Essential oils are by definition produced in one of two ways:
- Steam distillation (most herbs)
- Cold pressing (citrus and a few herbs)
Different brands of essential oils made from the same species of plant made appropriately by either of the two methods listed above are by definition the same grade. Of course, like any botanical, the sensory qualities but not the basic chemistry of a species can change a bit depending on where it's grown, just as olives and grapes do, without affecting quality or grade.
There are three basic analytical tests to determine quality and identity of essential oils:
- Sensory testing performed by trained testers
- Infrared spectrum analysis (IR) that produces a chemical fingerprint of the oil
- Gas chromatography (GC) that produces a separation and quantification of individual chemical components in the oil
If the original plant is edible then the essential oil is by definition food grade. Some examples are the citrus oils, peppermint, oregano, etc. If the plant is not edible then the oil is also not considered food grade; as with cedarwood, pennyroyal, or ylang ylang. Due to the small bottle size and the need for cautions, there is not typically a food label on these oils.
Much caution needs to be taken with these oils, as they are extremely concentrated and most can burn the skin and mouth if not diluted adequately, even if food grade. Don't swig them or apply undiluted to the skin if you don't know how to use the specific type of oil in question.