C. CONCENTRATED, TERPENELESS AND SESQUITERPENELESS ESSENTIAL OILS
Most essential oils consist of mixtures of hydrocarbons
(terpenes, sesquiterpenes, etc.), oxygenated compounds (alcohols, esters,
ethers, aldehydes, ketones, lactones, phenols, phenol ethers, etc.), and a small
percentage of viscid or solid nonvolatile residues (paraffins, waxes, etc.). Of
these the oxygenated compounds are the principal odor carriers, although the terpenes
and sesquiterpenes, too, contribute in some degree to the total odor and flavor
value of the oil. The oxygenated substances possess the added advantage of
better solubility in dilute alcohol and, with the exception of some aldehydes, of
greater stability against oxidizing and resinifying influences. Due to their unsaturated
character, the terpenes and sesquiterpenes oxidize and resinify easily under the
influence of air and light or under improper storing conditions which means spoilage
of odor and flavor, and lowering of the solubility in alcohol.
For many years, therefore, it has been the endeavor
of the essential oil industry to supply the users with concentrated, terpeneless
and sesquiterpeneless oils. Such oils consist mainly of oxygenated compounds; they
are more soluble, more stable, and much stronger in odor, yet retain most of the
odor and flavor characteristics of the original oil.
The degree of concentration is automatically limited
by the amount of oxygenated compounds present in the natural oil. For example,
an orange oil containing only 2 per cent of oxygenated constituents and 98 per
cent of terpenes, sesquiterpenes and waxes can, theoretically, be concentrated fifty
times at the most, whereas a bergamot oil containing 50 per cent esters, alcohols,
lactones, etc., and 50 per cent hydrocarbons can be concentrated only to double
strength.
Before discussing in more detail the methods of
manufacturing these concent rated, torpcnclcss and sesquiterpenoless oils, we should
point out for clarity's sake that they must not be confused with the so-called isolates
or aromatic isolates, or commonly but incorrectly called "synthetics"
which are isolated from certain essential oils. For instance, citral can be isolated
by fractionation or by chemical means from lemongrass oil, eugenol from clove oil,
safrol from sassafras oil or camphor oil fractions, citronellal from citronella
oil. These isolates may be converted chemically into other compounds, real synthetics,
viz., citral into ionones, eugenol into vanillin, safrol into holiotropin, citronellal
into citronellol, citronellyl acetate, hydroxycitronellal or synthetic menthol.
Terpeneless and sesquiterpeneless oils have nothing to do with these isolates as
the latter consist usually of only one well defined chemical substance, while
the former are composed of several, often many, oxygenated compounds as present
in the normal essential oil.
Because of the different composition, the
deterpcnation of each essential oil requires a special process. The general method
is based upon two principles: (a) removal of the terpenes, sesquiterpenes and paraffins
by fractional distillation in vacuo or (b) by extraction of the more soluble oxygenated
compounds with dilute alcohol or other solvents. In many cases, especially with
citrus oils, a combination of the two methods may be employed.
The commercial term "sesquiterpeneless"
oils conventionally includesalso the terpeneless oils. In some cases, especially
when the content of sesquiterpenes in the natural oil is small, the two terms are
employed synonymously. The trade designations and the names of the many brands on
the market, however, are not always correct from the scientific point of view.
It would be more appropriate to name these products
"Concentrated Oils," "Terpeneless Oils," and "Terpeneless
and Sesquiterpeneless Oils."
"Concentrated oils" are those from
which only a part of the hydrocarbons have been removed. This can be done by simple
fractional distillation in vacuo. According to the process applied and the intended
concentration, a wide range of concentrated oils, with different properties, may
be obtained. Thus, we speak of a twofold lemon or orange oil, a fivefold oil, etc.
"Terpeneless oils" are those from which all or most of the terpenes and
waxes have been removed, usually by fractional distillation. "Terpeneless and
Sesquiterpeneless oils" are those from which the terpenes, the
sesquiterpenes and the waxes have been eliminated. The common manufacturing practice
is to distill off in vacuo first the terpenes, and then to extract the terpeneless
oil with dilute alcohol, or other solvents, whereby the sesquiterpenes and waxes
are eliminated ; or, the sesquiterpenes and waxes may be removed by further fractionation
of the terpeneless oil in vacuo. The resulting terpeneless and Sesquiterpeneless
oil represents the highest possible concentration of a natural essential oil.
The manufacture of these products requires that
the operator be well acquainted with the chemical composition, especially with the
boiling ranges of the various terpenes, sesquiterpenes and oxygenated compounds
occurring in the natural oil which he expects to concentrate. The boiling range
of terpenes varies in most cases from 150o to 180o at atmospheric
pressure; that of sesquiterpenes from 240o to 280o. The boiling
points of most oxygenated compounds (terpene alcohols, aldehydes, esters, etc.;
lie between those of the terpenes and sesquiterpenes. Phenols, phenol ethers, and
a few aromatic aldehydes form an exception, also the sesquiterpene alcohols, esters,
etc., their boiling range falling into that of the sesquiterpenes or above.
As far as solubility in dilute alcohol is concerned,
the terpenes are, in general, only sparingly soluble, the paraffins and sesquiterpenes
practically insoluble. The oxygenated compounds, on the other hand, possess in general
much better solubility : the alcohols, aldehydes, ketones, and phenols are most
soluble, the esters and phenol ethers somewhat less soluble.
As pointed out, the terpenes may be removed by
fractional distillation of the natural oil under reduced pressure. Most constituents
of essential oils being deleteriously affected by heat, the distillation temperature
must be kept as low as possible, which can be achieved with the aid of a good vacuum.
For best results a well-constructed fractionation still as described in the section
on "Distillation of Essential Oils" should be employed. It must be
equipped with an efficient fractionation column.
It should be borne in mind that the terpenes cannot
be removed quantitatively from a natural oil by mere fractional distillation; indeed,
one of the greatest disadvantages of fractional distillation lies in the incomplete
separation of the constituents, especially if their boiling points lie close
together. A typical example is lemon oil which, aside from citral, contains also
lower boiling aldehydes, such as octyl, nonyl and decyl aldehyde. If natural lemon
oil is fractionated at 2 mm. pressure, the lower boiling terpenes should come over
first, and theoretically the terpene fraction should contain no citral. However,
even with a very efficient fractionation column, the aldehyde content of the
terpene fraction will amount to about 1.0 per cent. The terpene fraction may be
refractionated, but it will still retain small quantities of aldehydes;
furthermore, repeated heating affects the flavor. Separation of the oxygenated compounds
by chemical means is limited to certain cases only.
Repeated fractionation results in several intermediary
fractions which consist of terpenes and a slight amount of oxygenated compounds,
the latter increasing in proportion as the distillation temperature rises. Fractionation
may be conducted in such a way that the residual oil is free from terpenes, but
in this case the residual oil will be deprived also of those portions of the oxygenated
constituents which have been carried over into the intermediary fractions. In
order to recover these compounds, it will be necessary to refractionate the
intermediary fractions, but, as said, prolonged heating is likely to have a deleterious
effect upon the odor and especially the flavor of the fractions. Fractionation may
be controlled by testing each fraction for solubility and for its rotatory
power.
The elimination of the sesquiterpenes
presents even more difficulties than that of the terpenes. In some cases the
sesquiterpenes may be separated from the terpeneless oils by mere fractionation
in vacuo, provided that the oils are not affected by the relatively high boiling
temperature required for the distillation of scsquiterpenes (about 120o-140o
at 10 mm. pressure) and by the partial overheating in the still which easily takes
place. A vacuum of 3 to 5 mm. is desirable. In this case, too, the manufacturer
must be familiar with the boiling points, at reduced pressure, of the various oil
constituents. In some cases the differential in the boiling points of two compounds,
as prevailing at atmospheric pressure, does not remain constant at reduced pressures;
it may even be reversed. Too, every fraction should be tested for its rotatory power
and for solubility in dilute alcohol, the insoluble ones to be rejected as
containing mainly sesquiterpenes. Refractionation of the rejected fractions may
be necessary. Even at a pressure of only 1 mm., a relatively high temperature is
required to distill over the oxygenated compounds, most of them boiling between
90o-110o. Moreover, the temperature in the still itself will
usually be about 10o and even 20o higher than the boiling
point of the liquid, and intense local heating occurs especially along the walls
of the still. All these factors tend to impart to the oil a note which the expert
easily recognizes as "distilled" or slightly "burnt," as it
does not occur in natural cold-pressed citrus oils, for example. Furthermore, the
influence of heat seems to decompose the so-called "molecular compounds"
which some authorities assume to occur in natural oils. It is a well-known fact
that, upon aging, the odor of a perfume or flavor mixture changes and improves considerably.
This may be caused by chemical reactions of functional groups for example, by the
interaction of alcohols and aldehydes which form acetals. Such compounds may exist
in the natural oil and be decomposed upon heating and distilling.
Another method of removing the high boiling sesquiterpenes
and waxes from the terpeneless oil consists in steam distilling the terpeneless
oil at reduced pressure. This process is more gentle than dry distillation in
vacua and leaves the high boiling sesquiterpenes anc! waxes as residues in the still.
In this case the distillate should be tested for solubility; any sesquiterpenes
distilled over may be removed by treating the fractions with dilute alcohol. This
method, however, has the inherent disadvantage that compared with dry vacuum distillation
it takes much longer, especially in the case of oils containing a large percentage
of high boiling compounds. Also, certain constituents of an oil are liable to dissolve
in the distillation water e.g., phenylethyl alcohol or eugenol. In this case
the distillation water has to be returned into the still for cohobation.
In view of these inadequacies, some manufacturers
remove all remaining terpenes and sesquiterpenes from concentrated oil by extracting
the latter with dilute alcohol. The strength of the alcohol to be employed for this
purpose depends primarily upon the solubility of the oxygenated compounds. Thus,
the concentrated oil from which most of the torpenes and sesquiterpenes have been
eliminated by fractionation in vacua or by steam distillation under reduced pressure
is shaken for some time with fifteen to twenty times its volume of dilute
alcohol, for instance, with 60 per cent alcohol by volume ; or the concentrated
oil is first dissolved in the corresponding volume of strong alcohol and then the
required amount of distilled water is gradually added with continuous stirring until
the desired degree of alcohol dilution is reached. In both cases the turbid mixture
should be cooled for a prolonged period and set aside until clarified. Thus, the
oxygenated constituents dissolve in the dilute alcohol, while the terpenes and sesquiterpenes
remain undissolved and (together with traces of oxygenated compounds) may be separated.
Because of the small differential in the specific
gravity of the undissolved parts of the oil and that of the solution, emulsions
may form and the separation of the two layers may require some time. In order
to break the emulsion, small quantities of low boiling petrol ether are added, or
the emulsion may be separated by centrifuging. The undissolved oil is repeatedly
treated with dilute alcohol in order to extract any quantities of oxygenated compounds
which it might still retain.
The clear solution of oxygenated compounds in
dilute alcohol is then transferred into a still and the alcohol fractionated off
at reduced pressure, until only oil and water remain in the still. The two layers
of oil and water can easily be separated. The employment of an efficient condenser
will prevent losses of alcohol. The recovered alcohol and the residual water may
be used again for treating the next batch of oil.
The literature on essential oils contains many
references to the preparing of terpeneless and sesquiterpeneless oils, one of the
most comprehensive ones being the paper by Littlejohn.46 As stated, no
standard method has been adopted yet and every manufacturer uses his own process.
Romeo47 reported that terpeneless and sesquitcrpeneless citrus oils are
manufactured in Sicily by first removing the terpenes by fractional distillation.
The sesquiterpenes are then eliminated from the terpeneless oil by extracting the
oil with dilute alcohol, the strength of which should be somewhat lower than that
in which the sesquiterpeneless oils must finally be soluble. The sesquiterpeneless
oil is separated from the alcoholic solution by the addition of water or by distilling
off the alcohol under reduced pressure. This constitutes the general method described
previously and with some modifications it forms, today, the basis of most commercial
processes.
A more novel method has been described and patented
by van Dijek and Ruys.48 In this process the natural oil is extracted
by two solvents which are only partially soluble in one another for instance, pentane
and dilute methyl alcohol. The two solvents are made to flow, according to the countercurrent
principle, through a horizontal glass cylinder and the oil is entered in the
middle. The terpenes dissolve in the pentane phase, the oxygenated compounds in
the methylalcohol phase. After separation of the two phases, the solvents are removed
by distillation, only low temperatures being necessary. This, according to the inventors,
is the principal advantage of thoir method, aside from the fact that high-grade
terpeneless oils are obtained in almost quantitative yield. The principal difficulties
of this process lie in the necessity of working with large volumes of solvents,
furthermore, in the tendency toward formation of emulsions which, however, might
be broken in some cases by the addition of 0.1 per cent of citric or tartaric acid.
After having discussed the various methods of
manufacturing terpeneless and sesquiterpenoless oils, it might be advisable to add
a few words about their concentration, as there exists a great deal of confusion
regarding this point. The price lists and tables on the concentration of terpeneless
and sesquiterpeneless oils issued by the various essential oil houses differ widely
in regard to their concentration value. Yet, the theoretical concentration could
be calculated only from the actual yield of terpeneless or sesquiterpeneless oil
as obtained from a given weight of natural oil.
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46 Flavours 3, No. 4, August (1940), 7.
47 La deterpenazione delk essenze di agrumi. Estr. dagli Atti del H. Congresso
Nazionak di Chimica pura ed applicata. Palermo, May (1926). Ber. Schimmel &
Co. (1928), 38.
48 Perfumery Essential Oil Record 28 (1937), 91.
However, the actual odor and flavor strength of
two oils, although of the same theoretical concentration, may differ, concentration
not being necessarily proportionate to odor and flavor strength. Let us assume,
for instance, that 100 kg. of natural lemon oil are converted into terpeneless oil
and that the yield is 8 kg. of terpeneless oil containing about 40 to 45 per
cent of citral. (Some citral has been destroyed by distillation and, besides, the
oxygenated compounds cannot be completely freed of terpenes.) In this case the
actual concentration of the oil, but not necessarily of the flavor, is obviously
twelve and one-half times.
It is difficult, if not impossible, to indicate
general and definite limits for the physicochemical properties of concentrated,
terpeneless and sesquiterpeneless oils because of the fact that these properties
depend upon the degree of concentration and upon the relative proportions of oxygenated
constituents originally present. Furthermore, every manufacturer has his own standards
which are based upon his particular manufacturing process. Littlejohn49
listed the physicochemical properties of more than fifty terpeneless and sesquiterpeneless
oils. Accordinging to this author, the specific gravity affords a valuable clue
to the presence of any remaining terpenes. These hydrocarbons possess a low specific
gravity and refractive index and their complete removal should raise the specific
gravity and refractive index of the terpeneless oil relative to that of the original
oil. The determination of the optical rotation, too, provides a good indication
regarding the extent to which the terpenes have been eliminated.
By far the most important criterion for a terpeneless
and sesquiterpeneless oil is its solubility in dilute alcohol, 70 per cent
ethyl alcohol usually being employed for this purpose. A terpeneless oil should
usually be soluble in 3 to 10 volumes of 70 per cent alcohol, while a sesquiterpeneless
oil should be more soluble.
Aside from the determination of the physicochemical
properties, it is advisable to test a terpeneless or sesquiterpeneless oil also
for its content of oxygenated compounds, especially for alcohols, esters and aldehydes,
which can be done by the usual analytical methods. Bocker60 suggested
a method of evaluating and examining terpeneless lemon oils which is based on treating
the aldehyde-free oil with 51 per cent alcohol to remove all oxygenated compounds,
and on measuring the quantity of terpenes and sesquiterpenes left. For this purpose,
10 cc. of the oil is first treated with a solution of neutral sodium sulfite which
removes the aldehydes. The remaining, unabsorbed oil is shaken with 100 times its
volume of 51 per cent alcohol in a separatory funnel, cooled to about 2 and left
for a period of 6 hr. or more, until the liquids have completely separated when
the lower layer can be removed.
------------------------------------------
49 Flavours 3, No. 4, August
(1940), 7.
50 J. prakt. Chem. [2], 89 (1914),
199; [2], 90 (1914), 393.
After washing the oil layer with a further quantity
of 51 per cent alcohol, all undissolved oil is transferred to a burette tube and
its volume carefully measured. From this amount the percentage of nonoxygenated
constituents of the original oil can be determined. In order to obtain more exact
results the terpeneless oil is first fractionated and the process applied to both
the first and the last fraction. Docker's method is not absolutely quantitative,
as some terpenes will dissolve in the weak spirit, also because the transfer of
the oils from the separatory funnel to the measuring burette always causes some
loss.
The main advantage of the terpeneless and^
especially of the sesquiterppnoless oils consists in their better solubility in
dilute alcohol. The employment of these oils, therefore, ejects a considerable saving
of alcohol in the finished goods; odor and flavor of the oil are better utilized.
A further advantage consists in the fact that, by the process of concentration,
the oils are also freed of any products by decomposition or resinification which
might result from improper handling or aging of the natural oils. Another merit
of the terpeneless and sesquiterpeneless oils lies in their better stability. While
natural citrus oils are apt to resinify, primarily due to polymerization of certain
hydrocarbons, the concentrated oils are much more stable. Thus, they may be employed
in powders, for the flavoring of gelatin desserts, for example, or for the
scenting of bath salts.
The introduction of terpeneless and sesquiterpeneless
oils on the market has met with some resistance. Several authorities contend that
the elimination of terpcnes and sesquiterpenes removes also a part of the
characteristic odor and flavor of the natural oil. The application of heat undoubtedly
has some effect on the delicate flavoring constituents of the oil and, if improperly
prepared, concentrated oils may not display the freshness and bouquet of the original
oil. Furthermore, the terpeneless and sesquiterpeneless oils contain a lower
proportion of natural fixatives such as waxes and stearoptenes which contribute
to the retaining of the flavor on the palate. Weighing the advantages against
the disadvantages, the conclusion may be drawn that concentrated, terpeneless and
sesquiterpeneless oils have their definite place in many formulas where highest
possible concentration, solubility, and stability are required, but that they cannot
replace the natural oils for all purposes.
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SUGGESTED ADDITIONAL LITERATURE
"Concentrated Citrus Oils," by A. H. Bennett, Perfumery Essential
Oil Record 25 (1934), 111.
"Preparation of Terpeneless Oils," by
A. M. Burger, Riechstoff Ind. 13 (1938), 217. Chem. Abstracts 33 (1939), 1089.
"Removal of Terpenes from Essential Oils,"
by Pietro Leone, Riv. ital, essenze profumi 28 (1946), 5, 39, 82. Chem. Abstracts
40 (1946), 5201.
"Terpeneless and Sesquiterpeneless Oils,"
by Y. R. Naves, Mfg. Chemist 18, No. 4 (1947), 173.
1 Comment:
"Terpeneless and sosquiterpojieloss oils have nothing to do with these isolates as the latter consist..." in the 4th para.
"sosquiterpojieloss"
miss typing?
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