Subcritical Water Extraction Of Natural Products Part 3

Please contact oscar.xiao@wecistanche.com for more information

4.6. Essential Oils

Essential oils, also called volatile oils or ethereal oils, refer to the volatile chemical compounds derived from plants. Essential oils often consist of the parts of the flowers, leaves, wood, bark, root, seeds, or peel. Ethereal oils are usually lipophilic and easily soluble in oils, which enables them to easily penetrate the skin and enter the body through the rich capillaries under the subcutaneous fat. Essential oils have been used in folk medicine in ancient China, Egypt, Arabia, and Greece throughout centuries. Therefore, some researchers have attempted to utilize SBWE as a green solvent to extract essential oils from plants, and recent research is illustrated in Table 6 [12,22,28,174-206]. The most common essential oils are extracted by distillation [174,176,180,181], n-hexane [174]or supercritical carbon dioxide [181], or dichloromethane [181].

Please click here to know more

Coriander(Coriandrum satioum L.)seeds contain an essential oil (up to 1%) and are increasingly used as condiments in the food industry. Eikani et al.[174] and Zekovic et al. [175,181] have extracted and isolated the essential oil by SBWE. Extraction temperatures (100,125, 150, and 175°C), mean particle sizes (0.25,0.50, and 1 mm), and water flow rates(1,2, and 4 mL/min) were investigated by Eikani et al. Separation and identification of the components were carried out by GC-FID and GC-MS. They concluded that hydro distillation and Soxhlet extraction showed higher extraction efficiency, but the SBWE resulted in the essential oils more being concentrated in valuable oxygenated components. Zoran et al. also concluded that the SBWE process would have an advantage in terms of time consumption since 20 min for this process was significantly lower than the 2 h, 4-5h, and 4 h required for herbal decoction, Soxhlet, and supercritical fluid extraction, respectively.

Most essential oils extracted by SBWE have no research on the activities, except for Ma et al. [176], who extracted essential oils from K. galangal using ultrasound-enhanced SBWE and investigated the antioxidant ability of the essential oils. The effects of temperature, extraction time, pressure, ultrasonic power density, and frequency on the extraction yield were investigated. The antioxidant activity of the essential oils was evaluated by the assays of the 2,2-diphenyl-1-picryl-hydrazyl(DPPH) scavenging ability and the superoxide anion radical scavenging activity. The result demonstrated that the antioxidant effects of UAE

extracted essential oil were better than that of herbal decoction and SBWE. As shown in Figure 9, the optimal extraction temperature of essential oils is between 120 and 160°C and the time is between 10 and 35 min.

4.7. Alkaloids

Alkaloids are a class of basic nitrogen-containing organic compounds with a great structure diversity, most of which are heterocyclic compounds, and the nitrogen atom is in the heterocyclic ring. Most alkaloids are alkaline and have therapeutic and recreational activities. Alkaloid-containing plants have been used in folk medicine for centuries. Therefore, many researchers are paid attention to the extraction and separation of alkaloids from plants. Traditional extractions include organic solvents, such as methanol [183,184],

Cistanche can improve immunity

ethanol [183], or an acidic solution [187]. Recently, SBWE was successfully applied to the extraction or separation of alkaloids from plants or animals, as illustrated in Table 6.

Due to the diversity and relatively poor thermal stability of alkaloids, the optimal extraction methods depend on the pHysio-chemical characteristics of alkaloids. Liu et al. [16l used SBWE and capillary electrophoresis (CE) to extract and determine cytisine, sopho-carpine, marine, sophoridine, and oxymatrine in Sophora flavescens Ait., which is traditional Chinese medicine. The extraction yields obtained using SBWE, ASE, UAE, and chloroform soaking extraction methods were co Compared. SBWE needs a short extraction time, as there is no need for organic solvent consumption and it exhibited the highest extraction efficiency for the total alkaloid yield. Similarly, Torto et al.[184] concluded that both SBWE and conventional methods achieved co Mparable extraction vields, while reflux and UAE were slower (over 6h) and employed large quantities of organic solvents. Therefore, the SBWE method was simple and relatively fast for extraction. However, Liu et al. [183] showed that LC-ESI-orbitrap MS provides a powerful method for the identification and determination of hepatotoxic pyrrolizidine alkaloids, and reflux showed a higher extraction efficiency co Mpared with SBWE. Homes and his collaborators |186]investigated conventional and innovative extraction techniques (SBWE, UAE). They found both extracts of banana and beetroot peels obtained by THD(100°C, 20 min) exhibited the highest total phenolic content and antioxidant capacity. Extraction by infusion (80 °C, 30 min) yielded a beetroot peel extract with the highest total betacyanin content. The optimal extraction temperature and time for alkaloids is also listed in Figure 9.

4.8.QuinoneS

The quinones are a kind of organic components that have quinone structures, which can mainly be divided into four types: benzoquinone, naphthoquinone, pHenanthraquinone, and anthraquinone. Anthraquinone and its derivatives widely exist in plants and can be obtained from many plants, especially conifers. Some quinones have desirable pHarmaco-logical properties, such as purgative, antimicrobial and antiparasitic, anti-cardiovascular roles, etc. Extraction of quinones from natural products has utilized ethyl acetate [190], SFE[190], ethanol [191,193], or water as extraction solvents, shown in Table 6.

Mahmud et al. [188] performed an extraction of pHenolic compounds from pericarps of mangosteen by subcritical water treatment at temperatures and pressures of 120 to 160℃C and 1 to 10 Mpa in batch and semi-batch systems. They added 10 to 30% deep eutectic solvent (DES)to subcritical water, and the results showed that with 30%DES, the yields of xanthone and pHenolic compounds content were 24.87 mg/g dried sample and 179.54 mg of gallic acid equivalent/g dried sample at extraction temperatures of 160 and 120℃C in the batch system, respectively. The addition of DES in the SBWE process could accelerate hydrolysis reaction to extract plant biomass components matrix.

Morinda citrifolia(Noni), planted in tropical Asia, has been used in folk remedies to treat various kinds of diseases and symptoms. Shotipruk and coworkers have conducted a series of experiments on this plant[191-193]. They used a continuous flow system to extract damnacanthal, alizarin, and 1,2-dihydroxyanthraquinone. They found that pressure had no significant effect on the results for the range 110-220°C.Co Mpared with conventional extraction, SBWE and Soxhlet resulted in extracts that have the highest antioxidant activity. The data were fitted with mathematic models to determine the extraction mechanism. The results suggested that the overall extraction mechanism was influenced by solute partitioning equilibrium with external mass transfer through liquid film.

The optimal extraction temperature and time for quinones in SBWE are shown in Figure 10. The optimal extraction temperature of quinones is between 160 to 170°C and the time is between about 60 to 120 min.

4.9. Terpenes

Terpenes are a large and diverse class of organic compounds using isoprene as the basic structure unit, which widely exists in plants and some insects, and can be obtained from many plants, especially conifers. Terpenes have wide varieties and may be classified by the number of isoprene units in the molecule, such as monoterpenes, hemiterpenes, sesquiterpenes, diterpenes, triterpenes, tetraterpenes, and polyterpenes. Terpenes are of importance for use in food, cosmetics, and pHarmaceutical industries. However, the extraction of terpenes and terpenoids from natural products is often problematic, illustrated in Table 6. Consequently, they are mainly produced by industrial synthesis, usually from petrochemicals.

Chen et al. [197]employed SBWE for the efficient extraction of sesquiterpene lactones from I. racemose. Extraction time (23.2-56.8 min), temperature(129.5-230.5°C) and flow rate(1.3-4.7 mL/min) were investigated. A co Comparison of SBWE with traditional extraction technologies (Soxhlet, UAE, and SFE)showed that subcritical water could be a green and efficient substitution for the extraction of sesquiterpene lactones from I. racemose. Xiao et al. [13] developed SBWE for extraction of ursolic acid from Hedyotis diffusa. The RSM model proved to predict the experimental results very well and demonstrated that UA yield was mainly dependent on solvent/solid ratio, followed by particle size and temperature. Four extraction methods (UAE and SBWE)were co Mparatively analyzed, which indicated that SBWE was a time-saving, cost-saving, and environment-friendly extraction technology for the extraction of UA from Hedyotis fusa. Other researchers achieved the same experimental results, except for Falev et al. [200], who found that subcritical water is a poor solvent for pentacyclic triterpenes. Extraction with subcritical solvents (aliphatic alcohols, acetonitrile, and chloroform) is the most rapid and efficient way to isolate pentacyclic triterpenes from plant raw materials.

The optimal extraction temperature and time for terpenes in SBWE are summarized in Figure 10. The optimal extraction temperature of quinones is between 130 and 225C, and the time is between about 10 and 50 min.

4.10.Lianans

Lignans are a kind of natural compound formed by the polymerization of two molecules of phenylpropanoid derivatives (i.e., C3-C6 monomers), which exist in plants and belong to pHytoestrogens. The monomers composed of lignans include cinnamic acid, cinnamyl alcohol, acryl benzene, allylbenzene, and so on. Lignans are reported to have potential antitumor, anti-inflammatory, or antioxidant activity in the laboratory models of human diseases. Most lignans are lipophilic, and easily soluble in organic solvents. Therefore, the extraction of lignans from plants often utilized ethanol, ether, or acetone as extraction solvents. Researches about SBWE of lignans are only a few papers, as listed in Table 6.

Evrim [48,50] has conducted a detailed investigation of the material shape (flaxseed, ground flaxseed meal, and flaxseed meal sticks), temperature, extraction time, pressure, freshwater, and sample amount on the effect of secoisolariciresinol diglucoside lignan con-tent using an accelerated solvent extractor. The highest amount (12.94 mg/g) and extraction yield (72.57%)were obtained at 180°C for 15 min, 10.3 Mpa, and 40% freshwater using 5g of flaxseed meal sticks. Boudoir et al. [42] used water and ethanol under sub-critical conditions to extracted bioactive compounds from sesame(Sesamum indicum L.) defatted seeds. At 220°C,8 Mpa, and 63.5% ethanol as co-solvent conditions, the yields of lignans, TP, flavonoids compounds were maximized, and the antioxidants were similar to those reached by using synthetic antioxidants. Kinetic studies showed a high extraction rate of pHenolic compounds until the first 50 min of extraction, and it was in parallel with the highest scavenging capacity. SBWE could selectively extract different kinds of bioactive compounds only by changing process conditions. The optimal extraction temperature for lignans is about 180 °C, as shown in Figure 10.

4.11.Steroids

The asteroid is a class of natural chemical components widely existing in nature, including pHytosterin, bile acids, C2 steroids, insect allergic hormones, cardiac glycosides, steroidal saponins, steroidal alkaloids, bufogenin, etc. The steroid core structure has the basic skeleton structure of cyclopentane-perhydrophenthrene, bonded in four"fused" rings: three six-member cyclohexane rings and one five-member cyclopentane ring. Steroid compounds have various biological activities. They have a wide range of applications. The extraction methods of steroids from natural products including maceration, Soxhlet, and SBWE are illustrated in Table 6.

Ginseng is a well-known traditional Chinese medicine with numerous pHarmaco-logical effects [24-26]. These bioactive components are mainly ginsenosides, polyphenols, amino acids, and polysaccharides. Renata et al. [24]utilized SBWE to obtain fructooligosaccharides and beta-ecdysone from Brazilian ginseng root and aerial parts. Lee et al. [25]extracted red ginseng by varying the temperature (150-200°C) and extraction times (5-30 min) in SBWE. Co Compared to traditional heat extraction methods(ethanol, hot water, and methanol), extracts of red ginseng from SBWE had higher ginsenoside concentrations and antioxidative properties. Shivraj et al. [204] applied subcritical water to extract withanolides and withanolides from ashwagandha at varying temperatures (100-200°C) and extraction times(10-30 min). Various biological effects, including cytotoxicity, antioxidant, and enzyme inhibitory activities were quantified using HPLC. Withaferin A showed a significant reduction in cell viability of cervical cancer cells, with ICs0 values 10 mg/mL and 8.5 μM/mL， respectively， but no cytotoxic effect for normal cells. Thus， SBWE could be used for the extraction of pHarmacologically active fractions with therapeutic applications. The optimal extraction temperature for steroids is about 160°C, as shown in Figure 10.

5. Conclusions and Future Perspectives

Due to its nontoxic, nonflammable, and widely available nature, SBWE of natural products has gained greater attention during the last decade. Our review of over 200 articles shows that SBWE is a promising technology in extracting natural products. A wide variety of plant-related materials have been extracted by subcritical water. Sample materials include medicinal herbs, seasoning herbs, vegetables, fruits, algae, shrubs, tea leaves, grains, and seeds. The following natural products have been extracted by SBWE: Alkaloids, carbohydrates, essential oil, flavonoids, glycosides, lignans, organic acids, polyphenolics, quinones, steroids, and terpenes. Both static and dynamic modes are employed in SBWE. In general, the static extraction efficiency is lower than that of the dynamic model. Thus, most SBWE experiments were conducted using the static mode, and then, followed by the dynamic model. SBWE system was also coupled with HPLC, subcritical water chromatography, and high-temperature liquid chromatography. Co-solvents such as ethanol,

methanol, salts, and ionic liquids were also used to improve SBWEefficiency. It should be noted that temperature has the most significant effect on SBWE efficiency, and thus, it can be optimized. The optimal temperature ranges from 120 to 200°C for extracting the natural products mentioned above.

A major advantage of SBWE of natural products is that water is nontoxic, and therefore it is more suitable for the extraction of herbs, vegetables, and fruits since the extracts can be safely consumed by humans or animals. In addition, if no organic modifiers are used in SBWE, the liquid waste generated after SBWE does not require waste disposal. However, the high temperature used in SBWE may potentially cause analyte degradation. Thus, one must evaluate analyte stability under the temperature conditions to be used in SBWE to ensure that the analytes extracted do not undergo degradation during SBWE. SBWE employs high-pressure and high-temperature fluid, and great caution is required to ensure the safe operation of the SBWE system. In addition, frequent plumbing blockage may occur during the dynamic SBWE process.

A vast majority of SBWE research reported is studied at the bench scale. The next level of SBWE should be scaling up to the industrial level. Results of some pilot-scale studies have demonstrated the potential development of large-scale SBWE processes.