**9. Extraction of specific metabolites**

The chemical investigation profile of a plant extract, fractionation of a crude extract is suitable to isolate the major classes of compounds from each other before further chromatographic analysis. One procedure based on varying polarity that might be used on an alkaloids-containing plant is indicated in **Figure 10**. The type and quantity of components to be separate into different fractions will, vary from plant to plant. Such procedure can be modified when labile substances are investigated [47].

#### **9.1 Extraction of essential oils (EOs)**

Essential oils (EOs) are concentrated aromatic hydrophobic oily volatile liquids characterized by a strong odor and produced by all plant organs [91]. They are obtained from raw material by several extraction techniques such as water or steam distillation, hydrodiffusion, solvent extraction, Soxhlet extraction, expression under pressure or cold pressing method, also known as scarification method, microwave-assisted extraction, microwave hydrodiffusion and gravity, supercritical fluid or subcritical water extractions. The best extraction method to use depends on the ease of evaporating (volatility) and the hydrophilicity or hydrophobicity (polarity) of the desired components [92–96]. However, the three most commonly applied techniques to extract EOs are Soxhlet, hydrodistillation, and SFE [97]. The extraction method chosen significantly affects the chemical composition of EOs [91]. Benmoussa and collaborators have recently found that the microwave hydrodiffusion and gravity (MHG) appeared like a rapid process, a green technology, and a desirable alternative protocol to enhance both the quality and the quantity of the EOs extracted from medicinal and aromatic plants [92].

#### **9.2 Extraction of fats and oils**

Lipids contain a broad category of non-polar molecules that are barely soluble or completely insoluble in water, but soluble in an organic solvent such *Extraction of Bioactive Compounds from Medicinal Plants and Herbs DOI: http://dx.doi.org/10.5772/intechopen.98602*

**Figure 10.**

*A general procedure for extracting fresh plant tissues and fractionating into different classes according to polarity.*

as *n*-hexane, diethyl ether, chloroform, and alcohol [98]. Fats are triglycerides that are solid or semi-solid at room temperature, while oil is also triglycerides that are liquid or clear liquid at room temperature, however, their chemistry is determined by the degree of solubility. Fats and oil may be of vegetable, animal, and marine origin [99]. Oilseeds and fats production requires several unitsoperations, starting with a pre-treatment stage. It is often necessary to dry the sample before oil extraction using solvents because many organic solvents are not miscible with water and cannot easily penetrate the matrix and extraction would be inefficient [100]. The processing methods used are usually neither specific to lipids, nor insure 100% recovery of the lipid material because of the nature of the matrix. Diethyl ether and petroleum ether stands as favorite solvents in the case of crude fat because they are relatively non polar, hence extract most non-polar components [98].

Extraction process of edible oils may have negative effects on taste, stability, appearance or nutritional value, preserve tocopherols, and prevent chemical changes in the triacylglycerol. Fats and oil can be extracted from plants using conventional and advanced techniques that include hot water extraction, cold pressing, solvent extraction, high-pressure solvent extraction, microwave –assisted extraction, and supercritical fluid extraction [99]. Extraction of oil involves several mechanisms for removing a liquid from a solid such as leaching, washing, diffusion and dialysis [98]. In the case of palm oil (seeds of *Elaeis oleifera*), crude oil is obtained after a digestion step followed by a pressing stage. Digestion helps the rupture or breaking down the oil-bearing cells, thus releasing the palm oil in the fruit [101–103]. Enzyme-assisted extraction (EAE) is an efficient method to improve lipid extraction from several different biomasses such as soybean, sunflower, and microalgae [104, 105].

The main side reactions reported during oil processing are (i) *trans* fatty acid formation, (ii) *cis*-t*rans* isomerization, (iii) and physical loss [99]. Before oilseeds processing, moisture must not exceed a certain limit to prevent growth of fungi and the occurring lipase formation, resulting in a free fatty acid increase [98].

#### **9.3 Volatile organic compounds**

Volatile organic compounds (VOCs) are odorant compounds emitted from plant tissues. Plants can produce a high diversity of VOCs. They are responsible for the distinct aroma of certain dried plants, including the tea, *Camellia sinensis*. VOCs can therefore be used as an indicator of tea quality [106, 107]. Several VOCs are emitted as a natural defense mechanism against arthropods and pathogen attacks [108, 109].

Hydro-distillation (HD), steam distillation (SD), simultaneous distillation solvent extraction (SDE), microwave-assisted hydro-distillation (MWHD), supercritical fluid extraction (SFE), purge and trap, and solid phase microextraction (SPME), are used to extract VOCs [110].

Verde and collaborators conducted a work to optimize the MAE of the volatile oil terpenes from *Pterodon emarginatus* fruits and characterize the volatile compounds. According to their study, MAE proved to be feasible with a particular interest in avoiding the need of organic solvents in volatile oil extraction from plants. They proved that a minimum amount of water could be enough to bring result in extraction. That green methodology appears to be an excellent alternative to extract terpenes from aromatic plants [111].

#### **9.4 Alkaloids**

The alkaloids are low molecular weight nitrogen-containing compounds found mainly in plants and a lesser extent in microorganisms and animals. They contain one or more nitrogen atoms, typically as primary, secondary, or tertiary amines, which usually confers basicity on the alkaloids. If the free electron pair on the nitrogen atom is not involve in mesomerism, the salt formation can occur mineral acids. This fundamental property of alkaloids is used in their extraction and further clan-up. According to the nature of the nitrogen-containing structure, alkaloids are classified as pyrrolidine, piperidine, quinoline, isoquinoline, indole, etc. [27].

Two methods may be used for alkaloids extraction. One is to basify the plant material using diethylamine or ammonia and extract with an organic solvent [112, 113]. Alkaloids are substances with a basic character and their solubility is a function of pH. They are soluble in low polar organic solvents in basic medium, while in acidic medium, they are soluble in water.

#### *Extraction of Bioactive Compounds from Medicinal Plants and Herbs DOI: http://dx.doi.org/10.5772/intechopen.98602*

Alkaloids containing basic amines can be selectively extracted using a modified version of the classical "acid–base shake-out" method (**Figure 11**).

As recommendations, mineral acids and strong bases should be avoided in extracting alkaloids (and plant material in general) because of the risk of artifact formation [3, 114, 115].
