**3.7 Limonene**

The predominant element of essential oils derived from citrus fruit peels is d-limonene, which belongs to the terpene family. Since its cleaner and degreaser properties were discovered and considered, d-limonene has sparked a surge of interest. In this sense, this chemical has been classified as a viable alternative to halogenated carbon hydrates or traditional degreasing chemicals commonly used in industry and households. Several authors have attempted to create a commercial application for d-limonene. Sustainable chemistry has generated a lot of study into the processing of renewable fuels due to the demand for environment-friendly techniques and products [27].

*Green Extraction Techniques for Phytoconstituents from Natural Products DOI: http://dx.doi.org/10.5772/intechopen.105088*

Because d-limonene has a higher boiling point (175°C) than n-hexane (69°C), it uses more energy to recover the solvent by evaporation. To minimize the difficulty of solvent recovery caused by high d-limonene's boiling point, a technique based on steam or hydro-distillation employing Clevenger can be used. Distilled water was added to the extracted oil and d-limonene mixture after Soxhlet extraction with d-limonene. D-limonene and extracted oil were separated using a Clevenger device and azeotropic water distillation at less than 100°C [28]. It is a valuable and practical method for determining the lipids and oils in olive seeds. Waste minimization, rapid operation, and energy saving are all possible with Soxhlet microwave-integrated with limonene and microwave Clevenger distillation [29]. Limonene has a dielectric constant that is very similar to that of hexane and has been used to extract rice bran oil, oil from olive leftovers, carotenoids from tomatoes or algae and, more recently, algal lipids from wet algae [30].

#### **3.8 Solvent-free extraction**

Solvent-free extraction of a variety of important natural products (essential oils, fragrances, edible oils, antioxidants, and other organic compounds) eliminates the price and threats correlated with large amounts of solvent. It minimizes the amount of wastewater after extraction and uses a fraction of the energy that a traditional solvent-solid extraction process does.

In 2008, Chemat et al. developed the MHG method, which uses *in situ* dielectric heating on plant cell water to stretch the structure and cause membrane and wall ruptures. As a result, plant matter is used to gather primary and secondary metabolites, as well as the water in the cells. The behavior described is known as hydro diffusion. Gravity then drops the diffused components into a container. A continuous condensation system is maintained using a perforated Pyrex disc. MHG has been used to extract pigments, aroma components, and antioxidants from a variety of natural sources on a lab and commercial scale [21].

### **4. Pre-treatment techniques**

Crude drugs can be extracted in fresh or dried form. Grinding and drying of plant materials are examples of pre-preparation. This has an impact on the preservation of phytochemicals in final extracts. Air drying takes anywhere from 3 to 7 days. To optimize extraction operations and save energy, mechanical disruption pre-treatments can be employed alone or in combination. Bead milling, high-pressure homogenization, and hydrodynamic cavitation are all methods for mechanical disruption. The extraction of lipids has been demonstrated to be aided using a bead mill. Powdered samples, on the other hand, have a more homogenized and smaller particle size, developing in substantial surface contact with extraction solvents [31].

Nanotechnologies, including microwave, ultrasound, and pulse electric field, were found to improve operation efficacy as a pre-treatment before drying. After size reduction and before extraction, microwave pre-treatments upgraded the extraction of polyphenols, sugars, and other compounds. Pre-treatments with a pulsed electric field (PEF) improved extraction efficiencies in terms of yield and extract standard. PEF pre-treatment of rapeseed, apple, and sugar beet fruit extracts before mechanical expression resulted in higher yields [26]. Oven-drying is one more pre-extraction

method that uses heat energy to eliminate moisture from substances. This procedure for preparing a sample is regarded as particular easiest and most rapid thermal processing method available for phytochemicals.

Costly drugs can be dried by freeze-drying. In freeze-drying before use, the sample is frozen at −80°C to −20°C to lyophilize any liquid (e.g., solvent, moisture) in the body samples. The mouth of the test tube or other container holding the sample is wrapped in needle-poked-parafilm to avoid sample loss during the operation. Freezedrying resulted in a greater phenolic content compared to air-drying because most phytochemicals are preserved. This strategy is used to keep phytoconstituents safe. Freeze-drying, on the other hand, is a difficult process. Microwave drying is more expensive than traditional air drying. As a result, only fragile, heat-sensitive goods and high-quality materials are permitted [32].
