°/4° = 0.8944. Laurin is the chief constituent of palm-kernel oil, and also one of the principal components of cocoa-nut oil.

Fatty Acids.—When a fat or oil is saponified with soda or potash, the resulting soap dissolved in hot water, and sufficient dilute sulphuric acid added to decompose the soap, an oily layer gradually rises to the surface of the liquid, which, after clarifying by warming and washing free from mineral acid, is soluble in alcohol and reddens blue litmus paper. This oily layer consists of the "fatty acids" or rather those insoluble in water, acids like acetic, propionic, butyric, caproic, caprylic and capric, which are all more or less readily soluble in water, remaining for the most part dissolved in the aqueous portion. All the acids naturally present in oils and fats, whether free or combined, are monobasic in character, that is to say, contain only one carboxyl—CO.OH group. The more important fatty acids may be classified according to their chemical constitution into five homologous series, having the general formulæ:—

I. Stearic series C_nH_2n+1COOH
II. Oleic series C_nH_2n-1COOH
III. Linolic series C_nH_2n-3COOH
IV. Linolenic series C_nH_2n-5COOH
V. Ricinoleic series C_nH_2n-7COOH

I. Stearic Series.—The principal acids of this series, together with their melting points and chief sources, are given in the following table:—

Acid.	Formula.	Melting Point, °C.	Found in
Acetic	CH₃COOH	17	Macassar oil.
Butyric	C₃H₇COOH	...	Butter, Macassar oil.
Isovaleric	C₄H₉COOH	...	Porpoise and dolphin oils.
Caproic	C₅H₁₁COOH	...	Butter, cocoa-nut oil.
Caprylic	C₇H₁₅COOH	15	Butter, cocoa-nut oil, Limburg cheese.
Capric	C₉H₁₉COOH	30	Butter, cocoa-nut oil.
Lauric	C₁₁H₂₃COOH	44	Cocoa-nut oil, palm-kernel oil.
Ficocerylic	C₁₂H₂₅COOH	...	Pisang wax.
Myristic	C₁₃H₂₇COOH	54	Nutmeg butter, liver fat, cocoa-nut oil, dika fat, croton oil.
Palmitic	C₁₅H₃₁COOH	62.5	Palm oil, most animal fats.
Daturic	C₁₆H₃₃COOH	...	Oil of Datura Stramonium.
Stearic	C₁₇H₃₅COOH	69	Tallow, lard, most solid animal fats.
Arachidic	C₁₉H₃₉COOH	75	Arachis or earth-nut oil, rape and mustard-seed oils.
Behenic	C₂₁H₄₃COOH	...	Ben oil, black mustard-seed oil, rape oil.
Lignoceric	C₂₃H₄₇COOH	80.5	Arachis oil.
Carnaubic	C₂₃H₄₇COOH	...	Carnauba wax.
Pisangcerylic	C₂₃H₄₇COOH	...	Pisang wax.
Hyænic	C₂₄H₄₉COOH	...	Hyæna fat.
Cerotic	C₂₅H₅₁COOH	78	Beeswax, China wax, spermaceti.
Melissic	C₂₉H₅₉COOH	89	Beeswax.
Psyllostearylic	C₃₂H₆₅COOH	...	Psylla wax.
Theobromic	C₆₃H₁₂₇COOH	...	Cacao butter

Medullic and margaric acids, which were formerly included in this series, have now been shown to consist of mixtures of stearic and palmitic, and stearic palmitic and oleic acids respectively.

The acids of this group are saturated compounds, and will not combine directly with iodine or bromine. The two first are liquid at ordinary temperatures, distil without decomposition, and are miscible with water in all proportions; the next four are more or less soluble in water and distil unchanged in the presence of water, as does also lauric acid, which is almost insoluble in cold water, and only slightly dissolved by boiling water. The higher acids of the series are solid, and are completely insoluble in water. All these acids are soluble in warm alcohol, and on being heated with solid caustic alkali undergo no change.

II. Oleic Series:—

Acid.	Formula.	Melting Point, °C.	Found in
Tiglic	C₄H₇COOH	64.5	Croton oil.
Moringic	C₁₄H₂₇COOH	0	Ben oil.
Physetoleic	C₁₅H₂₉COOH	30	Sperm oil.
Hypogæic	C₁₅H₂₉COOH	33	Arachis and maize oils.
Oleic	C₁₇H₃₃COOH	14	Most oils and fats.
Rapic	C₁₇H₃₃COOH	...	Rape oil.
Doeglic	C₁₈H₃₅COOH	...	Bottle-nose oil.
Erucic	C₂₁H₄₁COOH	34	Mustard oils, marine animal oils, rape oil.

The unsaturated nature of these acids renders their behaviour with various reagents entirely different from that of the preceding series. Thus, they readily combine with bromine or iodine to form addition compounds, and the lower members of the series are at once reduced, on treatment with sodium amalgam in alkaline solution, to the corresponding saturated acids of Series I. Unfortunately, this reaction does not apply to the higher acids such as oleic acid, but as the conversion of the latter into solid acids is a matter of some technical importance from the point of view of the candle-maker, a number of attempts have been made to effect this by other methods.

De Wilde and Reychler have shown that by heating oleic acid with 1 per cent. of iodine in autoclaves up to 270°-280° C., about 70 per cent. is converted into stearic acid, and Zürer has devised (German Patent 62,407) a process whereby the oleic acid is first converted by the action of chlorine into the dichloride, which is then reduced with nascent hydrogen. More recently Norman has secured a patent (English Patent 1,515, 1903) for the conversion of unsaturated fatty acids of Series II. into the saturated compounds of Series I., by reduction with hydrogen or water-gas in the presence of finely divided nickel, cobalt or iron. It is claimed that by this method oleic acid is completely transformed into stearic acid, and that the melting point of tallow fatty acids is raised thereby about 12° C.

Another method which has been proposed is to run the liquid olein over a series of electrically charged plates, which effects its reduction to stearin.

Stearic acid is also formed by treating oleic acid with fuming hydriodic acid in the presence of phosphorus, while other solid acids are obtained by the action of sulphuric acid or zinc chloride on oleic acid.

Acids of Series II. may also be converted into saturated acids by heating to 300°C. with solid caustic potash, which decomposes them into acids of the stearic series with liberation of hydrogen. This reaction, with oleic acid, for example, is generally represented by the equation—

C₁₈H₃₄O₂ + 2KOH = KC₂H₃O₂ + KC₁₆H₃₁O₂ + H₂,

though it must be really more complex than this indicates, for, as Edmed has pointed out, oxalic acid is also formed in considerable quantity. The process on a commercial scale has now been abandoned.

One of the most important properties of this group of acids is the formation of isomeric acids of higher melting point on treatment with nitrous acid, generally termed the elaidin reaction. Oleic acid, for example, acted upon by nitrous acid, yields elaidic acid, melting at 45°, and erucic acid gives brassic acid, melting at 60°C. This reaction also occurs with the neutral glycerides of these acids, olein being converted into elaidin, which melts at 32°C.

The lead salts of the acids of this series are much more soluble in ether, and the lithium salts more soluble in alcohol than those of the stearic series, upon both of which properties processes have been based for the separation of the solid from the liquid fatty acids.

III. Linolic Series:—

Acid.	Formula.	Melting Point, °C.	Found in
Elæomargaric	C₁₆H₂₉COOH	...	Chinese-wood oil.
Elæostearic	C₁₆H₂₉COOH	71	Chinese-wood oil.
Linolic	C₁₇H₃₁COOH	Fluid	Linseed, cotton-seed and maize oils.
Tariric	C₁₇H₃₁COOH	50.5	Tariri-seed oil.
Telfairic	C₁₇H₃₁COOH	Fluid	Telfairia oil.

These acids readily combine with bromine, iodine, or oxygen. They are unaffected by nitrous acid, and their lead salts are soluble in ether.

IV. Linolenic Series:—

Acid.	Formula.	Found in
Linolenic	C₁₇H₂₉COOH	Linseed oil.
Isolinolenic	C₁₇H₂₉COOH	Linseed oil.
Jecoric	C₁₇H₂₉COOH	Cod-liver and marine animal oils.

These acids are similar in properties to those of Class III., but combine with six atoms of bromine or iodine, whereas the latter combine with only four atoms.

V. Ricinoleic Series:—

Acid.	Formula.	Melting Point, °C.	Found in
Ricinoleic	C₁₇H₂₂(OH)COOH	4-5	Castor oil.

This acid combines with two atoms of bromine or iodine, and is converted by nitrous acid into the isomeric ricinelaidic acid, which melts at 52°-53° C. Pure ricinoleic acid, obtained from castor oil, is optically active, its rotation being α_d +6° 25'.

Hydrolysis or Saponification of Oils and Fats.—The decomposition of a triglyceride, brought about by caustic alkalies in the formation of soap, though generally represented by the equation already given (pp. 6 and 7)—

C₃H₅(OR) + 3NaOH = C₃H₅(OH)₃ + 3RONa,

is not by any means such a simple reaction.

In the first place, though in this equation no water appears, the presence of the latter is found to be indispensable for saponification to take place; in fact, the water must be regarded as actually decomposing the oil or fat, caustic soda or potash merely acting as a catalytic agent. Further, since in the glycerides there are three acid radicles to be separated from glycerol, their saponification can be supposed to take place in three successive stages, which are the converse of the formation of mono- and diglycerides in the synthesis of triglycerides from fatty acids and glycerine. Thus, the above equation may be regarded as a summary of the following three:—

		{	OR	{	OH
(i.)	C₃H₅	{	OR + NaOH = C₃H₅	{	OR + RONa
		{	OR	{	OR

		{	OH	{	OH
(ii.)	C₃H₅	{	OR + NaOH = C₃H₅	{	OR + RONa
		{	OR	{	OH

		{	OH	{	OH
(iii.)	C₃H₅	{	OR + NaOH = C₃H₅	{	OH + RONa
		{	OH	{	OH

Geitel and Lewkowitsch, who have studied this question from the physical and chemical point of view respectively, are of opinion that when an oil or fat is saponified, these three reactions do actually occur side by side, the soap-pan containing at the same time unsaponified triglyceride, diglyceride, monoglyceride, glycerol and soap.

This theory is not accepted, however, by all investigators. Balbiano and Marcusson doubt the validity of Lewkowitsch's conclusions, and Fanto, experimenting on the saponification of olive oil with caustic potash, is unable to detect the intermediate formation of any mono- or diglyceride, and concludes that in homogeneous solution the saponification is practically quadrimolecular. Kreeman, on the other hand, from physico-chemical data, supports the view of Geitel and Lewkowitsch that saponification is bimolecular, and though the evidence seems to favour this theory, the matter cannot be regarded as yet definitely settled.

Hydrolysis can be brought about by water alone, if sufficient time is allowed, but as the process is extremely slow, it is customary in practice to accelerate the reaction by the use of various methods, which include (i.) the application of heat or electricity, (ii.) action of enzymes, and (iii.) treatment with chemicals; the accelerating effect of the two latter methods is due to their emulsifying power.

The most usual method adopted in the manufacture of soap is to hydrolyse the fat or oil by caustic soda or potash, the fatty acids liberated at the same time combining with the catalyst, i.e., soda or potash, to form soap. Hitherto the other processes of hydrolysis have been employed chiefly for the preparation of material for candles, for which purpose complete separation of the glycerol in the first hydrolysis is not essential, since the fatty matter is usually subjected to a second treatment with sulphuric acid to increase the proportion of solid fatty acids. The colour of the resulting fatty acids is also of no importance, as they are always subjected to distillation.

During the last few years, however, there has been a growing attempt to first separate the glycerol from the fatty acids, and then convert the latter into soap by treatment with the carbonates of soda or potash, which are of course considerably cheaper than the caustic alkalies, but cannot be used in the actual saponification of a neutral fat. The two processes chiefly used for this purpose are those in which the reaction is brought about by enzymes or by Twitchell's reagent.

I. Application of Heat or Electricity.—Up to temperatures of 150° C. the effect of water on oils and fats is very slight, but by passing superheated steam through fatty matter heated to 200°-300° C. the neutral glycerides are completely decomposed into glycerol and fatty acids according to the equation—

C₃H₅(OR)₃ + 3H.OH = C₃H₅(OH)₃ + 3ROH.

The fatty acids and glycerol formed distil over with the excess of steam, and by arranging a series of condensers, the former, which condense first, are obtained almost alone in the earlier ones, and an aqueous solution of glycerine in the later ones. This method of preparation of fatty acids is extensively used in France for the production of stearine for candle-manufacture, but the resulting product is liable to be dark coloured, and to yield a dark soap. To expose the acids to heat for a minimum of time, and so prevent discoloration, Mannig has patented (Germ. Pat. 160,111) a process whereby steam under a pressure of 8 to 10 atmospheres is projected against a baffle plate mounted in a closed vessel, where it mixes with the fat or oil in the form of a spray, the rate of hydrolysis being thereby, it is claimed, much increased.

Simpson (Fr. Pat. 364,587) has attempted to accelerate further the decomposition by subjecting oils or fats to the simultaneous action of heat and electricity. Superheated steam is passed into the oil, in which are immersed the two electrodes connected with a dynamo or battery, the temperature not being allowed to exceed 270° C.

II. Action of Enzymes.—It was discovered by Muntz in 1871 (Annales de Chemie, xxii.) that during germination of castor seeds a quantity of fatty acid was developed in the seeds, which he suggested might be due to the decomposition of the oil by the embryo acting as a ferment. Schutzenberger in 1876 showed that when castor seeds are steeped in water, fatty acids and glycerol are liberated, and attributed this to the hydrolytic action of an enzyme present in the seeds. No evidence of the existence of such a ferment was adduced, however, till 1890, when Green (Roy. Soc. Proc., 48, 370) definitely proved the presence in the seeds of a ferment capable of splitting up the oil into fatty acid and glycerol.

The first experimenters to suggest any industrial application of this enzymic hydrolysis were Connstein, Hoyer and Wartenburg, who (Berichte, 1902, 35, pp. 3988-4006) published the results of a lengthy investigation of the whole subject. They found that tallow, cotton-seed, palm, olive, almond, and many other oils, were readily hydrolysed by the castor-seed ferment in the presence of dilute acid, but that cocoa-nut and palm-kernel oils only decomposed with difficulty. The presence of acidity is essential for the hydrolysis to take place, the most suitable strength being one-tenth normal, and the degree of hydrolysis is proportional to the quantity of ferment present. Sulphuric, phosphoric, acetic or butyric acids, or sodium bisulphate, may be used without much influence on the result. Butyric acid is stated to be the best, but in practice is too expensive, and acetic acid is usually adopted. The emulsified mixture should be allowed to stand for twenty-four hours, and the temperature should not exceed 40° C.; at 50° C. the action is weakened, and at 100° C. ceases altogether.

Several investigators have since examined the hydrolysing power of various other seeds, notably Braun and Behrendt (Berichte, 1903, 36, 1142-1145, 1900-1901, and 3003-3005), who, in addition to confirming Connstein, Hoyer and Wartenburg's work with castor seeds, have made similar experiments with jequirity seeds (Abrus peccatorius) containing the enzyme abrin, emulsin from crushed almonds, the leaves of Arctostaphylos Uva Ursi, containing the glucoside arbutin, myrosin from black mustard-seed, gold lac (Cheirantus cheiri) and crotin from croton seeds. Jequirity seeds were found to have a stronger decomposing action on lanoline and carnauba wax than the castor seed, but only caused decomposition of castor oil after the initial acidity was first neutralised with alkali. Neither emulsin, arbutin nor crotin have any marked hydrolytic action on castor oil, but myrosin is about half as active as castor seeds, except in the presence of potassium myronate, when no decomposition occurs.

S. Fokin (J. russ. phys. chem. Ges., 35, 831-835, and Chem. Rev. Fett. u. Harz. Ind., 1904, 30 et seq.) has examined the hydrolytic action of a large number of Russian seeds, belonging to some thirty different families, but although more than half of these brought about the hydrolysis of over 10 per cent. of fat, he considers that in only two cases, viz., the seeds of Chelidonium majus and Linaria vulgaris, is the action due to enzymes, these being the only two seeds for which the yield of fatty acids is proportional to the amount of seed employed, while in many instances hydrolysis was not produced when the seeds were old. The seeds of Chelidonium majus were found to have as great, and possibly greater, enzymic activity than castor seeds, but those of Linaria are much weaker, twenty to thirty parts having only the same lipolytic activity as four to five parts of castor seeds.

The high percentage of free acids found in rice oil has led C. A. Brown, jun. (Journ. Amer. Chem. Soc., 1903, 25, 948-954), to examine the rice bran, which proves to have considerable enzymic activity, and rapidly effects the hydrolysis of glycerides.

The process for the utilisation of enzymic hydrolysis in the separation of fatty acids from glycerine on the industrial scale, as originally devised by Connstein and his collaborators, consisted in rubbing a quantity of the coarsely crushed castor seeds with part of the oil or fat, then adding the rest of the oil, together with acidified water (N/10 acetic acid). The quantities employed were 6-1/2 parts of decorticated castor beans for every 100 parts of oil or fat, and 50 to 60 parts of acetic acid. After stirring until an emulsion is formed, the mixture is allowed to stand for twenty-four hours, during which hydrolysis takes place. The temperature is then raised to 70°-80° C., which destroys the enzyme, and a 25 per cent. solution of sulphuric acid, equal in amount to one-fiftieth of the total quantity of fat originally taken, added to promote separation of the fatty acids. In this way three layers are formed, the one at the top consisting of the clear fatty acids, the middle one an emulsion containing portions of the seeds, fatty acids and glycerine, and the bottom one consisting of the aqueous glycerine. The intermediate layer is difficult to treat satisfactorily; it is generally washed twice with water, the washings being added to glycerine water, and the fatty mixture saponified and the resultant soap utilised.

The process has been the subject of a considerable amount of investigation, numerous attempts having been made to actually separate the active fat-splitting constituent of the seeds, or to obtain it in a purer and more concentrated form than is furnished by the seeds themselves. Nicloux (Comptes Rendus, 1904, 1112, and Roy. Soc. Proc., 1906, 77 B, 454) has shown that the hydrolytic activity of castor seeds is due entirely to the cytoplasm, which it is possible to separate by mechanical means from the aleurone grains and all other cellular matter. This active substance, which he terms "lipaseidine," is considered to be not an enzyme, though it acts as such, following the ordinary laws of enzyme action; its activity is destroyed by contact with water in the absence of oil. This observer has patented (Eng. Pat. 8,304, 1904) the preparation of an "extract" by triturating crushed castor or other seeds with castor oil, filtering the oily extract, and subjecting it to centrifugal force. The deposit consists of aleurone and the active enzymic substance, together with about 80 per cent. of oil, and one part of it will effect nearly complete hydrolysis of 100 parts of oil in twenty-four hours. In a subsequent addition to this patent, the active agent is separated from the aleurone by extraction with benzene and centrifugal force. By the use of such an extract, the quantity of albuminoids brought into contact with the fat is reduced to about 10 per cent. of that in the original seeds, and the middle layer between the glycerine solution and fatty acids is smaller and can be saponified directly for the production of curd soap, while the glycerine solution also is purer.

In a further patent Nicloux (Fr. Pat. 349,213, 1904) states that the use of an acid medium is unnecessary, and claims that even better results are obtained by employing a neutral solution of calcium sulphate containing a small amount of magnesium sulphate, the proportion of salts not exceeding 0.5 per cent. of the fat, while in yet another patent, jointly with Urbain (Fr. Pat. 349,942, 1904), it is claimed that the process is accelerated by the removal of acids from the oil or fat to be treated, which may be accomplished by either washing first with acidulated water, then with pure water, or preferably by neutralising with carbonate of soda and removing the resulting soap.

Lombard (Fr. Pat. 350,179, 1904) claims that acids act as stimulating agents in the enzymic hydrolysis of oils, and further that a simple method of obtaining the active product is to triturate oil cake with its own weight of water, allow the mixture to undergo spontaneous proteolytic hydrolysis at 40° C. for eight days, and then filter, the filtrate obtained being used in place of water in the enzymic process.

Hoyer, who has made a large number of experiments in the attempt to isolate the lipolytic substance from castor seeds, has obtained a product of great activity, which he terms "ferment-oil," by extracting the crushed seeds with a solvent for oils.

The Verein Chem. Werke have extended their original patent (addition dated 11th December, 1905, to Fr. Pat. 328,101, Oct., 1902), which now covers the use of vegetable ferments in the presence of water and manganese sulphate or other metallic salt. It is further stated that acetic acid may be added at the beginning of the operation, or use may be made of that formed during the process, though in the latter case hydrolysis is somewhat slower.

Experiments have been carried out by Lewkowitsch and Macleod (Journ. Soc. Chem. Ind., 1903, 68, and Proc. Roy. Soc., 1903, 31) with ferments derived from animal sources, viz., lipase from pig's liver, and steapsin from the pig or ox pancreas. The former, although it has been shown by Kastle and Loevenhart (Amer. Chem. Journ., 1900, 49) to readily hydrolyse ethyl butyrate, is found to have very little fat-splitting power, but with steapsin more favourable results have been obtained, though the yield of fatty acids in this case is considerably inferior to that given by castor seeds. With cotton-seed oil, 83-86 per cent. of fatty acids were liberated as a maximum after fifty-six days, but with lard only 46 per cent. were produced in the same time. Addition of dilute acid or alkali appeared to exert no influence on the decomposition of the cotton-seed oil, but in the case of the lard, dilute alkali seemed at first to promote hydrolysis, though afterwards to retard it.

Fokin (Chem. Rev. Fett. u. Harz. Ind., 1904, 118-120 et seq.) has attempted to utilise the pancreatic juice on a technical scale, but the process proved too slow and too costly to have any practical use.

Rancidity.—The hydrolysing power of enzymes throws a good deal of light on the development of rancidity in oils and fats, which is now generally regarded as due to the oxidation by air in the presence of light and moisture of the free fatty acids contained by the oil or fat. It has long been known that whilst recently rendered animal fats are comparatively free from acidity, freshly prepared vegetable oils invariably contain small quantities of free fatty acid, and there can be no doubt that this must be attributed to the action of enzymes contained in the seeds or fruit from which the oils are expressed, hence the necessity for separating oils and fats from adhering albuminous matters as quickly as possible.

Decomposition of Fats by Bacteria.—Though this subject is not of any practical interest in the preparation of fatty acids for soap-making, it may be mentioned, in passing, that some bacteria readily hydrolyse fats. Schriber (Arch. f. Hyg., 41, 328-347) has shown that in the presence of air many bacteria promote hydrolysis, under favourable conditions as to temperature and access of oxygen, the process going beyond the simple splitting up into fatty acid and glycerol, carbon dioxide and water being formed. Under anærobic conditions, however, only a slight primary hydrolysis was found to take place, though according to Rideal (Journ. Soc. Chem. Ind., 1903, 69) there is a distinct increase in the amount of free fatty acids in a sewage after passage through a septic tank.

Experiments have also been made on this subject by Rahn (Centralb. Bakteriol, 1905, 422), who finds that Penicillium glaucum and other penicillia have considerable action on fats, attacking the glycerol and lower fatty acids, though not oleic acid. A motile bacillus, producing a green fluorescent colouring matter, but not identified, had a marked hydrolytic action and decomposed oleic acid. The name "lipobacter" has been proposed by De Kruyff for bacteria which hydrolyse fats.

III. Use of Chemical Reagents.—Among the chief accelerators employed in the hydrolysis of oils are sulphuric acid and Twitchell's reagent (benzene- or naphthalene-stearosulphonic acid), while experiments have also been made with hydrochloric acid (Journ. Soc. Chem. Ind., 1903, 67) with fairly satisfactory results, and the use of sulphurous acid, or an alkaline bisulphite as catalyst, has been patented in Germany. To this class belong also the bases, lime, magnesia, zinc oxide, ammonia, soda and potash, though these latter substances differ from the former in that they subsequently combine with the fatty acids liberated to form soaps.

Sulphuric Acid.—The hydrolysing action of concentrated sulphuric acid upon oils and fats has been known since the latter part of the eighteenth century, but was not applied on a practical scale till 1840 when Gwynne patented a process in which sulphuric acid was used to liberate the fatty acids, the latter being subsequently purified by steam distillation. By this method, sulpho-compounds of the glyceride are first formed, which readily emulsify with water, and, on treatment with steam, liberate fatty acids, the glycerol remaining partly in the form of glycero-sulphuric acid. The process has been investigated by Fremy, Geitel, and more recently by Lewkowitsch (J. Soc. of Arts, "Cantor Lectures," 1904, 795 et seq.), who has conducted a series of experiments on the hydrolysis of tallow with 4 per cent. of sulphuric acid of varying strengths, containing from 58 to 90 per cent. sulphuric acid, H₂SO₄. Acid of 60 per cent. or less appears to be practically useless as a hydrolysing agent, while with 70 per cent. acid only 47.7 per cent. fatty acids were developed after twenty-two hours' steaming, and with 80 and 85 per cent. acid, the maximum of 89.9 per cent. of fatty acids was only reached after fourteen and fifteen hours' steaming respectively. Using 98 per cent. acid, 93 per cent. of fatty acids were obtained after nine hours' steaming, and after another seven hours, only 0.6 per cent. more fatty acids were produced. Further experiments have shown that dilute sulphuric acid has also scarcely any action on cotton-seed, whale, and rape oils.

According to Lant Carpenter, some 75 per cent. of solid fatty acids may be obtained from tallow by the sulphuric acid process, owing to the conversion of a considerable quantity of oleic acid into isoleic acid (vide p. 12), but in the process a considerable proportion of black pitch is obtained. C. Dreymann has recently patented (Eng. Pat. 10,466, 1904) two processes whereby the production of any large amount of hydrocarbons is obviated. In the one case, after saponification with sulphuric acid, the liberated fatty acids are washed with water and treated with an oxide, carbonate, or other acid-fixing body, e.g., sodium carbonate, prior to distillation. In this way the distillate is much clearer than by the ordinary process, and is almost odourless, while the amount of unsaponifiable matter is only about 1.2 per cent. The second method claimed consists in the conversion of the fatty acids into their methyl esters by treatment with methyl alcohol and hydrochloric acid gas, and purification of the esters by steam distillation, the pure esters being subsequently decomposed with superheated steam, in an autoclave, with or without the addition of an oxide, e.g., 0.1 per cent. zinc oxide, to facilitate their decomposition.

Twitchell's Reagent.—In Twitchell's process use is made of the important discovery that aqueous solutions of fatty aromatic sulphuric acids, such as benzene- or naphthalene-stearosulphonic acid, readily dissolve fatty bodies, thereby facilitating their dissociation into fatty acids and glycerol. These compounds are stable at 100° C., and are prepared by treating a mixture of benzene or naphthalene and oleic acid with an excess of sulphuric acid, the following reaction taking place:—

C₆H₆ + C₁₈H₃₄O₂ + H₂SO₄ = C₆H₄(SO₃H)C₁₈H₃₅O + H₂O.

On boiling the resultant product with water two layers separate, the lower one consisting of a clear aqueous solution of sulphuric acid and whatever benzene-sulphonic acid has been formed, while the upper layer, which is a viscous oil, contains the benzene-stearosulphonic acid. This, after washing first with hydrochloric acid and then rapidly with petroleum ether, and drying at 100° C. is then ready for use; the addition of a small quantity of this reagent to a mixture of fat (previously purified) and water, agitated by boiling with open steam, effects almost complete separation of the fatty acid from glycerol.

The process is generally carried out in two wooden vats, covered with closely fitting lids, furnished with the necessary draw-off cocks, the first vat containing a lead coil and the other a brass steam coil.

In the first vat, the fat or oil is prepared by boiling with 1 or 2 per cent. of sulphuric acid (141° Tw. or 60° B.) for one or two hours and allowed to rest, preferably overnight; by this treatment the fat is deprived of any dirt, lime or other impurity present. After withdrawing the acid liquor, the fat or oil is transferred to the other vat, where it is mixed with one-fifth of its bulk of water (condensed or distilled), and open steam applied. As soon as boiling takes place, the requisite amount of reagent is washed into the vat by the aid of a little hot water through a glass funnel, and the whole is boiled continuously for twelve or even twenty-four hours, until the free fatty acids amount to 85-90 per cent. The amount of reagent used varies with the grade of material, the smaller the amount consistent with efficient results, the better the colour of the finished product; with good material, from 1/2 to 3/4 per cent. is sufficient, but for materials of lower grade proportionately more up to 2 per cent. is required. The reaction appears to proceed better with materials containing a fair quantity of free acidity.

When the process has proceeded sufficiently far, the boiling is stopped and free steam allowed to fill the vat to obviate any discoloration of the fatty acids by contact with the air, whilst the contents of the vat settle.

The settled glycerine water, which should amount in bulk to 50 or 60 per cent. of the fatty matter taken, and have a density of 7-1/2° Tw. (5° B.), is removed to a receptacle for subsequent neutralisation with milk of lime, and, after the separation of sludge, is ready for concentration.

The fatty acids remaining in the vat are boiled with a small quantity (0.05 per cent., or 1/10 of the Twitchell reagent requisite) of commercial barium carbonate, previously mixed with a little water; the boiling may be prolonged twenty or thirty minutes, and at the end of that period the contents of the vat are allowed to rest; the water separated should be neutral to methyl-orange indicator.

It is claimed that fatty acids so treated are not affected by the air, and may be stored in wooden packages.

Hydrochloric Acid.—Lewkowitsch (Journ. Soc. Chem. Ind., 1903, 67) has carried out a number of experiments on the accelerating influence of hydrochloric acid upon the hydrolysis of oils and fats, which show that acid of a specific gravity of 1.16 has a very marked effect on most oils, cocoa-nut, cotton-seed, whale and rape oils, tallow and lard being broken up into fatty acid and glycerol to the extent of some 82-96 per cent. after boiling 100 grams of the oil or fat with 100 c.c. of acid for twenty-four hours. The maximum amount of hydrolysis was attained with cocoa-nut oil, probably owing to its large proportion of the glycerides of volatile fatty acids. Castor oil is abnormal in only undergoing about 20 per cent. hydrolysis, but this is attributed to the different constitution of its fatty acids, and the ready formation of polymerisation products. Experiments were also made as to whether the addition of other catalytic agents aided the action of the hydrochloric acid; mercury, copper sulphate, mercury oxide, zinc, zinc dust, aluminium chloride, nitrobenzene and aniline being tried, in the proportion of 1 per cent. The experiments were made on neutral lard and lard containing 5 per cent. of free fatty acids, but in no case was any appreciable effect produced.

So far this process has not been adopted on the practical scale, its chief drawback being the length of time required for saponification. Undoubtedly the hydrolysis would be greatly facilitated if the oil and acid could be made to form a satisfactory emulsion, but although saponin has been tried for the purpose, no means of attaining this object has yet been devised.

Sulphurous Acid or Bisulphite.—The use of these substances has been patented by Stein, Berge and De Roubaix (Germ. Pat. 61,329), the fat being heated in contact with the reagent for about nine hours at 175°-180° C. under a pressure of some 18 atmospheres, but the process does not appear to be of any considerable importance.

Lime.—The use of lime for the saponification of oils and fats was first adopted on the technical scale for the production of candle-making material, by De Milly in 1831. The insoluble lime soap formed is decomposed by sulphuric acid, and the fatty acids steam distilled.

The amount of lime theoretically necessary to hydrolyse a given quantity of a triglyceride, ignoring for the moment any catalytic influence, can be readily calculated; thus with stearin the reaction may be represented by the equation:—

CH₂OOC₁₈H₃₅						CH₂OH
\|						\|
2CHOOC₁₈H₃₅	+	3Ca(OH)₂	=	3Ca(OOC₁₈H₃₅)₂	+	2CHOH
\|						\|
CH₂OOC₁₈H₃₅						CH₂OH
stearin		milk of lime		calcium stearate		glycerol

In this instance, since the molecular weight of stearin is 890 and that of milk of lime is 74, it is at once apparent that for every 1,780 parts of stearin, 222 parts of milk of lime or 168 parts of quick-lime, CaO, would be required. It is found in practice, however, that an excess of 3-5 per cent. above the theoretical quantity of lime is necessary to complete the hydrolysis of a fat when carried on in an open vessel at 100°-105° C., but that if the saponification be conducted under pressure in autoclaves the amount of lime necessary to secure almost perfect hydrolysis is reduced to 2-3 per cent. on the fat, the treatment of fats with 3 per cent. of lime under a pressure of 10 atmospheres producing a yield of 95 per cent. of fatty acids in seven hours. The lower the pressure in the autoclave, the lighter will be the colour of the resultant fatty acids.

Magnesia.—It has been proposed to substitute magnesia for lime in the process of saponification under pressure, but comparative experiments with lime and magnesia, using 3 per cent. of lime and 2.7 per cent. of magnesia (Journ. Soc. Chem. Ind., xii., 163), show that saponification by means of magnesia is less complete than with lime, and, moreover, the reaction requires a higher temperature and therefore tends to darken the product.

Zinc Oxide.—The use of zinc oxide as accelerating agent has been suggested by two or three observers. Poullain and Michaud, in 1882, were granted a patent for this process, the quantity of zinc oxide recommended to be added to the oil or fat being 0.2 to 0.5 per cent. Rost, in 1903, obtained a French patent for the saponification of oils and fats by steam under pressure in the presence of finely divided metals or metallic oxides, and specially mentions zinc oxide for the purpose.

It has also been proposed to use zinc oxide in conjunction with lime in the autoclave to obviate to some extent the discoloration of the fatty acids.

Other catalytic agents have been recommended from time to time, including strontianite, lead oxide, caustic baryta, aluminium hydrate, but none of these is of any practical importance.

Soda and Potash.—Unlike the preceding bases, the soaps formed by soda and potash are soluble in water, and constitute the soap of commerce. These reagents are always used in sufficient quantity to combine with the whole of the fatty acids contained in an oil or fat, though doubtless, by the use of considerably smaller quantities, under pressure, complete resolution of the fatty matter into fatty acids and glycerol could be accomplished. They are, by far, the most important saponifying agents from the point of view of the present work, and their practical use is fully described in Chapter V.

CONSTITUTION OF OILS AND FATS, AND THEIR SAPONIFICATION.

CHAPTER III.