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Review
. 2021 Jun 30;26(13):4004.
doi: 10.3390/molecules26134004.

Subcritical Water Extraction of Natural Products

Yan Cheng  1   2   3 , Fumin Xue  1   2 , Shuai Yu  1   2 , Shichao Du  1   2 , Yu Yang  3
Affiliations
Review

Subcritical Water Extraction of Natural Products

Yan Cheng et al. Molecules. .

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 similar 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. Compared 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 and with higher extraction efficiency. Since SBWE employs high-temperature and high-pressure, great caution is needed for safe operation. Another challenge for 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: alkaloids; carbohydrates; essential oils; flavonoids; glycosides; lignans; natural products; organic acids; polyphenolics; quinones; steroids; subcritical water extraction; terpenes.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Dielectric constant of water at 27 to 527 °C and 10 to 100 MPa, acetonitrile, methanol, ethanol, liquid ammonia, and dichloromethane at 20 °C and 0.1 MPa.
Figure 2
Figure 2
Types of sample matrices extracted by subcritical water.
Figure 3
Figure 3
Parts of medicinal herbs extracted by subcritical water.
Figure 4
Figure 4
Subcritical water extraction system without solid trapping (top) and with solid trapping (bottom).
Figure 5
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.
Figure 6
Figure 6
Optimum extraction conditions flavonoids.
Figure 7
Figure 7
Optimum extraction conditions for polyphenols and glycosides.
Figure 8
Figure 8
Optimum extraction conditions for organic acids.
Figure 9
Figure 9
Optimum extraction conditions for carbohydrates, essential oils, and alkaloids.
Figure 10
Figure 10
Optimum extraction conditions for quinones, terpenes, lignans, and steroids.
See this image and copyright information in PMC

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