Subcritical Water Extraction Of Natural Products Part 1

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Abstract: Subcritical water refers to high-temperature and high-pressure water. A unique and useful characteristic of subcritical water is that its polarity can be dramatically decreased with increasing temperature. Therefore, subcritical water can behave similarly to methanol or ethanol. This makes subcritical water a green extraction fluid used for a variety of organic species. This review focuses on the subcritical water extraction (SBWE)of natural products. The extracted materials include medicinal and seasoning herbs, vegetables, fruits, food by-products, algae, shrubs, tea leaves, grains, and seeds. A wide range of natural products such as alkaloids, carbohydrates, essential oil, flavonoids, glycosides, lignans, organic acids, polyphenolics, quinones, steroids, and terpenes have been extracted using subcritical water. Various SBWE systems and their advantages and drawbacks have also been discussed in this review. In addition, we have reviewed co-solvents including ethanol, methanol, salts, and ionic liquids used to assist SBWE. Other extraction techniques such as microwave and sonication combined with SBWE are also covered in this review. It is very clear that temperature has the most significant effect on SBWE efficiency, and thus, it can be optimized. The optimal temperature ranges from 130 to 240°C for extracting the natural products mentioned above. This review can help readers learn more about the SBWE technology, especially for readers with an interest in the field of green extraction of natural products. The 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. Another advantage is that no liquid waste disposal is required after SBWE. Co Mpared with organic solvents, subcritical water not only has advantages in ecology, economy, and safety, but also its density, ion product, and dielectric constant can be adjusted by temperature. These tunable properties allow subcritical water to carry out class selective extractions such as extracting polar compounds at lower temperatures and less polar ingredients at higher temperatures. SBWE can mimic the traditional herbal decoction for preparing herbal medication with higher extraction efficiency. Since SBWE employs high-temperature and high-pressure, great caution is needed for safe operation. Another challenge for the application of SBWE is potential organic degradation under high-temperature conditions. We highly recommend conducting analyte stability checks when carrying out SBWE. For analytes with poor SBWE efficiency, a small number of organic modifiers such as ethanol, surfactants, or ionic liquids may be added.

Keywords: natural products; subcritical water extraction; alkaloids; glycosides; flavonoids; essential oils; quinones; terpenes; lignans; organic acids; polyphenolics; steroids; carbohydrates

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1. Introduction

For thousands of years, herbal medicine has played a vital role in treating diseases, especially in East Asia. The bioactive components in herbs and plants are the basis for the prevention and treatment of many diseases [1. Due to its relatively low side effects against chemically synthesized drugs, much attention is given to the extraction and separation of a wide range of bioactive compounds from herbs and plants. The 1000-year-old ex-traction process of the active pharmaceutical ingredients (APIs) from medicinal herbs is the traditional herbal decoction(THD)method. However, there are some defects in THD, such as a long extraction time and decomposition of active pharmaceutical ingredients. Methanol, ethanol, n-hexane, petroleum ether, diethyl ether, chloroform, ethyl acetate, and glycerol are often used as extraction solvents to increase the extraction efficiency and reduce extraction time. Obviously, these organic solvents are volatile, flammable, mostly toxic, and expensive. Thus, they are not safe extraction fluids for herbs, plants, fruits, vegetables, and the like.

Among the various new green extraction and separation technologies developed recently, SBWE is the most promising one. Subcritical water refers to the liquid water at temperature and pressure below its critical point (Tc= 374.15°C, Pc=22.1 Mpa). The pres-sure of the subcritical water must be higher than the vapor pressure at a given temperature to keep water in the liquid state. With the increase of temperature, the pHysical-chemical properties of subcritical water change drastically. It's dielectric constant, viscosity, and surface tension all decrease steadily, and its diffusion coefficient is improved with increasing water temperature [2-5].

As shown in Figure1, at 27°C and 100 Mpa water is a polar solvent, and its dielectric constant is 81.2. Fortunately, when the temperature is raised to 350 °C at 100 Mpa, the dielectric constant of water decreases to around 20. In another word, water's polarity changes from strong polar to much less polar[2]. For example, the dielectric constant of water at 200°C, 250°C, and 300°C are equivalent to that of acetonitrile, methanol, and ethanol, respectively(Figure 1). Thus, even much fewer polar compounds can be extracted by subcritical water at high temperatures. Theoretically, based on its tunable polarity, subcritical water can extract many substances by adjusting extraction temperature and preSSure Conditions.

In the actual extraction processes, since pressure has minimal influence on the dielectric constant of water, water's temperature is adjusted to control the dielectric constant of water in order to mimic various organic solvents. This unique characteristic of subcritical water allows water as the sole extraction fluid without any co-solvents such as acids, alkalis, catalysts, or organic solvents; this meets the principles of green chemical extraction since water is nontoxic.

Subcritical water applications can be found in the following areas:(1)reversed-phase liquid chromatography using subcritical water as the sole mobile pHase-subcritical water chromatography [3];(2)extraction of environmental samples, such as the determination of organic pollutants in solid wastes, soils, sediments, and atmospheric particles [5,6];(3)hydrolysis, degradation, polymerization, and synthesis reactions using subcritical water as both a solvent or a reactant [7l;(4)environmental remediation such as cleaning contaminated sewages and soils, decomposing pollutants (pesticides, polycyclic aromatic hydrocarbons, and polychlorinated biphenyls) and explosives [8];(5)extraction of active ingredients from medicinal and seasoning herbs, vegetables, fruits, and other plant-related matrices [4,9].

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This review focuses on the SBWE of natural products in recent years. Principles, mechanisms, static and dynamic extraction modes, and factors affecting SBWE efficiency have been described by Gbashi and coworkers [10]. Since various research has focused on the extraction of flavonoids, carbohydrates, glycosides, organic acids, polyphenolics, alkaloids, essential oils, quinones, terpenes, lignans, and steroids[11,12], we provide a systematic and comprehensive overview on SBWE conditions, the function and activities of the active ingredients and the subcritical extracts, analysis methods, and co Comparison with other extraction methods for the above-mentioned natural products. Subcritical water can simultaneously extract several active ingredients from natural products. Separation, identification, and quantification of each natural product compound in the subcritical water extracts are achieved by liquid chromatography, gas chromatography, infrared spectrum, and/ or mass spectrometry.

2. Sample Matrices Extracted by Subcritical Water

In general, the sample matrices extracted by subcritical water include the following four groups: Plants, food by-products, fungi, and marine algae. Based on the analysis and statistics of the references cited in this review, we generated Figure 2 to show the percentages of each type of sample matrices extracted by subcritical water. As shown in Figure 2,62.3% of sample matrices extracted by subcritical water come from plants, followed by food by-products with 29.0%, marine algae with 4.9%, and fungi with 3.8%.

For some medicinal herbs, their whole plants have pHarmaceutical values and are prescribed in traditional herbal medicine, while the only part or parts of other medicinal herbs are used in herbal medicine. The parts of medicinal herbs include roots, stems, leaves, flowers, seeds, and fruits. For example, the whole plants of Hedyotis diffusa Willd. [13]and Centella asiatica L.[14] have medicinal values and the leaves, nodes, petioles, and roots of these two herbs were grounded before SBWE. Stems and leaves of ginseng [15], the root of Sophora flavescens Ait. [16] and Rheum tanguticum [17] and flowers of chamomile ligulate [18] were extracted using subcritical water. We carefully sorted out the medicinal herbs, and the percentage of each medicinal part is illustrated in Figure 3. One can see from Figure 3 the leaves of medicinal herbs that are most widely investigated in SBWE. Although the peels, hulls, brans, barks, shells, epicarps, pericarps, sorghums of fruits and seeds are the byproducts, they also contain many APIs. For example, onion, one of the most frequently consumed vegetables, is known to have many health benefits because of its flavonoid contents. Some results demonstrated that the onion peel extracts produced by the SBWE technique have great potential as a source of useful antioxidants [19,20]. Grains, seeds, corn, and fruits have also been investigated using SBWE [21]. Among all papers reviewed, only one article reported that the wood of Aquilaria malaccensis has useful medicinal applications, and is used in traditional medicine to treat pain, fever, rheumatism, and asthma [22].

2.1.Plants

Plants extracted by subcritical water mainly include herbs [15,23-29], vegetables with medicinal values [30,31], fruits 32-36], oilseed crops [37-40], shrubs, grains, tea leaves and beans [41-46]. These plants have been used in traditional medicine for thousands of years in some countries and have been proved to possess plentiful pHarmacologically active components. For example, ginseng is a valuable Chinese medicine, which has been used in China, Korea, Japan, and Brazil. Ginseng has been reported to contain a variety of bioactive chemical compounds including terpenes, polyacetylenes, alkaloids, vitamins, minerals, phenolics, flavonoids, and triterpenes [15]. These active components possess antioxidant, anti-inflammatory, antidiabetic, antineoplastic, cardiovascular, immunoregulatory, and neuroregulatory activities [26]. Oregano is an herbaceous plant native to the Mediterranean regions [4748]. Some healthy properties have been attributed to this plant, such as its powerful anti-bacterial and anti-fungal effects related to carvacrol and thymol compounds and some antioxidant activity.

2.2.Food By-Products

In essence, by-products are the wastes produced by fruit peels, tea filters, seed residues, vegetable peels, chestnut barks, cocoa shells, grain bran, lotus seed epicarp, and stems [49]. Many kinds of fruits have peels, such as bananas, oranges, and apples. Although we often throw them away, fruit peels may contain many APIs. Therefore, research on the by-products is worth doing. For example, orange peel is the main waste by-product of the juice extraction industry [50]. Nonetheless, orange peel is an attractive source of bioactive compounds, which include plenty of pHenolic and flavonoid compounds.

2.3. Marine Algae

Marine algae studied in SBWE include microalgae, seaweeds, and Haematococcus Pluvialis [51-53]. Algae are rich in saturated fatty acid, monounsaturated fatty acid, and polyunsaturated fatty acid, which is healthy for the cardiovascular system. Omega-3, which plays a significant role in the body's inflammatory pathways and cell health, is especially used for cancer prevention and therapy. Using SBWE technology, algae could step up as one of the potential sources for future generation of omega-3.

2.4.Fungi

Some fungi are also responsible for some diseases in plants and animals, which also can be as vegetables, such as the edible-medicinal fungi, which include mushrooms (Lentinus edodes [54-57]; Grifola frondosa [58], Sagittaria sagittifolia L. [50-61]), and Cordyceps militaris(C.militaris)[62]. C.militaris is a type of precious edible-medicinal fungi widely distributed around the world. Extensive research demonstrated that extracts of C.militaris have multiple pHarmacological actions, such as anti-inflammation, improvement of insulin resistance, and antioxidant activity.

3. SBWE Systems and Extraction Mechanism

In the beginning, supercritical water (T>374°C, P>22.064 Mpa)was widely involved in the extraction of coal, oil sand bitumen [63,64], and oxidation [65]. Supercritical water requires harsh experimental conditions such as extremely high temperature and high pressure, which often bring severe corrosion on experimental equipment and connection tubing. Since the 1990s, subcritical water has been gradually replacing supercritical water in the field of extraction because subcritical water requires relatively mild temperature and pressure. Thus, subcritical water has been extensively employed in the extraction of organic pollutants [66,67], and researchers have attempted to couple SBWE with liquid chromatography to reduce the analytical steps in the extraction and chromatographic analysis process [68-70]. To improve the extraction efficiency, ultrasonic-assisted SBWE [71]and microwave-assisted SBWE [72] have also been reported.

3.1.Modes of SBWE

There are mainly two SBWE modes, namely static extraction, and dynamic extraction. Static extraction refers to an extraction method in which subcritical water and the sample to be extracted are added to an extraction vessel, as shown in Figure 4, and then heated to the desired temperature under a moderate pressure to keep water in the liquid state. After the set extraction time is reached, the extractant is collected for chromatographic analysis. This extraction process is similar to accelerated solvent extraction. The static extraction efficiency is normally lower than that of dynamic extraction.

Dynamic SBWE is a continuous extraction process, which means that after the samples are added to the extraction vessel, water is continuously fed into the extractor with a pump, and the extraction is carried out at a fixed temperature or in gradient temperature conditions. The pump can be set either under constant pressure mode or constant flow mode. Dynamic SBWE not only accelerates the mass transfer efficiency and shortens the extraction time, but also achieves selectively continuous extraction. However, dynamic SBWE may cause blockage to the SBWE system.

In many cases, both static and dynamic SBWE modes are used. SBWE operators normally conduct a period of static extraction, then followed by a dynamic extraction. Although a majority of the SBWE work is conducted without solid trapping of the extracted analytes (as shown in Figure 4, top), SBWE with a solid trap(Figure 4, bottom), which collects the extracted analytes can be easily coupled with liquid chromatography for analysis, as discussed further below.

3.2. Online and Offline Coupling of SBWE with Liquid Chromatography

SBWE system was coupled with high-performance liquid chromatography via solid-phase trapping [69]. The offline coupling of SBWE with HPLC was reported [70]. The solid trap was physically removed from the SBWE system after each extraction and connected to an HPLC system for analysis. However, this step is eliminated in the online SBWE-HPLC approach. After SBWE, the analyte trap stays in place while the HPLC analysis is carried out by simply switching valves to connect the analyte trap with the HPLC unit.

Offline coupling of SBWE with subcritical water chromatography (SBWC) has also been reported [70]. SBWCrefers to reversed-phase liquid chromatography using subcritical water as the sole mobile phase component. As shown in Figure 5(top), extracted analytes are collected in the sorbent trap during the SBWE step. After SBWE extraction, the sample trap is connected to a subcritical water chromatography system (Figure 5, bottom). The analytes collected in the sorbent trap during SBWE are thermally desorbed by heating the sample trap and then they are delivered into the SBWC system for analysis. The obvious advantage of SBWE-SBWC coupling is that organic solvents are eliminated in both extraction and chromatographic analysis steps. The challenge is, however, that the thermal desorption normally requires a high temperature, which may potentially cause analyte degradation.

3.3. Mechanisms of Subcritical Water Extraction

The main SBWE extraction mechanism follows the"like dissolves like"rule. As mentioned in the Introduction section, the polarity of water can be tuned by temperature [2,3]. As shown in Figure 1, the dielectric constant of water is dramatically lowered (becoming less polar)with increasing temperature. This means that water's polarity is tunable by changing temperature. Because of this, polar compounds can be efficiently extracted by water at"lower" temperatures while less polar analytes require higher temperatures to achieve a reasonable extraction efficiency. Therefore, the less polar the analyte, the higher the temperature required. In addition, larger and more complex molecules require higher temperatures. Pressure does not have a significant effect on SBWE efficiency as long as the pressure is high enough to keep water in the liquid state.

Figure 5. Offline coupling of SBWE with SBWC. Adapted with permission from [70] (Lamm, L.; Yang, Y. Off-line coupling of SBWE with subcritical water chromatography via a sorbent trap and thermal desorption.Anal. Chem.2003,75,2237-2242.).Copyright 2003 American Chemical Society.

This article is extracted from Molecules 2021, 26, 4004. https://doi.org/10.3390/molecules26134004 https://www.mdpi.com/journal/molecules