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:--
CH_{2}OOC_{18}H_{35} CH_{2}OH
| |
CHOOC_{18}H_{35} + 3NaOH = 3NaOOC_{18}H_{35} + CHOH
| |
CH_{2}OOC_{18}H_{35} CH_{2}OH
stearin sodium sodium glycerol
hydroxide stearate
CH_{2}OOC_{16}H_{31} CH_{2}OH
| |
CHOOC_{16}H_{31} + 3NaOH = 3NaOOC_{16}H_{31} + CHOH
| |
CH_{2}OOC_{16}H_{31} CH_{2}OH
palmitin sodium sodium glycerol
hydroxide palmitate
CH_{2}OOC_{18}H_{33} CH_{2}OH
| |
CHOOC_{18}H_{33} + 3NaOH = 3NaOOC_{18}H_{33} + CHOH
| |
CH_{2}OOC_{18}H_{33} CH_{2}OH
olein sodium sodium glycerol
hydroxide oleate
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:--
3C_{18}H_{35}O_{2}H + C_{3}H_{5}(OH)_{3} = C_{3}H_{5}(C_{18}H_{35}O_{2})_{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) C_{18}H_{35}O_{2}H + C_{3}H_{5}(OH)_{3} =
C_{3}H_{5}(OH)_{2}C_{18}H_{35}O_{2} + H_{2}O
monostearin
(2) C_{18}H_{35}O_{2}H + C_{3}H_{5}(OH)_{2}C_{18}H_{35}O_{2} =
C_{3}H_{5}(OH)(C_{18}H_{35}O_{2})_{2} + H_{2}O
distearin
(3) C_{18}H_{35}O_{2}H + C_{3}H_{5}(OH)(C_{18}H_{35}O_{2})_{2} =
C_{3}H_{5}(C_{18}H_{35}O_{2})_{3} + H_{2}O
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_:--
CH_{2}OR CH_{2}OH
| |
(alpha) CHOH and (beta) CHOR
| |
CH_{2}OH CH_{2}OH
_Diglyceride_:--
CH_{2}OR CH_{2}OR
| |
(alpha) CHOH and (beta) CHOR
| |
CH_{2}OR CH_{2}OH
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--
CH_{2}OR
|
CHOR
|
CH_{2}OR
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,
C_{3}H_{5}(OC_{18}H_{35}O)(OC_{18}H_{35}O)_{2}, obtained by Heise in 1896
Mkani fat. Hansen has since found that tallow contains oleodipalmitin,
from C_{3}H_{5}(OC_{18}H_{35}O)(OC_{16}H_{31}O), stearodipalmitin,
C_{3}H_{5}(OC_{18}H_{35}O)(OC_{16}H_{31}O), oleopalmitostearin,
C_{3}H_{5}(OC_{18}H_{33}O)(OC_{16}H_{31}O)(OC_{18}H_{35}O) and
palmitodistearin, CH(OC_{16}H_{31}O)(OC_{18}H_{35}O)_{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, C_{3}H_{5}(OC_{18}H_{33}O)(OC_{17}H_{33}O)_{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:--
[Transcriber's note: Table split to fit on page better.]
---------------------------------------------------------------------
Glyceride. | Formula. | Chief Occurrence.
---------------------------------------------------------------------
Butyrin | C_{3}H_{5}(O.C_{4}H_{7}O)_{3} | Butter fat
---------------------------------------------------------------------
Isovalerin | C_{3}H_{5}(O.C_{5}H_{9}O)_{3} | Porpoise, dolphin
---------------------------------------------------------------------
Caproin | C_{3}H_{5}(O.C_{6}H_{11}O)_{3} | Cocoa-nut and
| | palm-nut oils
---------------------------------------------------------------------
Caprylin | C_{3}H_{5}(O.C_{8}H_{15}O)_{3} | Do. do.
---------------------------------------------------------------------
Caprin | C_{3}H_{5}(O.C_{10}H_{19}O)_{3} | Do. do.
---------------------------------------------------------------------
Laurin | C_{3}H_{5}(O.C_{12}H_{23}O)_{3} | Do. do.
---------------------------------------------------------------------
Myristin | C_{3}H_{5}(O.C_{14}H_{27}O)_{3} | Nutmeg butter
---------------------------------------------------------------------
Palmitin | C_{3}H_{5}(O.C_{16}H_{31}O)_{3} | Palm oil, lard
---------------------------------------------------------------------
Stearin | C_{3}H_{5}(O.C_{18}H_{35}O)_{3} | Tallow, lard,
| | cacao butter
---------------------------------------------------------------------
Olein | C_{3}H_{5}(O.C_{18}H_{33}O)_{3} | Olive and
| | almond oils
---------------------------------------------------------------------
Ricinolein | C_{3}H_{5}(O.C_{18}H_{33}O_{2})_{3} | Castor oil
---------------------------------------------------------------------
---------------------------------------------------------------------
Glyceride. | Melting | Refractive | Saponification
| Point, °C. | Index at 60° C. | Equivalent.
---------------------------------------------------------------------
Butyrin | Liquid at -60 | 1.42015 | 100.7
---------------------------------------------------------------------
Isovalerin | ... | ... | 114.7
---------------------------------------------------------------------
Caproin | -25 | 1.42715 | 128.7
---------------------------------------------------------------------
Caprylin | -8.3 | 1.43316 | 156.7
---------------------------------------------------------------------
Caprin | 31.1 | 1.43697 | 184.7
---------------------------------------------------------------------
Laurin | 45 | 1.44039 | 212.7
---------------------------------------------------------------------
Myristin | 56.5 | 1.44285 | 240.7
---------------------------------------------------------------------
Palmitin | 63-64 | ... | 268.7
---------------------------------------------------------------------
Stearin | 71.6 | ... | 296.7
---------------------------------------------------------------------
Olein | Solidifies at -6 | ... | 294.7
---------------------------------------------------------------------
Ricinolein | ... | ... | 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, _d_60°/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_{n}H_{2n+1}COOH
II. Oleic series C_{n}H_{2n-1}COOH
III. Linolic series C_{n}H_{2n-3}COOH
IV. Linolenic series C_{n}H_{2n-5}COOH
V. Ricinoleic series C_{n}H_{2n-7}COOH
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 | Found in
| | Point, |
| | °C. |
-------------------------------------------------------------------------------
Acetic | CH_{3}COOH | 17 | Macassar oil.
------------------------------------------------------------------------------
Butyric | C_{3}H_{7}COOH | ... | Butter, Macassar oil.
------------------------------------------------------------------------------
Isovaleric | C_{4}H_{9}COOH | ... | Porpoise and dolphin oils.
------------------------------------------------------------------------------
Caproic | C_{5}H_{11}COOH | ... | Butter, cocoa-nut oil.
------------------------------------------------------------------------------
Caprylic | C_{7}H_{15}COOH | 15 | Butter, cocoa-nut oil,
| | | Limburg cheese.
------------------------------------------------------------------------------
Capric | C_{9}H_{19}COOH | 30 | Butter, cocoa-nut oil.
------------------------------------------------------------------------------
Lauric | C_{11}H_{23}COOH | 44 | Cocoa-nut oil, palm-kernel oil.
------------------------------------------------------------------------------
Ficocerylic | C_{12}H_{25}COOH | ... | Pisang wax.
------------------------------------------------------------------------------
Myristic | C_{13}H_{27}COOH | 54 | Nutmeg butter, liver fat,
| | | cocoa-nut oil, dika fat,
| | | croton oil.
------------------------------------------------------------------------------
Palmitic | C_{15}H_{31}COOH | 62.5 | Palm oil, most animal fats.
------------------------------------------------------------------------------
Daturic | C_{16}H_{33}COOH | | Oil of Datura Stramonium.
------------------------------------------------------------------------------
Stearic | C_{17}H_{35}COOH | 69 | Tallow, lard, most solid
| | | animal fats.
------------------------------------------------------------------------------
Arachidic | C_{19}H_{39}COOH | 75 | Arachis or earth-nut oil,
| | | rape and mustard-seed oils.
------------------------------------------------------------------------------
Behenic | C_{21}H_{43}COOH | ... | Ben oil, black mustard-seed
| | | oil, rape oil.
------------------------------------------------------------------------------
Lignoceric | C_{23}H_{47}COOH | 80.5 | Arachis oil.
------------------------------------------------------------------------------
Carnaubic | C_{23}H_{47}COOH | ... | Carnauba wax.
------------------------------------------------------------------------------
Pisangcerylic | C_{23}H_{47}COOH | ... | Pisang wax.
------------------------------------------------------------------------------
Hyænic | C_{24}H_{49}COOH | ... | Hyæna fat.
------------------------------------------------------------------------------
Cerotic | C_{25}H_{51}COOH | 78 | Beeswax, China wax, spermaceti.
------------------------------------------------------------------------------
Melissic | C_{29}H_{59}COOH | 89 | Beeswax.
------------------------------------------------------------------------------
Psyllostearylic| C_{32}H_{65}COOH | ... | Psylla wax.
------------------------------------------------------------------------------
Theobromic | C_{63}H_{127}COOH | ... | 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 | Found in
| | Point, |
| | °C. |
--------------------------------------------------------------------------
Tiglic | C_{4}H_{7}COOH | 64.5 | Croton oil.
--------------------------------------------------------------------------
Moringic | C_{14}H_{27}COOH | 0 | Ben oil.
--------------------------------------------------------------------------
Physetoleic | C_{15}H_{29}COOH | 30 | Sperm oil.
--------------------------------------------------------------------------
Hypogæic | C_{15}H_{29}COOH | 33 | Arachis and maize oils.
--------------------------------------------------------------------------
Oleic | C_{17}H_{33}COOH | 14 | Most oils and fats.
--------------------------------------------------------------------------
Rapic | C_{17}H_{33}COOH | ... | Rape oil.
--------------------------------------------------------------------------
Doeglic | C_{18}H_{35}COOH | ... | Bottle-nose oil.
--------------------------------------------------------------------------
Erucic | C_{21}H_{41}COOH | 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--
C_{18}H_{34}O_{2} + 2KOH = KC_{2}H_{3}O_{2} + KC_{16}H_{31}O_{2} + H_{2},
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 | Found in
| | Point, |
| | °C. |
--------------------------------------------------------------------------
Elæomargaric | C_{16}H_{29}COOH | ... | Chinese-wood oil.
--------------------------------------------------------------------------
Elæostearic | C_{16}H_{29}COOH | 71 | Chinese-wood oil.
--------------------------------------------------------------------------
Linolic | C_{17}H_{31}COOH | Fluid | Linseed, cotton-seed and
| | | maize oils.
--------------------------------------------------------------------------
Tariric | C_{17}H_{31}COOH | 50.5 | Tariri-seed oil.
--------------------------------------------------------------------------
Telfairic | C_{17}H_{31}COOH | 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 | C_{17}H_{29}COOH | Linseed oil.
--------------------------------------------------------------------
Isolinolenic | C_{17}H_{29}COOH | Linseed oil.
--------------------------------------------------------------------
Jecoric | C_{17}H_{29}COOH | 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 | Found in |
| | | Point, | |
| | | °C. | |
|------------|----------------------|---------|-------------|
| | | | |
| Ricinoleic | C_{17}H_{22}(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 [alpha]_{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)--
C_{3}H_{5}(OR) + 3NaOH = C_{3}H_{5}(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.) C_{3}H_{5} | OR + NaOH = C_{3}H_{5} | OR + RONa
|_OR |_OR
_ _
| OH | OH
(ii.) C_{3}H_{5} | OR + NaOH = C_{3}H_{5} | OR + RONa
|_OR |_OH
_ _
| OH | OH
(iii.) C_{3}H_{5} | OR + NaOH = C_{3}H_{5} | 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--
C_{3}H_{5}(OR)_{3} + 3H.OH = C_{3}H_{5}(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, H_{2}SO_{4}. 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:--
C_{6}H_{6} + C_{18}H_{34}O_{2} + H_{2}SO_{4} =
C_{6}H_{4}(SO_{3}H)C_{18}H_{35}O + H_{2}O.
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:--
CH_{2}OOC_{18}H_{35} CH_{2}OH
| |
2CHOOC_{18}H_{35} + 3Ca(OH)_{2} = 3Ca(OOC_{18}H_{35})_{2} + 2CHOH
| |
CH_{2}OOC_{18}H_{35} CH_{2}OH
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.