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Fluid Extraction of Hops Spices and Tobacco with Supercritical Gases.

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Fluid Extraction of Hops, Spices, and Tobacco with Supercritical
Gases[**]
By Peter Hubert and Otto G. Vitzthum[*]
This report provides an introduction to the principal methods of extraction with supercritical
gases, illustrated by the processing of some natural products with carbon dioxide. The combination of pressure and temperature as process parameters makes it possible to vary the solvent
power of the medium within certain ranges as desired without having to change the composition
of the solvent, as would be necessary in conventional solvent extraction. The problem-free
handling of carbon dioxide and also of the products obtained open up possibilities whose
development has only just started.
1. Introduction
Extraction with organic solvents is a well-established
method of selective separation of specific constituents from
food and semi-luxury products. Extractants with a low boiling
point such as ethyl acetate, methanol, dichloromethane, etc.
are suitable on the one hand for isolating valuable constituents
from such products as hops, spices, and oil seeds, and on
the other hand for removing or reducing the level of less
desirable accompanying substances (nicotine, caffeine). The
extracts, such as those obtained from hops and spices, have
a number of advantages over the starting material:
- the constituents are better utilized
- the products have a longer shelf-life
- greater uniformity is achieved because of the possibilities
of standardization
- the products are sterile.
The solvents used for this purpose must meet the requirements of foodstuffs legislation, which can vary from country
to country. The main requirements are as follows:
a high degree of purity
- chemical stability
considerable inertness, i.e. the extraction agent or traces
of impurities present in it must not react chemically with
the food constituents
- a low boiling point to facilitate removal of the solvent
residue from the treated product
- freedom from toxic effects.
In recent years national and international bodies (Fremdstoffkommission der Deutschen Forschungsgemeinschaft;
Food and Drug Administration (FDA), USA; EC Codex Committee; FAO/WHO, Geneva) have increasingly emphasized
the importance of complying with such criteria.
~
~
2. Extraction with Supercritical Systems
2.1. Applications and Properties of Compressed Gases
The fact that compressed gases can dissolve solids was
first demonstrated experimentally by Hannay and Hogarth”]
[*I
Dr. 0. G. Vitzthum, P. Hubert
HAG AG
Hagstrasse, D-2800 Bremen
[**I Based on a paper presented at the symposium “Extraction with Supercritical Gases”, held at Essen on June 5, 1978.
710
in 1879, who dissolved potassium iodide in supercritical ethanol and reprecipitated the salt by reducing the pressure.
Over the years there has been much discussion of the part
played by the dissolving effects of supercritical gases in natural
processes, for example the influence of methane in the production of crude oil[2]or the influence of water on rock formation
in geological processes[3].Only recently, however, has industrial exploitation of this phenomenon been proposed, for
example for removing asphaltenes from crude
for the
extraction of ~ o a l [ ~and
~ ~ for
] , the separation of mixtures
of substancesL7-“I. A number of authors have described the
use of supercritical gases as carriers in supercritical fluid chromatography (SFC)“’ - and also recently in combination
with thin-layer chromatography[l6I. In the early seventies
advances were made in the solvent extraction of foods as
a result of the introduction of separation with supercritical
carbon dioxide. This meant that for the first time natural
products such as coffee, tobacco, tea, cocoa, hops, spices,
and oil seeds could be extracted without the use of organic
solvents[”- 261 . W e shall not deal here in detail with the
physicochemicalprinciples of the solvent effects obtained with
supercritical gases; for further information the reader is
referred to relevant publications[y, -361.
’’
2.2. Methods of Processing Natural Products with Supercritical
Carbon Dioxide
To assess the dissolving power of a supercritical gas it
is necessary to determine the phase equilibria with the substances to be dissolved. However, on its own this does not
provide sufficient information for estimating the extractability
from vegetable material, since apart from the solubility the
way in which the substance to be extracted is bound to the
vegetable matrix has a considerable influence. For example,
the strength of the binding of nicotine in raw tobacco varies:
whereas some of it can be removed very easily by extraction,
the rest is probably present as a complex salt with tobacco
constituents such as chlorogenic acid, citric acid, etc. and
is more difficult to extract.
For natural products that can be subjected to only moderate
heating it is possible to use gases with appropriate critical
temperatures, such as COz, as the extraction agents. This
is generally the case when supercritical extraction can be
carried out at about 10-80°C above T,; 100°C above T,
should not be much exceeded. A selection of suitable gases
has been given by Stahl et a1.[I6].
Angew. Chem. Int. Ed. Engl. 17, 710-715 (1978)
Russian authors in particular have used liquid carbon dioxide to obtain extracts of natural products. The processing
of tobacco has been describedr3'], as has the production of
extracts from hops[38,391 and spices[40].
Extraction with supercritical in contrast to liquid COz
generally offers the following advantages: the substances to
be extracted are much more soluble, and there is a wider
range of possible operating temperatures. Density and dielectric constants can be changed by varying the pressure. Figure
1 shows how these two properties vary as a function of the
pressure of carbon dioxide at 50°C. It can be seen that the
greatest changes take place between about 70 and 150bar,
and that both parameters increase only slightly above about
300 bar.
2oo
100
/
t
304 bar
h r
:
20
m
bar
Y
A10
- 5.0
4 2.0
c
.-
3
1.0
0.5
0.2
0.1
bar
bar
7
17
27
-
37 47
T[TI
57
Fig. 2. Solubility of naphthalene in compressed ethylene as a function of
temperature.
matically in Figures 3 and 4 in the case of extract recovery
and in Figure 5 in the case of recovery of the carrier substance.
t
c
150
wi
V
0
v
Fig. 3. High-pressure extraction with extract separation by variation of the
pressure. Left, extraction stage; right, separation stage.
1.25
"
I
0
100
200 300
400
500
600
Ptborl-
Fig. 1. Density and dielectric constant of C 0 2 as a function of pressure
(50°C).
As the density increases, i. e. with increasing pressure, the
solubility of less volatile components in the gas generally
increases. According to our own measurements, the extraction
rate, which also depends on the temperature-dependent diffusion of the constituents within the plant material, is at least
2.5 times as high with supercritical COz as with liquid C 0 2 .
The influence of the temperature can vary, depending on
the pressure range. For example, Figure 2 shows that increasing
the temperature at higher pressures results in an increased
solubility of naphthalene in compressed ethylene, while at
lower pressures such an increase in temperature reduces the
solubility. Hence specific dissolution or separation effects can
be achieved by selecting suitable combinations of pressure
and temperature.
The method and apparatus used must be adapted to the
raw material and to the purpose of the process. In one case
the aim may be to obtain an extract and the remaining vegetable matrix or carrier substance may be either of no value
(spices) or subjected to a further treatment with water (hops),
while in another case it is the carrier substance that is wanted
(tobacco) while the extracts are of secondary interest. The
principle of extraction with supercritical gases is shown scheAngew. Chem. Inr. Ed. Engl. 17, 710-715 (1978)
Fig. 4. High-pressure extraction with extract separation by variation of the
temperature. Left, extraction stage; right, separation stage.
Fig. 5. High-pressure extraction with extract separation by adsorption. Left,
extraction stage; right, adsorption.
In the extraction step the soluble constituents are solved
by the compressed supercritical gas. The homogeneous phase,
regarded as a solution, is decompressed as it passes through
the throttle valve into the separation vessel (Fig. 3), with
the result that the density of the gas decreases and the extract/
solvent mixture separates into its component parts. The gas
is then compressed again and returned to the extraction step.
Separation can also be achieved by changing the temperature
of the supercritical solution (Fig. 4). Figure 5 shows a process
in which the dissolved extract is adsorbed onto suitable sorbents. This process, which can be run isobarically and isothermally in the whole circulation, is generally used when it is
the carrier substance that is required (tobacco, tea, coffee).
711
2.3. Process Parameters in Extraction and Separation
Extraction pressures in the range of 80-300 bar are generally necessary to achieve adequate densities. This pressure
range has so far been investigated most frequently. The occurrence of further dissolution phenomena at higher pressures
cannot be excluded, but their use would probably be rejected
for economic reasons. The preferred temperature range for
extractions with C 0 2 is about 35--80°C.
There are several possible combinations of extraction and
separation conditions that can be used to obtain hop and
spice extracts[22.231. If we denote the extraction pressure by
P1 and the separation pressure by P2, with corresponding
temperatures Tl and 7’2, the different cases listed in Table 1
can be distinguished.
Table 1. Extraction with supercritical gases. Adjustment of the parameters
for extraction (subscript 1) and separation (subscript 2).
Case 1 :
Case 2 :
Case 3:
Case 4:
In case 1 the supercritical gas phase is decompressed to
a subcritical pressure and a subcritical temperature. Given
these thermodynamic conditions, depending on P1 and TI
between 10 and 100% of liquid C 0 2 will be found in the
separation vessel together with the previously dissolved
extract. Providing this extract is not also partly soluble in
liquid COz, it will be deposited as a further phase. The C 0 2
vapor in equilibrium with its liquid phase, and whose density
is approximately 0.2 kg/l, has virtually no dissolving properties; to maintain the extraction circulation, it is drawn up
by the feed pump, compressed again, and tempered. The
method used in the case 1 means therefore that the liquid C 0 2
in the separation vessel must be continuously evaporated,
necessitating the supply of heat.
Since it is generally unnecessary to reduce the separation
pressure and temperature to below their critical values in
order to achieve separation (it is sufficient to adjust the density
to a suitably low value), for practical purposes cases 2 and
3 are more realistic. Here P 2 is reduced either to below the
critical pressure (case 2) or to a value between the extraction
pressure and the critical pressure (case 3), while in both cases
the separation temperatures are between the extraction temperature and the critical temperature. This also means that
drier extracts are obtained. In general, if suitable pressure/temperature combinations are selected it is possible to achieve
fractionation effects during the separation, using a multistage
pressure cascade for example. Thus, it would appear to be
possible in the case of pepper, for example, to obtain fractions
enriched with essential oils or the hot flavor components.
This also applies to hop extracts. However, it is also possible
to achieve similar fractionation effects during the actual extraction with the aid of pressure/temperature programs. In case
4 the separation temperature depends on the pressure. Figure
2 illustrates the different effects of temperature on the solubility
712
of naphthalene in ethylene. According to this, the temperature
must be reduced when the pressure is high and raised when
the pressure is low. Although case 4 would be of value because
of the isobaric method of operation, it remains to be shown
whether it is suitable for practical application, since the separation may not be as complete in this case as it is in cases
1-3 and the maximum thermal load that the extracts can
withstand limits the rise of the temperature.
3. Propertiesof Carbon Dioxide as an Extraction Agent
and Its Assessment from the Point of View of Food
Regulations
The advantages of carbon dioxide, which is unobjectionable
from the health point of view, over organic solvents make
it appear an ideal extraction agent. It is ubiquitous in nature,
e. g. in the air and mineral waters. It is what the plants breathe,
and is produced when organic substances ferment. It is formed
in many manufacturing processes in the food industry, e. g.
in the production of beer and wine, in the baking of bread,
etc., and remains in these food products. Moreover, it is also
used in artificially carbonating mineral waters. Being an “inert”
gas, even in the supercritical state, C 0 2 does not react in
any way with the food constituent^^^'! This is also the reason
for its universal use for quick-freezing foods in the liquid
and solid state. Even under the previous Federal German
food legislation, C 0 2 was not regarded as a foreign substance,
and this classification has been retained in more recent legislation. Hence, together with air, nitrogen, and distilled or
demineralized water, COz is expressly excluded from the prohibition of additives. Table 2 suggests purity specifications for
COa used for extraction purposes.
Table 2. Specification for carbon dioxide for extraction purposes. Determination methods cf. [a-d].
Odor
Color
CO, content
PH,, H I S and reducing
organic substances
Foreign acids, e.g. S O z ,
HCI
Carbon monoxide
Oxidizable constituents
Evaporation residue
[a]
[b]
[c]
[d]
neither burnt nor
pungent
colorless
minimum 99.9 vol-%,
measured in the
liquid or gas phase
not detectable in 1001
(STP) of the gas
not detectable in 1001
(STP) of the gas
< 2 vol.-ppm
max. 1 vol.-ppm
max. 1 vol.-ppm
la, b l
[a-c]
[a-c]
[c, d l
la1
[a1
W Fresenius, W Schneider, Mineralwasserzeitung 19, No. 4 (1966).
European Pharmacopeia, Vol. 2, pp. 175-177 (1971).
Zusatzstoff-Verkehrs-VO, Anlage 2, Liste 8.
Analysenmethoden HAG AG, Bremen. Vorschrift No. 081/’2/01 (1976).
In addition to its solvent properties, CO2 has the advantages
of being neither flammable nor toxic, not maintaining combustion, and also not exerting a corrosive effect in combination
with moisture on the materials used in processing natural
products (stainless steel o r plastics). It is also inexpensive,
and readily available in large quantities and high purity.
For all these reasons natural products have so far been
processed predominantly with C 0 2 .
Angrw. Chem. Int. Ed. Engl. 1 7 , 710-715 (197X)
4. Production of Extracts with Supercritical Carbon
Dioxide
4.1. Hop Processing
Hops have been used to make beer for some two thousand
years. The use of hop extracts by the brewing industry is
a more recent development, and is increasing in the interests
of rationalization.
The valuable constituents of the hop resins are the soft
resins which contain an cl-acid fraction (mixture of several
humulones) and a P-acid fraction (mixture of several lupulones). It is specifically the a-acids which, after isomerization
in the brewing process, give beer its characteristic bitter taste.
The structures of humulone and lupulone are shown in Figure
6.
toward rationalization in the food industry, have led to spice
extracts or oleoresins being used. The advantages of these
were mentioned in the introduction. The best extract or best
oleoresin is a preparation that has aroma, flavor, hotness,
i. e. all the organoleptic factors of the spice, and after dilution
reconstitutes fully the organoleptic charqcteristics of the starting materialr431.The applications extend throughout the food
industry. As there are so many spices, and the list of spice cons t i t u e n t ~is~ ~steadily
~]
growing as a result of the refinement
of analytical techniques, we shall not deal in detail with this
subject. We shall consider only black pepper, nutmeg, and
chilies, which can also be extracted with supercritical COz.
Figure 7 shows the main constituent (98 %[431) of the hot
0
Fig. 7. Piperine, the main component of the hot principle of pepper
Fig. 6. Soft resin components of hops. Left, humulone; right, lupulone
Conventional hop extraction generally employs dichloromethane, which has to be evaporated off after the resins have
been extracted. The resulting extract is a pasty, dark-green
to black-green mass, which is not permitted to contain more
than 2.2 % solvent residue (CHzC1z)r4zl.
To produce hop extract with supercritical COz commercial
hop pellets with a bulk density of approximately 0.65kg/l
were extracted under the conditions of case 3 (Table 1) without
any further preparation. An olive-green, pasty extract with
an intensive aroma of hops was obtained. The pellets had
disintegrated into a powder, which was easily shaken out
of the extraction vessel. Table 3 gives various analytical results.
Pellets
before after
CO, extraction
C02
extract
6.0
30.3
26.6
12.6
14.0
3.7
7.0
90.0
84.8
41.2
43.6
5.2
5.4
4.3
1.3
0.2
1.1
3.0
H
3
C
-
O
m
J
-
y
HO
Fig. 8. Main components of the hot principle of capsicum. Top, capsaicine;
bottom, dihydrocapsaicine.
The results of spice extraction with supercritical COz are
described for pepper, nutmeg, and chilies. A few analytical
results are compared in Table 4 with values for the commercial
extracts.
Table 3. Hop analysis, COz extraction.
Water content [%]
Total resins [ %]
Soft resins [ %]
cx-Acids [%I
p-Acids [ %]
Hard resins [ %]
principle of pepper, i. e. piperine. Other components are piperettine, piperoline A and B, and piperanine, which have similar
structures.
The most important constituent of capsicum spices is a
mixture of several amides, the capsaicine alkaloids, whose
main components are capsaicine and dihydrocap~aicine[~~]
(Fig. 8).
Degree of
extraction
89.9
96.5
98.9
94.4
Extract,
commercial
8.0
88.5
82.0
39.5
42.5
6.5
It can be seen that the extent of extraction of the a-acids,
almost 99 %, is above the required minimum of 95 %. The
analyzed extract was obtained by single-stage pressure reduction (cf. Fig. 3). Extract separation by releasing the pressure
in several stages is currently being investigated, this giving
a choice of green or yellow extracts of different compositions.
Table 4. Spice analysis, C 0 2 extraction
Material
before after
COz
extracted
Degree of
extraction
Extract,
commercial
(usually
CH2CI,)
44.0
12.7
7.0
97.7
81.1
41.2
15.0
0.2
COZ
extraction
Pepper
Piperine [XI
10.1
0.3
Essential oil [XI 3.5
t0.1
Water [%I
9.3
6.2
yellow
olive green
-.________
Nutmeg
Essential oil [%]
Hexane-soluble
matter [%]
Water [%]
Color of extract
7.4
<0.1
19.0
99.0
20.0
46.0
4.9
1.7
4.4
100
1.6
98.0
100
0.1
whitish
yellow
4.2. Processing of Spices
yellowishbrown
-.
Chilies
The important constituents of spices are their aroma and
flavor components. For industrial use, spices as such have
a number of disadvantages, which, together with the trend
Angew. Chem. Int. Ed. Engl. 1 7 , 710-715 (1978)
Capsaicine [ %]
Water [ %]
Color of extract
0.86
3.2
0.03
1.8
4.9
1.3
red
97.0
4.7
0.1
red
713
Ground black pepper was extracted under the conditions
of case 1 (Table I). The extract obtained was yellow and
pasty and had a very strong smell of pepper. The degree of
extraction of piperine was almost 98 %, i. e. above the minimum
required yield of 95 %, and the piperine content of the extract
(44 %)was roughly the same as that of the commercial product
(Table 4). The extraction of ground nutmegs under the conditions of case 3 yielded a pale yellow nutmeg butter with
a pronounced nutmeg aroma. The degree of extraction was
98 %. The extraction of ground chilies under the conditions
of case 1 gave a red oil, the degree of extraction, calculated
for capsaicine, being 97 %.
0
1
2
3
4
t
5
6
thl-
Fig. 9. Residual nicotine content as a function of processing time for three
tobaccos of different origins: 1, Burley; 2, Virginia; 3, Orient.
5. Obtaining the Carrier Material: Processing of
Tobacco with Supercritical Carbon Dioxide
The greater part of tobacco produced nowadays goes into
cigarettes, and the trend toward mild tobacco has risen steadily
over the last twenty years. The term “mild” is used here
in particular to describe a material whose smoke contains
relatively little nicotine and condensate when it is burnt.
Tobaccos of this type with a good aroma and a low nicotine
content are not available in unlimited quantities, and because
of the concern about damage to health that may be caused
by nicotine, attempts have already been made to remove
this constituent from
511. Since tobacco treated
with organic solvents, for example, often acquires a rubbery
structure, special processes have been suggested to avoid this
effect, which is extremely disadvantageous to further processing.
In the meantime interest has also grown in other potentially
harmful substances such as may appear in smoke condensates.
The primary aim of processing raw tobacco with supercritical COz is to reduce the nicotine content to the desired
levels with the minimum loss of aroma. This was found to
have the side effect of a certain expansion of the tobacco. It
is also possible to obtain a raw tobacco aroma equivalent
to that obtained with the method described for extract separation, to transfer aroma from one tobacco to another, or even
to impregnate a neutral matrix with the tobacco aroma.
In the single-stage process the moisture content of the
tobacco is increased up to 25 %, in special cases even higher,
and a stream of supercritical gas is passed through the material.
The pressures are approximately 300 bar, and the temperatures
are between the T, of the gas and about 100°C.The dissolved
nicotine is separated by reducing the pressure or by changing
the temperature (cf. Section 2.2) or is bound by adsorption
onto suitable sorbents[”]. After drying the tobacco is immediately ready for further processing. A multistage process[’9]
may prove suitable for tobaccos for which the single-stage
method has a detrimental effect on aroma. Figure 10 shows
a block diagram of a three-stage process as a section of a
continuous production process.
1
-
5.1. Preparation of the Material for Extraction
In principle raw tobacco in any form can be used, including
leaf ribs and tobacco dust. Apart from conditioning (moistening or drying), the tobacco requires no further preparation.
The additives commonly used in tobacco preparation, such as
moisture-retainers, sauces, flavors, binders, etc. are not added
until the tobacco has been treated by the described method.
5.2. SingleStage and Multistage Processing
The presence of water is essential in the extraction of nicotine. However, at the normal moisture content of tobacco
(ca. 10-13 %), supercritical COz extracts insufficient nicotine,
or none at all, but removes the aroma instead. It should
nevertheless be mentioned at this point that tobaccos of different origins behave very differently (Fig. 9), this also applying
in all other stages of the process.
714
Fig. 10. Multistage extraction of nicotine from tobacco with supercritical
C 0 2 .1, Tobacco (starting material); 2, nicotine adsorbent; 3, nicotine-reduced
tobacco from preceding cycle; 4, nicotine-reduced tobacco produced from
1 in the process shown for the next cycle; 5, transfer of aroma from 1
to 3; 6, aroma distribution; 7, conditioning; 8, removal of nicotine from
the (dearomatized)batch produced from 1 with supercritical C 0 2 ;9, drying:
10, regeneration of the adsorbent; 11, nicotine-reduced, re-aromatized tobacco
(finished product); 12, nicotine (by-product).
The arrow at the top indicates the intake of a previously
denicotinized batch, the arrow at the bottom discharge of
the nicotine-reduced batch produced in the section shown
to the next cycle. In the first stage the aroma is removed
from the fresh material. (As a rule, less nicotine will be extracted
the drier the tobacco.) The aroma could be isolated as an
extract in the same way as described for hop and spice extraction. In the present case, the aroma is used to impregnate
a previously denicotinized batch, i. e. the aroma-carrying gas
stream is allowed to expand in the nicotine-reduced batch.
Angew. Chem. Int. Ed. Engl. 17, 710-715 (1978)
The de-aromatized tobacco is moistened for the second stage
and the nicotine is then removed in an isobaric and isothermal
recycling operation. Selective sorbents are used for the nicotine.
Regeneration of these sorbents affords nicotine as a by-product. The third stage of the process involves a homogeneous
distribution of the transferred aroma in the bulk by repeated
dissolution and reprecipitation. The rearomatized tobacco can
then, after conditioning if necessary,be used directly for further
processing.
5.3. Quality Characteristics of the Treated Tobacco
When the treated tobacco is discharged from the extraction
vessel, a certain expansion of its fibers is observed as a result
of the removal of gas residue from the vegetable tissue. This
effect is dependent on the moisture content, the temperature,
and several other parameters and can be controlled within
certain limits. The external appearance of the product remains
unchanged or may change only slightly to a lighter or darker
color, this again depending on the origin and the treatment
process. Table 5 shows the results of analysis of smoke from
Table 5. Results of smoke analysis performed on cigarettes made from
untreated tobacco and tobacco treated by the three-stage process.
Length of cigarette
Length of filter
Length of total stub
Diameter
Effective sample weight
Cigarette paper
Filter and wrapping
Quantity of burnt tobacco
Air-flow resistance of cigarette
Air-flow resistance of filter
Air permeability of
cigarette paper
Moisture content of cigarette
Nicotine content of tobacco
Number of draws
Nicotine content of smoke
Nicotine in main smoke
[mm]
[mm]
[mm]
[mml
[mg]
lwl
Cmgl
lmgl
[torr]
[torr]
Untreated
tobacco
Treated
tobacco
84.0
20.0
28.0
8.03
1126
46
162
803
8.5
4.7
84.0
20.0
28.0
8.06
1070
42
162
758
8.6
4.7
99.6
12.4
1.52
9.9
1.08
1.34
100.0
12.5
0.08
8.5
0.06
0.07
[(l/h)]
[::,I
c "/.I
lmgl
[mg/gl
cigarettes made from untreated tobacco and from tobacco
treated by the three-stage process. The nicotine content of
the tobacco is reduced by 94.7 % and that of the main smoke
Table 6. Analysis of raw tobacco in various stages of processing, C 0 2 extraction (nicotine contents in the dry substance).
Starting material
after removal of the aroma
after reduction of nicotine
after re-aromatizing
Tobacco 1
Tobacco 2
[%I
["/.I
2.47
2.33
0.09
0.16
1.05
1.oo
0.04
0.08
by 94.8%. Table 6 compares the nicotine contents of two
tobaccos at intermediate stages of the multistage process.
Angew. Chem. Int. Ed. Engl. 17, 710-715 (1978)
Received: June 16, 1978 [A 234 IE]
German version: Angew. Chem. 90, 756 (1978)
Translated by Express Translation Service, London
~-
J . B. Hannay, J . Hogarth, Proc. Roy. S O ~(London)
.
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