a brief tutorial on how plant essences are collected
Enfleurage is an age-old technique used to pull the delicate essence of flowers like jasmine, rose, tuberose and isn't widely practiced outside of Grasse, France. The fat soluble constituents of the delicate blossoms are drawn into stable (rendered) fats and later drawn off.
Steam distillation, expressing, solvent extraction and Co2 are much more common.
Solvent extraction creates a concrete with the plant solids (minus chlorophyll and fiber). The concrete is dissolved in alcohol to create an absolute.
CO2 extraction produces interesting results. Under pressure, CO2 changes from gas to liquid, serving as an inert solvent. It also has been shown to draw out constituents not found in essential oils of some plants -for example with Frankincense, the anti-inflammatory properties are found in CO2 extraction, but not in any concentration in steam distilled essential oils.
More here: National Association of Holistic Aromatherapy