Methods of Extraction of Phytochemicals

Introduction

From ancient times plants have been exploited medicinally. In modern times herbal medicines have gained quite an importance owing to their safety and negligible side effect. Phytochemicals are the active principles of plants that are responsible for the medicinal attributes however their presence and secretions vary from species to species. These phytochemicals can be found in various plant organs which include root, bark, stem, leaves, flowers, seeds, seed coat, and pulp. Thus, these phytochemicals must be extracted for their pharmaceutical use. Extraction ensures obtaining the biologically active compound to the maximum extent and thereby reducing unwanted constituents (Ingle et al., 2017; Feng et al., 2019). Extraction is the primary and most crucial step before the separation and characterization of phytochemicals. Extraction uses steps like pre-washing, drying (which may be sun drying or freeze-drying), and grinding which helps to homogenize the sample and also increase the area of contact of the sample surface with the solvent. Proper care must be taken so that the phytochemicals are not destroyed, distorted, or lost during extraction. Different solvent systems exist that help in extraction however, the solvent system must be chosen based on the targeted bioactive compound. Polar solvents (methanol, ether, ethyl-acetate) are used for the extraction of hydrophilic compounds. Dichloromethane or a mixture of dichloromethane/methanol in the ratio of 1:1 is used to extract lipophilic compounds. To get rid of chlorophyll, hexane extraction is used. The target compound's composition such as polarity and thermal stability is also taken into consideration while extraction (Feng et al., 2019).

Extraction procedure is generally of three types: liquid/solid extraction, liquid/liquid extraction and acid/base extraction. Solvent extraction is the most prevalent type of extraction, followed by distillation extraction, pressing, and sublimation, all of which are based on extraction principles. The extraction procedure may be conventional or non-conventional which is further classified into various methods. The major factors based on which the extraction procedure varies are the time taken for extraction, the temperature of extraction, the dimension of the tissue or its part to be extracted, the ratio of solvent and sample, and the solvent’s pH (Stéphane et al., 2021).

Solid/liquid extraction is used in traditional or classical procedures. It is based on the fact that when a solid component comes into touch with a solvent (liquid), the soluble parts of the solid dissolve in the solvent, and this migration of the soluble ingredients into the solvent occurs along a concentration gradient. The rate of transfer is determined by the constituent concentrations and continues until equilibrium is attained. Different types of conventional techniques are maceration, digestion, infusion, elution (lixiviation), decoction, reflux extraction, tincture, percolation, steam distillation, hydrodistillation, soxhlet extraction, serial exhaustive extraction, and fermentation (aqueous-alcoholic extraction) (Zhang et al., 2018, Stéphane et al., 2021).

Non-conventional or innovative techniques have developed in recent years. It includes processes like microwave-assisted extraction (MAE), ultrasound-assisted extraction (UAE) or sonication, pressurized liquid extraction (PLE) or accelerated solvent extraction (ASE), supercritical fluid extraction (SFE), enzyme assisted extraction (EAE), pulse electric field extraction (PEF), turbo-distillation or turbolysis, counter-current extraction (CCE), solid-phase extraction (SPE), high-voltage-assisted extraction, and phytonics process (Zhang et al., 2018, Stephane et al., 2021). This chapter is an attempt to discuss the various extraction procedures of the phytochemicals. Efforts are taken to illustrate the procedures and technicalities involved in extraction and also to highlight the non-conventional methods of extraction.