Carrier oils, a term that usually relates to aromatherapy, are vegetable oils derived, pressed from the fatty portion of a plant (kernel, nuts, seeds). They're known as such for they are usually used as a base for essential, fragrant oils. Mostly of vegetable source, mineral oil and emu oil (animal source) can also be called as such. Although some are odorless, others have a faint sweet nutty or fruity aroma. If the odor is too strong, this may be a sign that the oil as gone rancid. Some are more delicate than others and require refrigeration after a certain period of time.
Oil extraction methods
Cold pressed, or expeller extraction, is done without the use of added heat. Also called mechanical extraction (crushing and/or pressing), the process itself can generate heat due to the friction, but cold pressed means that the heat is kept to a minimum. Without cool conditions, oils can lose some of their fragile nutrients.
The crushing is done in a large grooved circular stone with another stone on top. This second stone is rotated to force the oil out (fig.1). This produces a mash which it then transferred onto thin mats that are then stacked and placed into a press (fig.2). As pressure is applied, oil and water seep out. No heat is used, hence the term " first cold press". The oil can then be filtered to remove the left over bits of fruit and pits Some oils are actually left unfiltered by preference, reason why you can sometimes find a light deposit at the bottom of the bottle.
Cold pressed oils usually come in dark colored glass bottles because many of them are sensitive to light and will keep their freshness longer if exposed as little as possible to it. They are the best quality and the ones to be looking for. In the case of olive oil, first cold press is what is also called extra virgin olive oil.
If after the first press the color or the taste of the oil are not up to the quality standards, it is submitted to another process. It is brought to a refinery where defects in color, taste and aroma are totally removed. In the case of olive oil, to be known as such, some extra virgin olive oil is mixed back in.
The machinery for these processes has evolved thought the centuries but the ancient models of oil presses, as the ones shown below for olives could still be used today. Modern machinery is based on the same design, they are of course much larger and may have several wheels.
1. Ancient olive oil press, Israel
2. Oil press in the ancient city of Hazor, Israel
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction.(Wikipedia). It's used mostly for the extraction of essential oils.
Depending on the oil, if a different extraction method is used, it will be added.
Here is the start of my list. Important to keep in mind that these information where collected through several websites and some are listed as references. These are the notes I wanted to keep for myself but I figured it could be useful to some. No scientific approach, just a general overview with some historical tidbits.