Originally Posted by David Ruskin
Actually what you get is called a "Concrete". It contains not only the smelly bits that you are interested in but also other substances which are soluble in Hexane, such as waxes. To get the Absolute it is necessary to dissolve the Concrete in Absolute alcohol (the clue is in the name), then filter off those unwanted materials which are not soluble and remove the alcohol. Concretes tend to be cheaper than Absolutes (one less stage in manufacture, more product) and used to be used in cheaper, non alcoholic products like soap or detergents. Concretes are not fully soluble in alcohol and so cannot be used in fine fragrance perfumery.
You've made some excellent points. I'd like to add a few additional points, if I may. Concretes are extracted from fresh, undried plant materials, with non-polar solvents. The extracts are often called oleoresins when dried plant material is used or when the plant extract is of a more resinous nature. Concretes often have more of a fresh smell because the plant material that is used is fresh. However, as non-polar solvents are used to make concretes, there is the potential for some of the more polar aroma compounds to be left behind in the plant material. Sometimes the plant material that is used heavily favors non-polar aroma compounds, so the concrete will closely resemble the aroma of the fresh plant. Other times, the plant material less heavily favors non-polar aroma compounds and the concrete therefore less closely resembles the aroma of the fresh plant.
Depending on the plant material used, it is sometimes better to use a solvent that has both polar and non-polar qualities, such as ethanol. Ethanol is largely polar but it also has a non-polar carbon chain. This allows both polar and non-polar aroma compounds to be extracted in good measure. It may still leave behind some of the most non-polar aroma compounds but it will extract everything between the polar and moderately non-polar. The reason why alcohol is used to make absolutes, is because it can dissolve most of the non-polar aroma compounds in the concrete but much less so the oleaginous substances (waxes, fats), which are extremely non-polar. The alcohol extract is then chilled to around -20°C, to precipitate any remaining oleaginous substances.
Another thing that has a bearing on the end product is the temperature required for extraction. Obviously a fair amount of heat is needed to produce essential oils. Although, if a vacuum pump is used, much lower temperatures are required. The temperature and pressure required to remove the solvent in extracts is also important. The removal of hexane requires heat or a vacuum, which can lead to some of the aroma compounds being lost during evaporation of the solvent. The same is true with many other solvents, including alcohol. CO2 sublimates without the addition of any heat or a vacuum, so there's no chance for any aroma compounds to be lost during solvent removal. Refrigerant gases like hydro-fluorocarbons are sometimes used for extraction because they also require no heat or a vacuum during solvent removal. Such extracts are called phytols or florasols. Butane is also sometimes used for the same reasons and the extracts are then known as butaflors.
The other thing to consider is that during distillation of essential oils, some of the polar aroma compounds can become dissolved in the water and become separated from the essential oil fractions. The byproducts of which are called floral waters or hydrosols. If you were to combine both the essential oil and it's hydrosol together in a fragrance, you may end up with something that more closely resembles the aroma of the plant.