5. High-pressure carbon dioxide applications

The most cited drawback of using supercritical carbon dioxide as solvent is the high investment cost for equipment acquisition and operation. However, the extraction with supercritical carbon dioxide presents a lower extraction time because of its diffusivity and low surface tension, greater selectivity in the compounds of interest and little or no consumption of organic solvents [43–46].

As given above, it is observed that the process of extraction of essential oils using supercritical CO2 is ecologically a cleaner method than the conventional ones, and it has been seen as one of

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Phytosterols (plant sterols) are non-volatile triterpenes. The great majority of these compounds are formed by carbon with one or two carbon-carbon double bonds [62]. And the most common phytosterols found in plants are β-sitosterol, campesterol, and stigmasterol [63]. These compounds have various biological activities such as lowering the total serum or plasma cholesterol levels and the low-density lipoprotein cholesterol levels. In addition, they have

For the extraction of these phytosterols, the supercritical CO2 has been shown to be an efficient technique for extraction of fixed oils from vegetable matrices. Studies report that this solvent may be superior to obtain oils in relation to the conventional extraction, exhibiting a recovery rate of phytosterols of 836.5 mg/100 g versus 30.5/100 g using a Soxhlet-type extraction apparatus [66]. A very important parameter for the extraction of phytosterols with supercritical CO2 is the increase of the pressure, because it favors the solvation power and consequently the solubilization of these compounds, with a recovery rate of up to 7262.80 mg.kg<sup>1</sup> [67]. Table 6 presents some articles published in the literature on the extraction of phytosterols

Carotenoids are tetraterpenes present in plants that have several applications in food [72], cosmetic [73], and pharmaceutical [74] areas. Some of the benefits provided by these pigments are: antioxidant activity and strengthening of the immune system against degenerative diseases such as cancer, cardiovascular diseases, muscle degeneration, inflammation, hyperten-

Because of their hydrophobic characteristics, carotenoids are usually extracted using organic solvents such as hexane and petroleum ether. Carotenoids with hydrophilic characteristics can be obtained with more polar solvents such as acetone, ethanol, and ethyl acetate [77]. The

Plants Phytosterols References

Brassica napus β-sitosterol, campesterol and brassicasterol [69] Hippophae rhamnoides L. β-sitosterol [70]

stigmastatrienol, Δ7-stigmastenol, Δ7,25-stigmastadienol, and Δ7-avenasterol

cholesterol, Δ-5 avenasterol and stigmasterol, while lower levels of Δ-5,24 stigmastadienol, brassicasterol, clerosterol + Δ-5-23 stigmastadienol, Δ-7

[68]

[71]

Cucurbita pepo convar Desmosterol, campesterol, stigmasterol, β-sitosterol, spinasterol, Δ7,22,25-

Sesamum indicum L. β-sitosterol + sitostanol, cholesterol, campesterol + campestanol +24-methylene

avenasterol, eritrodiol and Δ-7 stigmasterol

antitumor activities inhibiting the development of colon cancer [64, 65].

the most viable alternatives.

5.2. Phytosterols extraction

using supercritical CO2.

5.3. Carotenoid extraction

sion, insulin resistance and obesity [75, 76].

Table 6. Phytosterols extracted using supercritical CO2

### 5.1. Essential oil extraction

Essential oils have been used to prevent or treat human diseases for several centuries. The extraction of the volatile compounds present in edible or medicinal aromatic plants is generally carried out by hydrodistillation; however, the authors report that some compounds may undergo hydrolysis during the extraction period [47]. Although there are other techniques for isolating essential oils, the use of CO2 as supercritical fluid has been considered a "chemically green" unconventional extraction technique that does not alter or degrade the substances present in oils because it uses relatively low temperatures in the extraction process.

Guan et al. [48] performed a comparison between conventional extraction methods and extraction with supercritical CO2 and observed that the extraction using supercritical CO2 as solvent was less effective with recovery rate of 57.36% for eugenol compared to steam distillation with 58.2%, but it was more effective when compared to hydrodistillation with recovery rate of 48.82% and Soxhlet extraction with 57.24%. However, when compared with the extraction of eugenol acetate, the extraction with supercritical CO2 presented higher yields in relation to the other extraction methods.

Extraction of chemically active volatile molecules with supercritical CO2 is very widespread [49–51]. This is due to the possible applications as agents that promote biological activities [52], such as antioxidant activity [53], anti-inflammatory activity [54], insecticidal activity [55], and phytotoxic activity [56]. In Table 5, some studies in the literature on the extraction of essential oils with supercritical CO2 can be observed.


Table 5. Published studies on extraction of essential oils using CO2 as supercritical fluid.

As given above, it is observed that the process of extraction of essential oils using supercritical CO2 is ecologically a cleaner method than the conventional ones, and it has been seen as one of the most viable alternatives.

### 5.2. Phytosterols extraction

5. High-pressure carbon dioxide applications

solvents [43–46].

5.1. Essential oil extraction

226 Carbon Dioxide Chemistry, Capture and Oil Recovery

other extraction methods.

oils with supercritical CO2 can be observed.

dibutyl phthalate

The most cited drawback of using supercritical carbon dioxide as solvent is the high investment cost for equipment acquisition and operation. However, the extraction with supercritical carbon dioxide presents a lower extraction time because of its diffusivity and low surface tension, greater selectivity in the compounds of interest and little or no consumption of organic

Essential oils have been used to prevent or treat human diseases for several centuries. The extraction of the volatile compounds present in edible or medicinal aromatic plants is generally carried out by hydrodistillation; however, the authors report that some compounds may undergo hydrolysis during the extraction period [47]. Although there are other techniques for isolating essential oils, the use of CO2 as supercritical fluid has been considered a "chemically green" unconventional extraction technique that does not alter or degrade the substances

Guan et al. [48] performed a comparison between conventional extraction methods and extraction with supercritical CO2 and observed that the extraction using supercritical CO2 as solvent was less effective with recovery rate of 57.36% for eugenol compared to steam distillation with 58.2%, but it was more effective when compared to hydrodistillation with recovery rate of 48.82% and Soxhlet extraction with 57.24%. However, when compared with the extraction of eugenol acetate, the extraction with supercritical CO2 presented higher yields in relation to the

Extraction of chemically active volatile molecules with supercritical CO2 is very widespread [49–51]. This is due to the possible applications as agents that promote biological activities [52], such as antioxidant activity [53], anti-inflammatory activity [54], insecticidal activity [55], and phytotoxic activity [56]. In Table 5, some studies in the literature on the extraction of essential

Aromatic plant Bioactive compounds References

[61]

Juniperus communis L. Germacrene D and 1-octadecene. [57] Satureja hortensis γ-Terpinene, thymol, and carvacrol [58] Myrtus communis L. Methyl eugenol, 1,8 cineole, and beta- caryophyllene [59] Leptocarpha rivularis α-thujone, β-caryophyllene, and caryophyllene oxide [60] Piper nigrum L. β-caryophyllene, limonene, sabinene, 3-carene, β-pinene, and α-pinene [53]

Camellia sinensis L. 9-Thiabicyclo[3.3.1]non-7-en-2-ol, tricosane, heneicosane, tetracosane, and

Table 5. Published studies on extraction of essential oils using CO2 as supercritical fluid.

present in oils because it uses relatively low temperatures in the extraction process.

Phytosterols (plant sterols) are non-volatile triterpenes. The great majority of these compounds are formed by carbon with one or two carbon-carbon double bonds [62]. And the most common phytosterols found in plants are β-sitosterol, campesterol, and stigmasterol [63]. These compounds have various biological activities such as lowering the total serum or plasma cholesterol levels and the low-density lipoprotein cholesterol levels. In addition, they have antitumor activities inhibiting the development of colon cancer [64, 65].

For the extraction of these phytosterols, the supercritical CO2 has been shown to be an efficient technique for extraction of fixed oils from vegetable matrices. Studies report that this solvent may be superior to obtain oils in relation to the conventional extraction, exhibiting a recovery rate of phytosterols of 836.5 mg/100 g versus 30.5/100 g using a Soxhlet-type extraction apparatus [66]. A very important parameter for the extraction of phytosterols with supercritical CO2 is the increase of the pressure, because it favors the solvation power and consequently the solubilization of these compounds, with a recovery rate of up to 7262.80 mg.kg<sup>1</sup> [67]. Table 6 presents some articles published in the literature on the extraction of phytosterols using supercritical CO2.

### 5.3. Carotenoid extraction

Carotenoids are tetraterpenes present in plants that have several applications in food [72], cosmetic [73], and pharmaceutical [74] areas. Some of the benefits provided by these pigments are: antioxidant activity and strengthening of the immune system against degenerative diseases such as cancer, cardiovascular diseases, muscle degeneration, inflammation, hypertension, insulin resistance and obesity [75, 76].

Because of their hydrophobic characteristics, carotenoids are usually extracted using organic solvents such as hexane and petroleum ether. Carotenoids with hydrophilic characteristics can be obtained with more polar solvents such as acetone, ethanol, and ethyl acetate [77]. The


Table 6. Phytosterols extracted using supercritical CO2

techniques used to extract this compound may be maceration, Soxhlet, microwave-assisted extraction [78, 79], ultrasound-assisted extraction [80], pressurized liquid extraction [81, 82], and supercritical fluid technology using low temperature. The process is performed in a short time in relation to conventional processes and does not use toxic solvents to collect the compound of interest [83]. In Table 7, some published works that used supercritical CO2 to obtain carotenoids are shown.

### 5.4. Fatty acids extraction

Fatty acids (FA) belong to the lipid class and differ according to the size of the C chain (2–80), the presence or absence of double bonds (saturated or unsaturated) or their radical function as the groups hydroxyl, epoxy, and halogen atoms [89]. Ingestion of FA is essential to have an adequate energy balance in the human organism in addition to reducing the risk of some diseases such as diabetes [90], hypertension [91], coronary diseases [92], and inflammation [93].

Some of its applications are in food, nutraceutical, and cosmetic industries, and in the production of lubricants, biodiesel, and glycerol [94–96]. Some of the extraction methods that can be used to obtain FA are mechanical extraction [97], extraction by supercritical fluids and organic solvent [98], microwave-assisted extraction [99], and supercritical CO2 extraction [98]. Table 8 shows some studies that used supercritical CO2 to obtain the main classes of the FA group.

non-polar molecule and does not have "power" to solubilizate polar substances as is the case of the phenolic compounds (phenolic acids and flavonoids) [105, 106]. As previously mentioned, parameters of processes, such as temperature and pressure, can influence the extraction of bioactive compounds. Besides these two parameters, anthocyanins are also important for the extraction of phenolic compounds. Solvent flow rate, percentage of co-solvent, co-solvent type (ethanol or water), and extraction time are parameters that directly implicate the yield of

3-O-galactoside, delphinidin 3-O-arabinoside, cyanidin 3-O-glucoside, petunidin 3-O-galactoside, cyanidin 3-O-arabinoside, petunidin 3-O-glucoside, peonidin 3-O-galactoside, petunidin 3-O-arabinoside, peonidin 3-O-glucoside, malvidin 3-O-galactoside, peonidin

3-O-arabinoside, malvidin 3-O-glucoside, malvidin 3-O-arabinoside and

Raw material Compounds References

[113]

229

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[114]

Myrciaria cauliflora Anthocyanin [8] Elderberry (Sambucus nigra) Anthocyanins [110] Vitis vinifera var. Malvasia nera Anthocyanins [111] Arrabidaea chica Anthocyanins and luteolin [112] Scutellaria lateriflora L. Baicalin, dihydrobaicalin, lateriflorin, ikonnikoside I, scutellarin, oroxylin A 7-O-glucuronide, oroxylin A, baicalein, wogonin

Vaccinium myrtillus L. Delphinidin 3-O-galactoside, delphinidin 3-O-glucoside, cyanidin

malvidin 3-O-xyloside

Table 9. Phenolic compounds extracted with supercritical CO2 modified with ethanol/water.

Further examples of extraction of phenolic compounds using supercritical CO2 modified with co-solvents can be analyzed in the studies [106, 108, 109]. They extracted various flavonoids like quercetin, catechin, epicatechin from cranberry, blueberry, and raspberry. Table 9 presents some studies in which supercritical CO2 modified with ethanol/water were used to extract

Carbon dioxide can be safely applied in high-pressure extraction processes due to its numerous advantageous characteristics. It is neither toxic nor inflammable, being able to act as solvent, cosolvent, or anti-solvent, which allows it to be used in natural products and foodstuff processing that require treatments intending to preserve their nutritional and sensory properties. Since it is a non-polar substance, it is suitable for extraction of non-polar bioactive compounds when used in pure form. When associated with a polar co-solvent, it can be used for extraction of polar compounds such as phenolic compounds and anthocyanins. Therefore, these characteristics make carbon dioxide the most important fluid used in high-pressure processes for extraction, separation, fractionation, micronization, and encapsulation, applied to obtain concentrated

extracts with bioactive compounds for food, pharmaceutical, and cosmetic applications.

these substances at the end of the extraction process [107].

chemically active phenolic compounds.

6. Conclusion
