Ultrasound assisted extraction (UAE)
In this method, the plant material is immersed in water or solvent and then exposed to ultrasound. The ultrasonic waves have a frequency of 20 kHz–1 MH depending on plant species. In this method, the mechanical vibration created by ultrasonic waves causes the walls and membranes of plants to vibrate and rupture. As a result, essential oil droplets are released. in other words, the cell walls are diffused, and once the walls are broken, the essential droplets washed out [6, 72]. The size of the plant material is an important factor in this method. The smaller the size of the plant material, the more cells are exposed to the waves [6]. Figure 11 shows Commonly used ultrasonic systems [73].
Compared to traditional methods, the UAE is improving time and temperature extraction and increases the range of solvent selection. It also offers high quality products with higher efficiency. This method is useful for compounds which sensitive to heat. The equipment is relatively simple and inexpensive compared to other new techniques [6, 74]. For example, citrus flowers of lemon were extracted by UAE. According to the result, the main component was Linalool with 11.3 % efficiency [75].