Which method is NOT commonly used for extracting essential oils?

The method of fermentation is not commonly used for extracting essential oils, making it the correct answer in this context. Essential oils are predominantly extracted using methods that effectively harness the aromatic compounds from plant materials without altering them significantly.

Distillation is one of the most widely used methods, particularly steam distillation, which involves passing steam through plant material to vaporize the volatile compounds. As the steam cools, the essential oil condenses and can be collected, allowing for a pure extract.

Expression, often referred to as cold pressing, is typically used for citrus fruits. This method involves mechanically pressing the rind of the fruit to release the essential oil contained within its surface.

Solvent extraction is another method employed, especially for delicate flowers that cannot withstand heat. It uses a solvent to dissolve the essential oil, which is then separated from the solvent by evaporation, yielding a concrete or absolute form of the essential oil.

In contrast, fermentation is a biological process predominantly involving the conversion of sugars to acids, gases, or alcohol using microorganisms, and is not suitable for extracting essential oils. Therefore, fermentation does not align with the established methods for obtaining essential oils.