**3.2 Hydrodistillation**

Hydrodistillation (HD) (**Figure 2**) is a standard EO extraction method. It enables the extraction of water-insoluble natural products with a high boiling point. The process involves complete immersion of the plant material in water, followed by boiling. This operation is generally carried out under atmospheric pressure. The steam formed is condensed by the refrigeration system at a water flow rate.

This method protects the extracted oils from overheating. The advantage of this technique is that the required material can be distilled at temperatures below 100°C.

Distillation may seem like a simple process for extracting essential oils, but it comes with several drawbacks. In developed countries, its use has become outdated due to the overheating of plant material and the production of burned-smelling oils. However, this method is still effective for powders and hard materials. It is important to note that exposure to boiling water for extended periods can cause weathering reactions and hydrolysis of esters into alcohols and acids, which can have serious consequences for oils with high ester levels. Rectification is often necessary to remove unwanted impurities or constituents responsible for unacceptable odor. Distillation time varies depending on the type of plant material, with woody plant organs requiring longer distillation times than herbaceous plants [18].

### **3.3 Hydrodiffusion**

Hydrodiffusion is another method conventional method for extracting essential oils from plant materials. It involves the use of steam and water to extract the oils.

#### **Figure 2.** *A schematic representation of hydrodistillation of essential oils.*

### *Essential Oil Extraction Process DOI: http://dx.doi.org/10.5772/intechopen.113311*

The plant material is placed on a grid above water in a distillation vessel, and steam is injected into the bottom of the vessel. The steam then passes through the plant material, carrying the essential oils with it. The steam and oil mixture then condenses on a cooled surface, with the oil and water separating into two layers. This method is particularly useful for extracting essential oils from delicate plant materials, as it uses lower temperatures and less pressure than other methods [19, 20].
