亚临界水是指高温高压水。亚临界水的独特而有用的特性是其极性可以随着温度的升高而急剧下降。因此,亚临界水的行为类似于甲醇或乙醇。这使得亚临界水成为用于各种有机物质的绿色萃取流体。本文对天然产物的亚临界水萃取(SBWE)进行了综述。提取的材料包括药用和调味草药,蔬菜,水果,食品副产品,藻类,灌木,茶叶,谷物,和种子。广泛的天然产物,如生物碱,碳水化合物,精油,黄酮类化合物,糖苷,木脂素,有机酸,多酚,醌,类固醇,和萜烯已经使用亚临界水提取。本综述还讨论了各种SBWE系统及其优缺点。此外,我们已经审查了共溶剂,包括乙醇,甲醇,盐,和离子液体用于辅助SBWE。本综述还涵盖了其他提取技术,例如微波和超声处理结合SBWE。很明显,温度对SBWE效率的影响最大,因此,它可以优化。提取上述天然产物的最佳温度范围为130至240°C。这篇评论可以帮助读者更多地了解SBWE技术,特别是对天然产品绿色提取领域感兴趣的读者。天然产物SBWE的主要优点是水无毒,因此,它更适合提取草药,蔬菜,和水果。另一个优点是在SBWE之后不需要液体废物处置。与有机溶剂相比,亚临界水不仅具有生态优势,经济,和安全,还有它的密度,离子产品,和介电常数可以通过温度调节。这些可调的性质允许亚临界水进行类选择性提取,例如在较低温度下提取极性化合物和在较高温度下提取极性较小的成分。SBWE可以模仿传统的中药汤剂制备中药,具有较高的提取效率。由于SBWE采用高温高压,安全操作需要非常谨慎。SBWE应用的另一个挑战是在高温条件下潜在的有机降解。我们强烈建议在进行SBWE时进行分析物稳定性检查。对于SBWE效率差的分析物,少量的有机改性剂,如乙醇,表面活性剂,或者可以添加离子液体。
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.