Question

Why is it useful to carry out steam distillation rather than simple distillation or fractional distillation to isolate essential oils from spices? Why not just use direct extraction of the spices with dichloromethane or other organic solvent?

Answer 1

Spices are plant parts or whole plants which are usually comprised of numerous hydrophobic molecules that keep out the water from entering the organism from outside without control. Therefore, if one simply uses DCM or such organic solvent to extract essential oils from spices, along with the oils, the other innumerable organic molecules that constitute the spices will also be dissolved, making their separation from the essential oil a Herculean task.

Fractional distillation helps isolate liquids with similar boiling points while for simple distillation, there should be a large difference between the boiling points of the desired liquid and its impurities for isolation. As mentioned before, spices contain many liquid organic compounds as well as solid ones and since only the essential oils are required, to separate out the other liquids of similar boiling point to the desired oils, without complex separation processes like column chromatography, fractional distillation is preferred over simple distillation for isolation of essential oils from spices.