The Journal of Supercritical Fluids, 1992, 5, 157-162 157

Supercritical Carbon Dioxide Extraction of Peppermint and Spearmint?

Paul Barton*

Department of Chemical Engineering, Pennsylvania State University, University Park, PA 16802

Robert E. Hughes, Jr.

A, M. Todd Company, I71 7 Douglas Drive, K&mazoo, MI 49007

Mamoun M. Hussein

Warner-Lambert Company, 175 Tabor Road, Morris Plains, NJ 07950

Received June 14, 1991; accepted in revised form February 2 1, 1992

Peppermint and spearmint oils were extracted from cut green plants and field-dried hay with liquid and supercritical carbon dioxide at 297 to 3 16 K and 6 to 18 MPa. Solvent treatment was varied from 6 to 30 g COz/g dry plant material. Extraction time was varied from 4 to 9 hours. Extraction vessel charge sizes were 4.4 and 33 L. Downflow of carbon dioxide through the bed of mint plants was more effective than upflow. Essential oil compositions and attainable yields were nearly the same as those by steam dis- tillation when single pass mode of CO;! with depressurization to atmospheric pressure was used for oil re- covery. The recovery of the terpene constituents was reduced when using depressurization to 3-6 MPa for oil recovery and recycle of CO 2. The flavor and fragrance of the carbon dioxide mint extracts were closest in quality to actual mint plant leaves, compared to mint oils produced by conventional steam distil- lation.

Keywords: supercritical fluid, peppermint, spearmint, carbon dioxide, extraction

INTRODUCTION Peppermint and spearmint are popular flavors used

in a wide variety of sugar confectioneries, chewing gums, toothpastes, chocolate fillings, pharmaceuticals, and liqueurs. The volatile oils from the peppermint and spearmint plants, rather than just the leaves, are used in most applications.

True peppermint consists of the aerial parts of the perennial herb Mentha piperita L., usually harvested when in flower.’ Peppermint odor is related to high men- thone and menthol contents. The designation “spearmint” is applied commercially to several species and varieties of the genus Mentha possessing a distinct and characteristic odor profile attributable to high carvone content.

YPaper presented at the 2nd International Symposium on Supercritical Fluids, May 20-22, 1991, Boston, MA, USA.

Spearmint plants resemble those of peppermint and are similarly cultivated and harvested.

Most of the mint crops are cultivated for steam dis- tillation of the essential oil. The plants are cut and the hay is partially dried in the field, loaded into portable steaming chambers, and transported to a distillation site. Steam is sparged into the bottom of the bed of hay and removed overhead and piped to a water-cooled condenser vented to the atmosphere. The essential oil is decanted from the steam condensate. The stripped hay is returned to the field.

Much of the steam-distilled mint oil production is redistilled and rectified to remove undesirable compounds such as dimethyl sulfide and to concentrate the flavor and fragrance constituents such as menthol, menthone, and carvone. These fractionations are done using steam or un-

0896-8446/92/0503-O 157$4.00/O 0 1992 PRA Press

158 Barton et al. The Journal of Su,vercriticoi Fluids, Vol. 5, No. 3, 1992

Figure 1. 4.5Liter apparatus for supercritical extraction from solid materials. CW, cold water; E, wire basket; F, fil- ter; FV, manual control valve; HW, hot water; HX, heat ex- changer; M, motor; P, plunger pump; PI, pressure indicator; PSS, pressure safety switch; TI, temperature indicator; V, vessel; WI, weight indicator.

der vacuum to minimize thermal exposure. Nevertheless, flavor and fragrance are degraded by these distillation pro- cesses.

Gerard* discussed using countercurrent extraction of essential oils with supercritical carbon dioxide to separate the terpene, oxy-compound, and sesquiterpene fractions. Temperatures in such extractive distillation processes can be maintained less than 320 K, so thermal degradation is avoided.

In our work, we use direct leaching of essential oil from cut mint plants using pressurized carbon dioxide at 297 to 316 K. Thus, thermal degradation is avoided in the mint oil production process right from the start.

EXPERIMENTAL SECTION The supercritical carbon dioxide batch extractions

were performed in (1) the small pilot plant shown in Figure 1, and (2) the large pilot plant of Supercritical Processing, Inc., sketched in Figure 2. The small plant has a 4.57-L extraction vessel (V-2) and is operated in the single-pass mode of CO;! either down or up through the bed of cut mint plants, with oil recovery by depressuriza- tion to atmospheric pressure. During the extraction, the pressure was cycled a few times between 14 and 8 MPa. The large pilot plant has a 50-L extraction vessel (V-l)

T 1 “9 f Q Hx-2

i

Figure 2. 50-Liter supercritical processing pilot plant. A. CO2 supply; B. vent: E. basket; HW, hot water: HX. heat ex- changer; P. pump; PCV, pressure control valve; R, refrigera- tion system; V. vessel; T, variable speed transmission.

and is operated with recycled CO? flowing down through the bed of cut mint plants, with oil recovery by partial depressurization in a 10-L separator (V-2) and finally by depressurization to atmospheric pressure at the end of the extraction. During the extraction, the extraction pressure was sometimes cycled between 14 and 11 MPa. The car- bon dioxide is condensed in a refrigerated condenser (HX- 1) and recycled back to the extractor.

In the small pilot plant, cut mint leaves and stems are placed in a wire basket (E-l) between two porous Teflon filters (F-2,3) that prevent carryout of particulates. A variable-displacement piston pump (P-1,2), with a maximum capacity of 57 cm”/min, takes liquid carbon dioxide from a cylinder (V-l) on a weigh scale (WI-l) and pumps it through a heat exchanger (HX-I) where the car- bon dioxide vaporizes and becomes superheated. The ex- traction is carried out in the semibatch mode (fill/continuous flow/discharge), where the continuous flow portion is done at a preset pump rate. If any aqueous or organic liquid extract phase is formed, it is expected to have a higher density than the supercritical carbon dioxide at the temperatures and pressures used. Operation with carbon dioxide in the downflow mode assures that this liquid extract phase will be washed down and exit with the dense gas phase. Also, any liquid carbon dioxide con- densed during depressurization is pushed out the exit.

Extraction temperature is controlled by indirect heat exchange (HX-1,2) with lukewarm water; no high skin temperatures are encountered by the extract. The pressure in the extraction vessel is controlled manually by a valve (FV-I) in the exit tube. Sometimes the pressure was purposely cycled between approximately 14 MPa and ap- proximately 8 MPa to superimpose convective mass transfer onto diffusive mass transfer within the pores of the plants and to have multiple flushings of extract from the void space in the extraction vessel. These pressure changes caused expansive self-cooling and compressive warming during the extractions. The temperatures and pressures reported in this work are the time-integrated av- erages.

Tjle Journal of Supercritical Fluids, Vol. 5, No. 3, 1992 Extraction of Peppermint and Spearmint 159

TABLE I Operating Conditions* and Oil Yields

Water Content,

wt %

Av. Temp.,

K

Av. Press., MPa

g co2

g dried hay Time,

h p oil

g dried hay

Plant Hay (4.4-L charges)

Spearmint 15 306 10 1.4 4.2 0.039" (Farwest scotch) 308 9 6.3 4.2 0.026

Spearmint 16 306 11 9.7 4.6 0.038b (Farwest scotch) 307 11 9.5 4.0 0.037

Peppermint (Idaho)

14 307 10 7.9 4.5 0.02lC 307 10 9.8 5.4 0.026

CO? Upflow 307 8 12 6.8 0.019 17 9.2 0.022

Frozen Green (4.4-L charges)

Peppermint 82 306 11 21 4.6 0.025" (Midwest) 13 32 6.3 0.025

Peppermint 69 307 10 13 3.9 0.009 (Idaho) 307 11 16 6.5 0.009

Frozen Hay (33-L charges)

Spearmint (Midwest)

50 302 12 30 5.6 305 12 30 5.8 307 13 28 6.2

0.008' 0.008 0.008 0.010 with wax

*: Direction of CO, flow through extractor is down except where noted.

Oil vield bv steam distillation a) 0.036 d) 0.027 b) 0.038 e) 0.009 c) 0.029 f-l 0.017

The pressure control valve and exit tubing are warmed by lukewarm water (HX-3) to prevent plugging by freezing. The depressurized extract is collected in product traps (V-3,4) cooled by ice water and dry ice, re- spectively. Carbon dioxide gas is vented to a hood in these single pass experiments.

In the large pilot plant, all heating is also done with warm water (HW), to avoid exposing the extract and recy- cled carbon dioxide to high temperature. The extraction pressure is automatically controlled by a back-pressure re- ducing valve (PCV). The pressure in the product receiver is controlled by the refrigerant temperature in a double- pipe condenser (HX- 1). The variable speed recycle pump (P-l) was operated at a capacity of 22 kg/h of liquid car- bon dioxide during the continuous flow portion of the ex- traction. The recycled carbon dioxide is heated to the su- percritical conditions in a double-pipe heat exchanger

(HX-2). Make-up carbon dioxide is provided from tanks (A) by a pump (P-2).

The recovered oil was analyzed by gas chromatogra- phy. For the samples from the small pilot plant, a HP 5790 gas chromatograph was used with a 30 M x 0.25 pm ID, 0.25ym film Carbowax-20 M fused silica capil- lary column programmed from 343 to 473 K. For the samples from the large pilot plant, a 0.25-mm ID, 0.25 pm film Supelcowax 10, fused silica capillary column was used.

RESULTS AND DISCUSSION The yields of oil from the various extractions and

distillations are included in Table I. The CO?-extracted oils are strongly colored, dark yellow to greenish yellow. They differ very slightly from the steam distilled oil in specific gravity and refractive index. The samples recov-

160 Barton et al. The Journal of Supercritical Fluids, Vol. 5, No. 3, 1992

TABLE II Comparative GC Analyses of Peppermint Oils from Steam Distillation and Supercritical CO2

Extraction of Field-Dried Hay and Green Plants

% of Peak Area

Field-Dried Green

Idaho Midwest Id&Q

Constituent Steam 02 (332 (32 co2 Distilled Downflow Upflow Downflow Down flow

Dimethyl Sulfide 0 - Isobutyraldehyde 0.01 0.09 0.01 0.01 Isovaleraldehyde 0.05 - 0.01 a-Pinene 0.75 0.51 0.42 0.32 0.18 P-Pinene 1.03 0.77 0.69 0.55 0.33 Sabinene 0.59 0.47 0.43 0.32 0.20 Myrcene 0.28 0.26 0.24 0.16 0.11 &Terpinene 0.16 0.08 0.04 0.01 0

L-Limonene 1.78 1.69 1.57 0.97 1.13 1,8-Cineole 5.41 4.39 4.34 4.11 3.58 cis-Ocimene 0.53 0.56 0.55 0.14 0.26 y-Terpinene 0.43 0.33 0.25 0.06 0.11 p-Cymene 0.12 0.07 0.06 0.03 0.04 Terpinolene 0.13 0.11 0.10 0.05 0.13 3-Octanol 0.26 0.23 0.24 0.34 0.30 1 -Octen-3-01 0.09 0.06 0.04 0.08 0.17

t-Sabinene Hydrate 2.67 3.00 3.47 2.89 3.09 L-Menthone 22.49 22.79 22.37 26.26 1 1.07 Menthofuran 4.01 4.78 4.80 11.05 7.81 Iso-menthone 2.96 2.79 2.80 3.20 1.94 P-Bourbonene 0.50 0.45 0.47 0.18 0.32 Linalool 0.42 0.41 0.43 0.28 0.34 Menthyl Acetate 2.19 2.19 2.36 0.96 7.28 Neo-menthol 5.43 5.18 5.28 4.75 3.93

P-Caryophyllene Terpinen-4-01 L-Menthol Pulegone cr-Terpineol Germacrene-D Piperitone Viridiflorol Later Unknowns

NR* 0.43

38.24 1.24 0.20 3.07 0.6 1 0.61

-

NR” NR” NR* NR” 0.30 0.28 0.15 0.3 1

37.21 37.34 30.12 46.79 1.45 1.48 5.06 2.09 0.22 0.19 0.29 0.17 3.41 3.51 3.59 2.16 0.57 0.70 0.48 0.65 0.38 0.68 0.59 0.20 0.90 1.24 0.44 0.53

* Not resolved from preceding peak.

ered from the separator in the small pilot plant contained lot plant, matching the yields obtained by steam distilla- 3-5% entrapped moisture separable by centrifugation. tion. The carbon dioxide was supercritical most of the The oils also had a heterogeneous appearance due to solid time, at an average temperature of 307 K and an average particulates. Upon warming to 303-308 K, the solids pressure of 10 MPa. An extraction time of 4 hours was melted and the oils had a clear homogeneous appearance. used, though the minimum has not been determined. Chemical analyses by gas chromatography are presented Based on the yield data in Table I, the carbon dioxide in Table II for the peppermint oils and in Table III for the treatment needs to be 7.5 g CO& dried hay in the single- spearmint oils. pass downflow mode.

Complete extraction of spearmint oil from field- Gas chromatographic analyses showed several minor dried hays was attained by carbon dioxide in the small pi- differences between the C&-extracted spearmint oil from

The Journal of Supercritical Fluids, Vol. 5, No. 3, 1992

TABLE III Comparative GC Analyses of Spearmint Oils from Steam Distillation and Supercritical CO2

Extraction of Field-dried Hay

Extraction of Peppermint and Spearmint 161

% of Peak Area

Constituent

Farwest Scotch

Steam (332

Distilled Downflow Steam

Distilled

Midwest

CO2 Downflow with Recycle

Dimethyl Sulfide 0 Isobutyraldehyde 0 Isovaleraldehyde 0.03 a-Pinene 0.73 P-Pinene 0.69 Sabinene 0.47 Myrcene 0.89 a-Terpinene 0.02

0.01

0.37 0.41 0.32 0.68 0.03

0.33 0.36 0.28 0.47 0.00

L-Limonene 17.69 15.82 30.01 14.65 1,8-Cineole 1.38 0.92 1.52 0.86 cis-Ocimene 0.02 0.02 0.02 0.01 FTerpinene 0.05 0.02 0.07 0.01 p-Cymene 0.01 0.01 0.02 0.01 Terpinolene 0.02 0.01 0.03 0.01 3-Octyl Acetate 0.10 0.07 0.23 0.11 3-Octanol 2.01 1.89 1.62 1.39 1 -Octen-3-01 0.04 0.03 0.02 0.02

t-Sabinene Hydrate 0.46 0.46 0.11 0.22 L-Menthone 1.07 0.93 1.44 1.22 /I-Bourbonene 1.07 0.92 0.76 0.54 Linalool 0.05 0.05 0.05 0.05 P-Caryophyllene 0.59 0.52 0.59 0.46 Terpinen-4-01 0.20 0.19 0.08 0.07 Dihydrocarvone 0.83 0.83 0.77 0.81 Dihydrocarvyl Acetate 0.13 0.05 0.10 0.14 trans-SFarnesene 0.24 0.44 0.21 0.19

a-terpineol 0.45 0.24 Germacrene-D 0.41 0.36 L-Carvone 67.22 69.01 Carvyl Acetate 0.04 0.03 Vans-Carve01 0.19 0.19 cis-Carve01 0.31 0.32 cis-Jasmone 0.19 0.24 Viridiflorol 0 0.09 Later Unknowns 0 1.48

0.21 0.18

54.51 0.10 0.28 0.19 0.13 0.00

-

0.29 0.21

71.77 0.08 0.42 0.33 0.23 0.01

the small pilot plant and the steam distilled counterpart (Table III). The C02-extracted oil contains slightly more L-carvone and less monoterpenes (pinenes, myrcene, ter- pinene, L-limonene) and 1,8-cineole. Iso-valeraldehyde and dimethyl sulfide are absent in the C02-extracted oil, which suggests that they are possibly heat-generated. The C02-extracted oils contain several minor unknown com- ponents which exhibited high retention time.

Extraction yields of peppermint oil from field-dried hay and cut green plants (shipped frozen) by downflow carbon dioxide in the small pilot plant reached 90 to 98%

of those by steam distillation. The carbon dioxide was Supercritical most of the time, at an average temperature of 307 K and average pressure of 10-l 1 MPa. An extrac- tion time of 4-5 hours was used. The carbon dioxide treatment needs to be 10 g C02/g dried hay in the single- pass downflow mode. The yield of peppermint oil in the CO2 upflow mode reached only 76%, even at higher sol- vent treatments and extraction times. During CO2 dis- charge in the pressure swing cycles, expansive self-cool- ing condensed liquid CO2 in the extractor and the pressure dropped to less than 6 MPa. The essential oil tended to

162 Barton et al. The Journal of Supercritical Fluids, Vol. 5, No. 3, 1992

concentrate in this boiling liquid rather than exit with the gaseous CO*, where the solubilities of the constituents are lower.2T3 It is not possible to transfer heat into a su- percritical extractor at a high enough rate to prevent con- densation from occurring during depressurization. In the downflow mode, the condensate is simply pushed out the bottom and its oil content is recovered.

Gas chromatographic analyses showed several minor differences between the COz-extracted.peppermint oil from field-dried hay in the small pilot plant and the steam dis- tilled counterpart (Table II). The C02-extracted pepper- mint oils contained less mono-terpenes (heads) and I$-ci- neole than the steam distilled oil. The former, however, contained more menthofuran. This is possibly due to heat sensitivity of menthofuran. The direction of the CO2 flow has not affected the composition of the extracted peppermint oil.

Composition of peppermint oil extracted from green plants is quite different from the oil extracted from weath- ered dry hay (Table II). The content of menthofuran is 2- 3 times higher from the green plants. The peppermint oil from mature stalky Idaho green plants harvested in late August contained appreciably lower menthones, higher menthofuran, lower menthyl acetate, and higher menthol than the oil from Idaho hay harvested in June.

Scale-up. The yield of spearmint oil from par- tially-dried stalky hay by C02-extraction in the large pilot plant was 50% of that obtained by steam distillation (Table I). This hay contained oil that was significantly high in L-limonene and low in L-carvone, as isolated by steam distillation (Table II). Yet the oil extracted by CO2 contained more normal concentrations of these two con- stituents. In addition to the essential oil, carbon dioxide extracted waxy material to the extent of 0.002 g/g dried hay.

In the large pilot plant, the carbon dioxide extract leaves the bottom of the extractor at average conditions of 303-307 K and 12-13 MPa and is depressurized to aver- age conditions of 296 K and 4.4 MPa in the separator. The vapor from the separator enters the condenser main- tained at average conditions of 259 K and 2.4 MPa. Based on the solubilities of limonene and carvone in carbon dioxide at various temperatures and pressures,2,3 all of the essential oil is not recovered in the separator but enters the condenser where some may precipitate and some will be recycled with the carbon dioxide fed back to the top of the extractor. Based on solubilities, limonene will enter this recycle path at higher concentration than carvone. The recycled material lowers mass transfer driving forces in the extraction step. Also, throughout the extractions performed, the yield of oil is low by just 770 grams (from 108 kg hay). How much of this can get lost in the con- denser system? These phenomena may explain why the COz-extracted oil is low in yield and limonene content.

In the runs in the large pilot plant, operating at subcritical and supercritical temperatures produced similar results. Also, operating at constant extraction pressure and cycling the pressure by 3 MPa during extraction pro- duced similar oil yields and compositions. However, the solvent treatments in these runs were four times higher than the minimum required and this tends to cover up any influences these parameters may have had.

ORGANOLEPTIC EVALUATION Odors of the C02-extracted mint oils were closer to

those of their parent plant. To some testers, however, this leaf-like character was considered foreign to the oil. The difference between the C02-extracted and the steam- distilled oils was more pronounced in spearmint than in peppermint oil. The CO?-extracted spearmint oil was considered to possess a true spearmint leaf character. This description was also noted for the spearmint chewing gum prepared with this oil. In addition, the spearmint gum with COz-extracted oil was considered to have a more en- hanced flavor release character, longer lasting flavor and sweetness, and less bitterness. The peppermint gum with CO,-extracted peppermint oil had more enhanced flavor re- lease. The overall flavor quality was not considered too different from the distilled oil, although it had a definite herbal character.

CONCLUSIONS Essential oils can be extracted quantitatively from

cut peppermint and spearmint plants by supercritical car- bon dioxide at 306 to 308 K and 8 to 13 MPa. The sol- vent-to-plant ratio required is 7.5 g C02/g dried hay for spearmint and 10 g COz/g dried hay for peppermint. Required extraction time may be 4 hours. Wet green plants as well as dry hay can be extracted. Downflow of carbon dioxide through the bed of cut plants is more effec- tive than upflow.

The advantages of supercritical carbon dioxide ex- traction over steam distillation include: no thermal degra- dation of odor and flavor compounds; no production of dimethyl sulfide; leaf-like herbal character is retained; odor of oil is closer to parent plant; and more enhanced and longer lasting flavor in chewing gum. In CO2 extraction, menthofuran is not destroyed.

ACKNOWLEDGMENT We thank Raymond J. Robey of Supercritical

Processing, Inc., Allentown, PA, for his assistance.

REFERENCES (1) Heath, H. B. Source Book qf Flosors; Avi Publishing

Co.: Westport, CN, 1981. (2) Gerard, D. Cltrnz. 1~. Tech. 1984,56, 794. (3) Maier. M.; Stephan. K. Chern. Irtg. Tech. 1964, 56,

222.