**3.4 Cold pressing**

One of the oldest extraction methods for essential oils from citrus peels such as lemon, orange, bergamot, and grapefruit is cold pressing. This technique mechanically tears the peels by simply pressing them to extract the volatile essences contained in the citrus pericarps. Until the early twentieth century, cold-pressed citrus oils were produced manually. The process produces an aqueous emulsion, which is then centrifuged to separate the essential oil. This method is preferred for citrus peel essential oil extraction because it avoids thermal alteration of the aldehydes. This process results in the production of an aqueous emulsion, which is then centrifuged to separate the EO [21].

### **3.5 Enfleurage**

Enfleurage is another conventional extraction method that dates back to antiquity. It is based on the affinity of fragrances for fats, and concerns plants that retain their fragrance after being picked (such as jasmine or tuberose). The flowers are spread out on frames coated with odorless grease. The flowers' fragrance is absorbed by the grease until saturation. The flowers are changed regularly (e.g., every 24 hours for jasmine). When the fat is saturated by the flowers, the operation is complete. Saturation can last up to a month. The resulting pomade is then melted. After decanting, the mixture is cold-treated with alcohol. The alcohol draws out the fragrance on its own, without taking on the fats. This extraction technique is virtually dying out due to its high cost, and the extracted oils have no applications in the food industry [14, 22, 23].

#### **3.6 Organic solvent extraction**

Solvent extraction is commonly employed to extract EOs that exhibit thermal labile properties, such as those extracted from flowers. The plant material is placed in a solvent bath. Successive washings charge the solvent with aromatic molecules. After separation by filtration, the emulsion is distilled to extract the EO.

Solvent extraction has been used for fragile or delicate floral materials, which cannot withstand the temperature of distillation. Various solvents, including hexane, acetone, petroleum ether, ethanol, or methanol, can be used for extraction [7].

Solvent extraction is relatively fast and inexpensive. The chosen solvent must be permissible, inert, and stable to heat, light, or oxygen. Its boiling temperature should preferably be low to facilitate elimination.

The produced EO contains a small amount of solvent residue, making it unsuitable for food applications. However, if alcohol is used as the solvent, it is considered "foodgrade" and safe for consumption. This method is commonly used in the perfume industry [24].

In practice, the solvent is mixed with the plant material, heated to extract the EO, and then filtered. The filtrate is then concentrated through solvent evaporation. It is later mixed with pure alcohol to extract the oil and distilled at low temperature.

However, this method is relatively time-consuming, making the oils more expensive than other methods. Additionally, solvent residues in the final product can cause allergies, toxicity, and affect the immune system [25].

The limited use of this extraction method is justified by its cost, toxicity and safety issues, and environmental protection regulations. However, HE yields are generally higher than with distillation. What is more, this technique avoids the hydrolyzing action of water vapor.
