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J. of Supercritical Fluids 59 (2011) 53–60

Contents lists available at ScienceDirect

The Journal of Supercritical Fluids

journal homepage: www.elsevier .com/ locate /supf lu

Extraction of caffeine from Robusta coffee (Coffea canephora var. Robusta) husksusing supercritical carbon dioxide

J. Tello, M. Viguera, L. Calvo ∗

Department of Chemical Engineering, Universidad Complutense de Madrid, 28040Madrid, Spain

a r t i c l e i n f o

Article history:

Received 15 June 2011

Received in revised form 22 July 2011Accepted 22 July 2011

Keywords:

Caffeine

Extraction

Coffee husks

Supercritical CO2

a b s t r a c t

This work evaluated the technical feasibility of supercritical CO2 extraction of caffeine from coffee husks,an abundant residue of the coffee industry. Different pre-treatments (initial humidity and milling) andoperational conditions (pressure, temperature, time and flow rate) were studied in a CO2 continuous

flow laboratory-scale unit. While prior wetting of the coffee husks was needed, milling was not requiredto extract the caffeine. The use of higher flow rates and/or operational times resulted in higher extrac-

tion rates. The process was favoured with increased operational pressure and temperature due to highersolubility. The maximum extraction yield obtained of this alkaloid was 84% when working at 373 K and

300 bar, using 197 kg CO2/kg husks. After water washing, the caffeine was at least 94% pure. Compar-ing world production data, the initial caffeine content and global extraction yield data of other natural

sources, this process could be very advantageous for its technological application.© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Caffeine (1,3,7-trimethylxanthine) is an alkaloid of the xan-thine group widely known worldwide due to its occurrence inextensively consumed beverages, drinks and food. Natural sourcesof caffeine include different varieties of coffee beans (Coffeacanephora, Coffea arabica), tea leaves (Camellia sinensis), guaraná

seeds (Paullinia cupana), maté leaves (Ilex paraguariensis), kolanut seeds (Cola nitida, Cola acuminata) and cocoa beans (Theo-broma cacao) [1]. Despite the fact that tea is globally consumedmore widely than coffee, coffee is the main source of caffeine

in daily consumption given its generally higher caffeine content[2]. Other dietary factors that contribute to daily overall caffeineconsumption include foods like certain soft and energy drinks,chocolate, candies and sweets, as well as that contained in some

medications [3], such as stimulants, diet aids, painkillers and coldremedies [4].

Numerous studies have reported the effects of caffeine con-sumption in humans, such as the well-known stimulant effect

of low doses of caffeine on the nervous system, which enhancesconcentration capacity and counteracts tiredness. However, inmore sensitive individuals (or under bigger doses of caffeine),this effect could generate episodes of insomnia, anxiety, nervous-

ness, irritability, hostility and mood swings. Some other reported

∗ Corresponding author. Tel.: +34 913944185.

E-mail address: [email protected] (L. Calvo).

physiological effects are stimulation of the gastric and urinary sys-tems and increased heart rate and blood pressure [4–8].

These negative effects have led to an increasing consumption of decaffeinated coffee, generating the development of various pro-cesses that remove this alkaloid from coffee beans. Among theseprocesses, organic solvents with high toxicity (methylene chloride,ethyl acetate), Swiss water decaffeination (which results in a less

flavourfulbrewthanothermethods)and theemployment ofcarbondioxide in supercritical conditions are commonly used [9]. The lat-ter was successfully developed on an industrial scale in the 1970s,based on Kurt Zosel’s patent [10], initially run by Cafe HAG and

General Foods [9]. Compared with other conventional methods,this process showed better results, both in terms of generating ahigher quality product and for being a betterprocess from an envi-ronmental point of view. CO2 is a non-flammable and non-toxic

solvent that can be easily removed from the final product. Theseadvantages have driven the subsequent investigation of supercriti-cal extraction of caffeine from other natural sources, such as tealeaves [11], stalks and fibre wastes from industrial tea process-

ing [12,13], guaraná seeds [14,15], maté leaves [15], cocoa beans[15,16] and coffee oil [17].

On the other hand, the low solubility of xanthines in super-critical CO2, due to its apolar character, makes the addition of

polar cosolvents (such as water or ethanol [13,15,18–20]) to thecompressed gas an interesting practice. Cosolvent effects are dueto specific chemical (hydrogen bonds and acid–base interactions)or physical interactions (dipole–dipole or dipole-induced dipole)

between the cosolvent and solute, and also to a possible interac-tion between the solvent and the cosolvent,affectingsolvent-solute

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54 J. Tello et al. / J. of Supercritical Fluids 59 (2011) 53–60

Fig. 1. High pressure apparatus used for the supercritical fluid extraction of caffeine from coffee husks. PI, pressure indicator; TI, temperatura indicator; TIC, temperature

indicator and controller; TC, temperature controller; BPR, back pressure regulator.

interactions. These effects result in accelerating and making theextraction easier [15]. Regarding the operational conditions, the

solvent power of supercritical CO2 is highly dependent on pressureand temperature, variables that can be independently modified.Thus, different authors have reported higher yields when work-ing at high pressures, due to the enhanced CO2 solvent power, and

when working at higher temperatures [14,16,20–22].Among all the natural sources previously cited, coffee could be

the most important worldwide from an economical point of view.FAO estimated its world production in 2009 at more than eight

million tons [23]. In order to be used and consumed, green cof-fee is processed by two different methods, the wet or dry method.On average, for every ton of clean coffee produced, 1 ton of husksare generated during dry processing, whereas, for wet process-

ing, 0.28tons of parchment husks, 2tons of pulp and 22.73tonsof wastewater are generated [24].

Obviously, this great amount of residues represents a serioussource of pollution and environmental problems. Recent efforts to

solve this problem include activities like the production of organicfertilisers [25], using these residues as a substrate for the synthe-sis of biomass [26] and bio-ethanol [27], for the production of adiversity of products through fermentation (citric acid [28], pectins,

pectic enzymes and vinegar [29]), as a natural source of antho-cyanins [30], for the growth of mushrooms, as an adsorbent [27]and as a source of caffeine by percolation with alcohols and water[31]. Alternatively, coffee husks have been used for animal feed,

because of their high cellulose content, which can be metabolisedby ruminants. However, given its structure and its high content of lignin, silica [31], polyphenols, tannins (5% [26]) and caffeine (1.3%[26]) [32], this use is limited. Thus, the most common final use for

husks is in coffee mills to provide the energy needed for the finaldrying of the coffee beans [31].

The aim of this work was to evaluate the feasibility of extract-ing caffeine from coffee husks using carbon dioxide at supercritical

conditions, asa meansof using this residue toobtainhigh-valuecaf-feine that may be subsequently used in the food, pharmaceutical,

veterinary or cosmetic industries.

2. Materials and methods

2.1. Materials

Whole coffee husks of C. canephora var. Robusta were obtainedfrom Manaus (Brazil). They were previously characterised in

Natraceutical group by standard methods, with a fat content of 1.0%, a moisture content of 16.4% and a caffeine concentration of

1.1%. This material was stored in dry conditions at room temper-ature during the investigation. Pressurised liquid carbon dioxide

(99.998% in purity) was kindly donated by Carburos Metálicos S.A.Caffeine, 99.99% in purity and used in the solubility experiments,was provided by the Natraceutical Group. The packing agents wereround perforated plastic beads, with an average diameter of 2 mm.

They were also used to create the inert bed in the net caffeine sol-ubility measurement experiment. Redistributors were made froma stainless steel mesh with same area as the cross section of theextractor.

2.2. Pre-treatment

To evaluate the influence of the condition of the rawmaterial on

the extraction rate, two different pre-treatments were performed.First, coffee husks were ground by the commercial stainless steelblades of a coffeegrinder. Second,the material (raw or ground) washumidified to the desired percentage of humidity (measured by

weight) with ultrapure water. Thus, the following pre-treatmentswere assayed:

1. Whole coffee husks with a natural humidity of 16%.

2. Whole coffee husks with a humidity of 32%.3. Whole coffee husks with a humidity of 48%.4. Whole coffee husks with a humidity of 64%.5. Ground coffee husks (32% humidity) loaded with redistributors.

6. Ground coffee husks (32% humidity) loaded with redistributorsand packing agents (36%, w/w).



J. Tello et al. / J. of Supercritical Fluids 59 (2011) 53–60 55

7. Ground coffee husks (32% humidity) loaded with redistributors

and packing agents (50% w/w).

2.3. Apparatus description and extraction process

Supercritical fluid extraction was conducted using the lab-

oratory scale system previously described by Calvo et al. [33],consisting of a CO2 feeding line, a 50mL capacity 316 ss extrac-

tor, a pressure control device and a sample collection system, asshown in Fig. 1.

The CO2 was fed from the pressured container in the liq-uid phase, then cooled in a temperature-controlled bath (Selecta,Frigiterm-30) to 258 K and impelled by a cooled-head mem-brane pump (Milroyal D, Dosapro Milton Roy). This refrigeration

prevented pump cavitation in the pressurisation process. The pres-surised CO2 is pre-heated in a coil located inside a heating jacketpriorto entering thevessel. Thevessel is also thermallyconditionedby another heating jacket; both with a temperature control of ±1 K

and recorded by a type K thermocouple placed inside the reactorin direct contact to the fluid and the solid. Pressure was read bya Bourbon manometer with an accuracy of ±5 bar. The control of the pressure and flow rate was achieved by both a heated micro-

metering valve (Tescom, Serie 26-1700) and the pump. A safetyrupture disk set at 380 bar prohibited the pressure from exceed-ing the prescribed values. The amount of CO2 per unit of time wasdetermined in a mass flow meter (Alicat Scientific, M-10SLPM-D)

connected at the end of the line with an accuracy of ±0.5 g/min.Coffee husks, previously weighed and adequately pre-treated,

were loaded in the extractor, forming a fix bed. When using wholehusks 8.01±0.28g were used. When using milled coffee husks, the

quantity of 15.75±1.64g wasloaded into the extractor. Redistribu-tors made of stainless steel mesh having the same size of the crosssection of the extractor, were introduced approx every 1.5 cm inthe tests done with the ground material. Then, the extractor was

closedand pre-heated. After that, theCO2 waspumped in,and,oncethe desired pressure was reached, the back pressure regulator (BPR)wasopened, providing a continuous flowthrough the bed.Afterthevalve, the CO2 was depressurised, the solvent power of the super-

critical fluid dropped and the extract precipitated in a previouslyweighedglass flask. When theestablished treatmenttime wasover,the apparatus was depressurised, the extract weighed and the cof-fee husks unloaded. The total amount of CO2 circulated was read

by the mass flow meter.Since caffeineis a solid product,part ofit stuck tothe walls ofthe

equipment. To recover it, after each experiment, a “washing” pro-cedure was conducted. To do this, a portion ofcotton was soaked in

40mL of ultrapure water and loaded in the extractor. Then, CO2 at323 K was flown through the extractor at a pressure of about 60bar(container pressure) with a flow rate of 2–3 g/min for 20min. Theprocedure was repeated twice. The collected water with the corre-

sponding dissolved caffeine was placed in an oven at 323 K. Once

the water was evaporated, the precipitate was weighted, properlylabelled and stored for its subsequent composition and purity anal-ysis. Similar washing methods with polar solvents were previously

used by Saldana [34].

2.4. Analysis of caffeine content on the extracts

The determination of caffeine in the extracts was performed

by HPLC following the protocols stated by Icen and Gürü [12].Briefly, extracts were diluted with ultrapure hot water, filteredunder vacuum, cooled and analyzed by means of an Agilent 1100Series chromatograph equipped with an UV–visible detector set

at 320nm, and a C-18 column. The mobile phase used was ace-

tonitrile (30%, v/v) (Merck) at 0.8 mL/min flow rate. Calibrations

were performed using standard caffeine solutions at different

concentrations.

2.5. Measurement of caffeine solubility

To measure the solubility of net caffeine in pure CO2, a givenamount of caffeine was charged in the extractor on an inert bedmade of plastic beads. The rest of the operation was performed as

described in Section 2.4.

2.6. Determination of results

Extraction yield data were expressed as the percentage of totalextract obtained with respect to the initial content of fats and caf-

feine.

Extraction Yield (%) :Extract ( g )

[Fats ( g ) + Caffeine ( g )]initial× 100

Caffeine yield data were expressed as the percentage of caffeinein theextracts(obtainedfrompuritydata) with respect to theinitial

content of caffeine.

Caffeine Yield (%) :

Extract ( g )× Caffeine content (%)

Initial Caffeine ( g )

2.7. Experimental standard deviation

To assess the reproducibility of the results, two experimentswere repeated six times each, establishing an experimental stan-dard deviation of 9% in extraction yield and 7% in caffeine purity.

All data are the average of at least two experiments.

3. Results and discussion

This section first presents the effect of pre-treatment of the raw

material on the yield of the extraction process. Thus, the initial

moisture content and the particle size of the raw material wereexplored. Theuse of agents to prevent the agglomeration of the bedwas then investigated to enhance contact. Regarding the variables

that affect the extraction process, different values of pressure, tem-perature and operational times combined with different CO2 flowrates were studied.

Caffeine extraction from coffee husks is shown as a curve rep-

resenting the yield results versus the circulated CO2 mass. Caffeinesolubility data were obtained to assess the outcome of our col-lection procedure, comparing our results with those previouslyreported by other authors [35,36]. Finally, a study of the compo-

sition of the extracts in terms of caffeine purity was done.

3.1. Caffeine solubility measurement

In order to verify the reliability of the experimental data

obtained with our apparatus, a measurement of the solubility of net caffeine in pure CO2 at 333K and 200barwas done, comparingthe results obtained with those previously reported by Johannsen

and Brunner [35] and Saldana et al. [36]. The results are shownin Fig. 2. Our result closely agreed, validating our extraction andwashing procedure.

Caffeine solubility showed a direct relationship with operation

conditions. The higher the pressure was, the higher the amountof extracted caffeine. This result was attributed to the increaseddensity of the solvent and, consequently, to its solvent power. Onthe contrary, the effect of temperature depended upon the work-

ing pressure, with retrograde behaviour at pressures lower than

about 200bar. This pressure, where the solubility isotherms cross,




0

1

2

3

4

5

6

400350300250200150100

Pressure (bar)

S o l u b

i l i t y ( g / k g C O 2 )

Fig.2. Solubility of caffeine in supercriticalCO2 at differentpressuresand tempera-tures (◦: 313K, Johannsenand Brunner [35]; : 333K, Johannsen and Brunner [35],

×: 353K, Johannsen and Brunner [35]; +: 313K, Saldanaet al. [36], 323K, Saldana

etal. [36], ♦: 343K, Saldanaet al. [36], and •: 333K, this work).

is known as the crossover pressure [37]. Below it, the caffeine sol-

ubility decreased as temperature increased. Above this value, anincrease in temperature increased solubility [17].It is well-known that this behaviour is a consequence of the

opposing effects of the solvent density variation and the solutevapour pressure. Below the crossover pressure (about 200 bar),

smallincreasesin temperature resultin drasticdecreases in solventdensity and consequently, on solvent capability. As an example, at180 bar, caffeine solubility decreases from 1.16 to 0.87g/kg CO2

when augmenting the temperature from 323 to 343K [36]. As the

process pressure moves away from the solvent crossover pressure,its density become less sensitive to temperature changes and theeffect that prevails is the increase in vapour pressure. Thus, impor-tant growths on caffeine solubility are seen over 200bar in the

isotherms as the pressure increases. This change in the influenceof temperature may also be seen in the isotherms obtained withCO2 mixed with cosolvents, although in this case, the crossoverpressure acquires a higher value. For example, for mixtures of

CO2–isopropanol (5%) or CO2–ethanol (5%), the crossover pressureincreases to 210 and 230 bar, respectively [37].

3.2. Effect of coffee husks initial moisture content

In industrial supercritical CO2 decaffeination processes, a previ-ous pre-wetting of the coffee beans is needed in order to make the

process feasible [9,38,39]. Similarly, we found that it was necessaryto wetthe coffeehusks prior to extraction, since hardlyany caffeinewas removed when working with coffee husks as received, even at

themost extreme conditionsallowedby theapparatus (350bar and

373K) (sample 1, Fig. 3).Knowing the initial moisture content of the coffee husks pro-

vided (16.4%), higherhydration valuesweretested, i.e. 32%,48% and64%(samples2,3and4,in Fig.3, respectively). The results indicated

that increasing the amount of water up to 32% resulted in higherextraction yields, but exceeding this percentage was not benefi-cial. For example, when the water content was 64%, the extractionresulted in the removal of only 5% of the extractible compounds

originally present in the husks. Moreover, this high moisture levelgenerated a malfunction of the apparatus, withthe formation of icecrystals in the outlet valve of the extractor, which interrupted thenormal flow of CO2. Because of this, coffee husks at 32% moisture

were chosenas the starting material forthe following experiments.In light of these results, it can be concluded that the hydration

of coffee husks prior to the extraction process is a critical factor.This fact could be attributable to the hydrolytic rupture of hydro-

gen bonds which link the caffeine to the natural matrix on whichit is adsorbed, liberating it to be swept out by the supercritical CO2

[39]. Furthermore, water could contribute to swelling of the cellmembrane, leading to the enhancement of solute diffusion out of

the plant tissues [11,40]. In fact, we observed an important volumeaugmentation of the husks after wetting. Another indirect reason

could be theentrainereffect of thewater dissolved by theCO2 dur-ing the extraction, facilitating the dissolution of polar compounds

[40] such as caffeine. Supportingthis hypothesis, Iwai et al. noticedthat solubility of caffeine in supercritical CO2 was found to increase22% when using CO2 with saturated water at 313.2 K and 150 bar[41]. Similarly, Park et al. found an important increase (from 9% to

75%) on caffeine extraction yield from green tea, when using CO2

wetted with 8.8% water [18].The lower extraction yield obtained in the test conducted at

64% moisture was presumably due to the preference of the caf-

feine to remain soluble in the excess aqueous phase, as suggestedby Pourmortazavi and Hajimirsadeghi [40].

3.3. Effect of grinding and use of packing and redistributors

In general, grinding is an operationthat improves the extractionof compounds because it decreases the particle size, augmentingthe contact area with the solvent [42,43]. However, it is a highenergy cost operation, so it is important to evaluate the need for

this procedure.In order to assess the impact of grinding the rawmaterial,yields

obtained at fixed conditions in both whole and ground coffee huskswere compared (samples 2 and5, in Fig.3, respectively).The results

show that grinding did not improve the process performance, butalso hindered the operation. This could be because milling the cof-fee husks together with the addition of water generated a very

0

5

10

15

20

25

30

W (16,4%

moisture)

W (32%

moisture)

W (48%

moisture)

W (64% moisture)

G (32% moisture) + R

G (32% moisture) + R +

P (36%)

G (32% moisture) + R +

P (50%)

E x t r a c t i o n Y i e l d ( % )

1

23

4 5

6

7

Fig. 3. Effect of different pre-treatments on the extraction yields with supercritical CO2 at 200bar and 333K with a mass ratio of 24g CO2 /g husks (sample 1 extracted at

350 bar and 373K) (W, whole coffeehusks; G, groundcoffee husks; R, redistributors;P, packing).




0

10

20

30

40

50

60

70

80

90

100

8006004002000

CO2 mass (g)


Fig.4. Comparison betweenthe extractioncurves obtainedfor purecaffeine()and

forthetotalextractfromcoffeehusks(•).Conditions:333 K and200bar (brokenline:

linear fitting; solid line: second grade polynomial fitting).

compact bed, so CO2 would pass through preferential channels

(probably around the bed) without making efficient contact withthe raw material. This negative effect is common in beds made of finely ground raw materials [44].

To eliminate this negative effect, mesh was inserted togetherwith the rawmaterialat differentlevelsin the fixed bed to facilitate

CO2 radialredistribution,as previouslyrecommended [45]. We alsostudied mixing of the ground husks with a packing material con-sisting of inert perforated beads used in two different proportionsby weight: 36% and 50% (samples 6 and 7 of Fig. 3, respectively).

Its purpose was to increase the void fraction of the bed facilitatingthe CO2 flow and contact. Our results showed that the use of theseagents improved the extraction yield compared to those samplesin which they were not employed, but, owing to the fact that the

results obtained were not better than those from the whole raw

material, we opted to continue the investigation with coffee huskswithout grinding, which would mean important energy savings atthe industrial scale.

3.4. Extraction curve

The course of the extraction process can be followed by deter-mining the amount of extract against the time of extraction or

the solvent used. If the extractible compounds are easily acces-sible, the obtained curve presents two different parts; the firstone is a straight line (corresponding to a constant extraction rate)while in the second part, the curves bends down, approaching a

limiting value which is given by the total amount of extractiblesubstances [46]. In cases of a low initial concentration of extract in

the solid substrate or an extract not readily available for the sol-vent, transport within the solid dominates from the beginning of

theoperation. In thiscase,the extraction curve does notshow thesetwo parts, but approaches an asymptote, given by the distributioncoefficient corresponding to the initial concentration of the extract

in the solvent [47]. Of the latter type are those obtained for theextraction of caffeine from coffee husks using supercritical CO2. Anexample obtained at 333K and 200baris given in Fig.4. The extrac-tion yield increased at an ever-decreasing rate as the mass of CO2

was increased. Increasingamounts of CO2 flowingthrough thefixedbed were achieved by increasing the CO2 flow rate and/or by usinglonger operational times (see Table 1).

Fig.4 alsocomparesthe extractionyieldwith theone that would

result if allthe caffeine initiallycontained in thecoffeehuskswould

be fully accessible, so compressed CO2 would leave the extractor

Table 1

Experimental conditions to obtain increasing amounts of CO2 for Fig.4.

CO2 flow (g/min) Time (min) CO2 mass (g)

0.6 180 116

2.5 120 295

2.4 180 436

5.8 120 702

saturated.(Thecurvewasdrawnbasedon thesolubilityvalueunderthe same operating conditions. This comparison demonstrates thedifficulty of caffeine extraction from coffee husks. For example, a

quantity of 150 g of CO2 was enough to achieve total extraction of pure caffeine if it was completelyaccessible, while for coffee husks,this amount could only retrieve 15% of the extractible compounds(that is, a mixture of caffeine, fats and other minor compounds.)

The low extraction yield achieved may be attributed to tworeasons. First of all, it could be due to the capacity of caffeine toform complexes with certain compounds naturally present in cof-fee husks (such as chlorogenic acids) which would prevent its total

removal [15,20] even in the presence of water. Second, it could berelated to the strong resistance of the internal mass transfer.

3.5. Effect of operating conditions

The density of a supercritical fluid varies dramatically withoperational pressure and temperature. Thus, one of the most char-acteristic properties of supercritical fluids is the wide range of

densities that can be attained depending on the pressure andtemperature of work. Table 2 shows the estimated density val-ues corresponding to the operational conditions used in this work[48]. Consequently, and due to the direct relationship between the

density of a fluid with its solvation capacity, gases at supercriticalconditions can vary enormously in terms of solventpower by smallvariations of pressure and/or temperature.

On the other hand, the change of these two variables gener-

ates significant changes in CO2 transfer properties, such as viscosityand diffusivity, which therefore impact on the penetrability of CO2

into solid matrices. In general, increasing the operational pressuregenerates an increment of viscosity and a reduction of the self-

diffusion coefficient, while an increase in temperature causes theopposite effect.Finally, when it comes to plant materials, it is espe-cially important to maintain the temperature in a certain rangesince it could alter the properties of both the natural source and

the solutes being extracted. For all these reasons, when evaluatinga supercritical fluid extraction process, it is mandatory to study theimpact of these two variables.

3.5.1. Effect of temperature on extraction yields

As discussed in Section 3.1, the impact of temperature onthe solubility of extractible compounds depends on the working

Table 2

Density of supercritical CO2 under the experimental conditions used in this work

[48].

Pressure (bar) Temperature (K) Density (kg/m3)

60 373 100100 373 189

150 373 333

200 373 481

250 373 589

300 373 662

300 363 703

300 353 746

300 343 788

300 333 830

300 323 870

300 313 910




5

10

15

20

25

30

35

40

383363343323303

Temperature (K)


Fig. 5. Influenceof temperature on the extractionyield obtainedfrom whole coffee

husks at a fixed pressure of 300bar, a fixed water content of 32% and a total CO2

massof 320g (: experimental data; solid line: linear fitting).

pressure, which depends on whether the operation is carried outabove or below the crossover point.

In Fig. 5, extraction yields from wet and whole coffee husks

using CO2 at 300 bar with increasing temperature from 313 to373 K are shown. The solvent to feed ratio was kept fixed at 39gCO2/g (experimental runs were obtained at constant flow rates).Under the same treatment conditions, higher yields were obtained

when working at higher temperatures, as was expected due tooperating above the crossover point, leading to an enhancementin caffeine solubility. This fact has been previously reported bySaldana et al. [15], who concluded that the extraction yield of

methylxanthines fromvarious natural sources at 400 bar (above thecrossover point) increased with increasing temperature, whereaswhen working at 100 bar, there was a retrograde behaviour. This

result also agrees with those obtained by Kim et al. [11], whose caf-feine extraction yields from green tea leaves increased from 54% to66%by increasing theworking temperature from 313to 353 K whileoperating at 400 bar. Similarly, in the extraction of caffeine fromcoffee beans, Saldana [34] f ound that by increasing the working

temperature from 313 to 343 K, the amount of material extractedwhen working at 280 bar increased, although much of the extractwas the essential oil of coffee, which in green coffee beans of theRobusta variety comprises between 9 and 13% (w/w) of the raw

material.Another reason for this behaviour is the increase in the mass

transfer rate when increasing operational temperature, due to animprovement in the diffusion coefficient [49]. Knaff and Schlün-

der showed that for a range of only 10K (from 323 to 333K), the

diffusion coefficient of caffeine in supercritical CO2 varied from1.506×10−8 m2/s to 1.992×10−8 m2/s when working at a fixedpressure of 137bar [50].

0

10

20

30

40

50

60

70

350300250200150100500

Pressure (bar)


Fig. 6. Influence of pressure on the extraction yield obtained from whole coffee

husks at a fixed temperature of 373K, a fixed water content of 32% and a total CO2

massof 429g (: Experimental data; Curve line: second grade polynomial fitting).

Furthermore, the colour of the extracts was influenced by the

working temperature; thus, samples obtained at higher tempera-tures showed a dark brown colour instead of the yellowish-green

seen in the samples obtained at lower temperatures. This colourtransformation was possibly due to the browning of some of thepigments naturally present in the coffee husks.

3.5.2. Effect of pressure on extraction yields

Pressure is another factor that sets the solvent capacity of supercritical CO2. As discussed in Section 3.1, an increment of theworking pressure causes an increase in the density of the com-pressed gas (see Table 3), which in turn enhances the gas solvent

capacity for the removal of the caffeine.To check whether this trend was maintained in the extraction

process, different pressure values were explored maintaining a

fixed temperature of 373K and a solvent to feed ratio between thecompressed gas and the coffee husks of 53g/g. Results are shownin Fig. 6.

Indeed, and as expected, an increase in the operating pressureexponentially increased the efficiencyof extraction. A raise in pres-

sure from 100to 300bar, multiplied by tenthe extractionyield. Thisresult is consistent with those found in literature. Thus, Saldanaetal. [15] confirmed the effect of variation of pressure on extractionyield: working at 100, 200 and 300 bar resulted in the removal of

0.26%, 60% and 96% of the initial caffeine present in guaraná seeds,respectively. Moreover, Kim et al. [11] improved the yield of caf-feine extractedfromtea leaves from 3% to 28%whenincreasing theoperational pressure from 200 to 400bar working at a fix tempera-

ture of 323K using compressed CO2 modified withwater(7%, w/w).

Similarly,Icen and Gürü [12] reported increments on the extractionof caffeine from teastalks from 13.6 to 14.7 mg/g when augmentingthe pressure from 150 to 200 bar at 333 K.

Table 3

Total extraction and caffeine yields obtained under different conditions.

T (K) P (bar) Solvent to raw material

mass ratio (gCO2/g)

Time (min) Extract yield (%) Purity of caffeine (%) Caffeine yield (%)

333 300 35 120 24 63 27

353 300 36 105 28 74 39

373 300 40 120 35 61 40

373 200 53 100 29 65 36373 300 58 105 65 64 78

373 300 197 300 59 77 84




Table 4

Conditions used and caffeine extraction yields from several natural sources obtained using pure supercritical CO2.

Raw material 2009 world

productiona

(MM tons)

Initial

caffeine (%)

Pressure

(bar)

Temperature

(K)

Solvent to raw

material mass

ratio (gCO2/g)

Yield (%) Ref.

Theobroma cacao (cocoa beans) 4.2 0.6–0.8b 400 343 43.7 66 [16]

Paullinia cupana (guaraná seeds) N. A. 4.3b 400 343 399 97 [15]

Ilex paraguariensis (maté leaves) 0.8 0.8–0.9b 400 343 1140 96 [15]

Camellia sinensis (tea leaves) 3.9 2.8b 300 343 102 9 [17]

Camellia sinensis (tea stalks) 3.9 1.16 250 333 924 62 [12]Camellia sinensis (fibre wastes) 3.9 0.92 250 333 924 66 [12]

Coffeacanephora (coffee husks) 8.3 1.1 300 373 197 84 This work

Coffeacanephora (coffee husks) 8.3 1.1 300 373 58 78 This work

N.A., Datum not available.a FAO Statistics [23].b Data obtained from Ref. [1].

3.6. Caffeine purity in the extracts

Along with the caffeine extracted, other unidentified com-

pounds naturallypresent in coffeehusks were recovered, primarilyfatty compounds. Table 3 shows the purity of selected extractsobtained under different conditions.

The main component in all the extracts obtained was caffeine,

with an average value of 67%. This high selectivity of supercriti-cal CO2 for caffeine in coffee beans has been previously reportedby Saldana [34]. This value was not substantially altered whenvarying experimental conditions. As a result, caffeine yields var-

ied proportionally to total extraction yields. In other words, anincrement in operational pressure, temperature and solvent to rawmaterial mass ratio resulted in higher extraction rates, achievingthe removal of more than 80% of the initial caffeine when working

under the most extreme conditions.Ontheotherhand,therestofthecompoundswereeasilyremov-

able by simple water washing, which generated two immisciblephases: a fatty one with the vast majority of the undesirable com-

pounds, and an aqueous one in which the caffeine was selectivelysoluble. After evaporating this water, extracts had purity higher

than 94%.

3.7. Comparisonwith other natural sources

Table4 shows a summary ofthe yieldsof caffeine extractedfromdifferent natural sources with pure supercritical CO2. The oper-ational conditions employed in the processes are also included.The highest extraction yield of caffeine obtained from coffee husks

(about 84%) was a good value, even though it was not as high asthose obtained in supercritical extraction from guaraná seeds andmaté leaves.

However, compared to these other sources, coffeehusks present

further advantages as a raw material for the production of caffeine.First, the world production of green coffee is higher than that of

the other natural sources cited, which generates (as commentedin Section 1) a large quantity of husks rich in caffeine during the

manufacturing process. Second, the procedure requires the use of much lower solvent to raw material mass ratios (60–200 g CO2/g).Consequently, this material is definitely an interesting option forobtaining caffeine on an industrial scale, as well as from an eco-

nomical and environmental point of view.

4. Conclusions

The present study evaluated the technical feasibility of extract-ingthe caffeine naturallypresent in Robusta coffeehusks using CO2

under supercritical conditions, as a novel use of this common and

abundant residue from the coffee industry.

The results revealed that to make the process feasible, pre-treatment was required, consisting of pre-wetting of the rawmaterial up to 32% moisture, which is close to that needed for cof-

fee beans (30%) and doubled the original amount of water presentin the coffee husks. Grinding the coffee husks seemed to be a pro-cess that hindered the removal of the compound of interest, due tothe strong compaction of the bed. It seems that coffee husks were

porous enough to allow the circulation of the gas, easily extractingthe available caffeine.

On the other hand, the use of greater quantities of CO2 resultedin a large increase in the amounts of caffeine extracted, as the CO2

did not reach saturation. These greater amounts could be achievedeither by increasing the CO2 flow rate or by increasing operationaltimes. Optimum relationships should be deducted through an eco-nomical analysis.

Regarding the operational conditions, it was observed that themore extreme the operational conditions were (pressure and tem-perature above the crossover point), the higher the extractionyields achieved. Pressure variation was more effective, because it

affected the density of the supercritical fluid and, therefore, its sol-vent capacity. Thus, 84% of the initial caffeine was extracted, after

treatingthematerialforfivehourswith197gofcompressedgasperg ofrawmaterialat 373 K and 300bar. However,usinga ratio aslow

asof58gCO2/g, 78%of the initial caffeine was recovered underthesame conditions. The remaining caffeine seemed to be not remov-able under the conditions studied, due to the strong interactionsexisting between caffeine and the plant matrix and the important

limitations to internal mass transfer.Caffeine was not extracted pure, but in a mixture of fats and

small quantities of pigments, which could be easily separated bysimple water washing and subsequent evaporation, resulting in

high purity extracts.Thus, coffee husks are a very interesting option as a raw mate-

rial from an economic and industrial point of view since they arean abundant residue of the coffee industry with a high caffeine

content, most of which is simply extracted. The low cost of theraw material and the employment of moderate CO2 ratios wouldresult in low production costs. However, an exhaustive economi-cal analysis should be made to establish if the combination of this

benefit together with the selling of the caffeine andthe husks couldcounterbalance the relatively high installation costs.

Acknowledgement

Theauthors thank NatraceuticalGroupfor thefinancial support.

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