Essential oils are distilled from the leaves, bark, roots and other aromatic portions of a botanical. Essential oils evaporate and have a concentrated aroma. Carrier oils, on the other hand, are pressed from the fatty portions (seeds, nuts, kernels) and do not evaporate or impart their aroma as strongly as essential oils. Carrier oils can go rancid over time, but essential oils do not. Instead, essential oils "oxidize" and lose their therapeutic benefits, but they don't go rancid.
The term carrier oil is generally limited to use within the practice of aromatherapy. In natural skin care, carrier oils are typically referred to as vegetable oils, fixed oils or base oils. Not all fixed oils/base oils are vegetable oils. Emu oil (from the emu bird) and fish (marine) oils are also classified as fixed/base oils.
Some carrier oils are odorless, but generally speaking, most have a faintly sweet, nutty aroma. If you come across a carrier oil that has a strong, bitter aroma, the carrier oil may have gone rancid.
Trends are changing, but most typical vegetable oils sold in grocery stores are not cold-pressed. Instead, the oils are processed using heat. For the most nourishing, freshest carrier oils, strive to shop with retailers and suppliers that specialize in the sale of aromatherapy or natural skin care ingredients. Your local health food/nutrition store may be a source for carrier oils, but the oils can often be pricier. Watch for dust on the bottles when buying oils locally. That can indicate the oil has been sitting around for awhile. Look for oils that are not blends of two or more oils and that have no additives.
Mineral oil and petroleum jelly are byproducts of petroleum production and are not used in aromatherapy. Mineral oil is used in baby oils and many commercially available moisturizers because it is an inexpensive oil to manufacture. It, however, can clog pores, prevent the skin from breathing naturally, prevent essential oil absorption, prevent toxins from leaving the body through the natural process of sweating, and I've read reports that it can be absorbed into the body and block vitamins from properly being utilized. These same concerns apply to petroleum jelly.
For fragile carrier oils or for those that you will be keeping for a long duration, store them in dark glass bottles with tight fitting tops, and store them in a cool, dark location. Amber or cobalt Boston round bottles are ideal.
Essential oils do not go rancid. Carrier oils, however, do become rancid over time. The level of natural fatty acids, tocopherols, method of extraction and other characteristics of an oil all can affect how quickly an oil becomes rancid. If you come across a carrier oil that has a strong, bitter aroma, the carrier oil may have gone rancid. If you can, compare the aroma of the oil that you suspect is rancid with the same botanical oil that you know is fresh.
Vegetable buttes are not carrier oils, but the beneficial properties of vegetable butters like Cocoa Butter and Shea Butter make them lipids that are suitable for use in aromatherapy.
Alpha linolenic acid is the primary Omega-3 Essential Fatty Acid found in plants. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are more beneficial than Alpha linolenic acid, but EPA and DHA are only present in fish oil.
Gamma linolenic acid (GLA) and linoleic acid are examples of Omega-6 Essential Fatty Acids found in plant oils.
You may occasionally read about Omega-7 or Omega-9 Fatty Acids. Omega-7 and Omega-9 Fatty Acids are not considered Essential Fatty Acids. The body can manufacture these fatty acids.
The easiest way to make an infused oil is by the use of a crock pot with a very low heat setting. Since the infused oil must be gently heated, it is essential that your crock pot does not overheat the oil. Do not use a crock pot that only has one heat setting as that crock pot most likely will overheat the oils. Add 2 ounces of your chosen carrier oil and 1/2 - 2/3 ounce of dried herbs (if you use fresh herbs, double the amount of herbs used) to your spotlessly-clean crock pot and stir well. Turn your crock pot to the lowest heat setting. Allow to heat on the lowest setting for two hours, stirring every 10-15 minutes (setting a timer to remind you to stir is important). After two hours, carefully strain the oil by using unbleached muslin (usually available at fabric stores). You should strain the oil at least twice. If any herbs remain in the infused oil, the oil can go rancid.
Mineral oil and petroleum jelly are byproducts of petroleum production and are not used in aromatherapy. Mineral oil is used in baby oils and many commercially available moisturizers because it is an inexpensive oil to manufacture.
I wrote the above paragraph several years ago. Recently, while reading Marge Clark's book Essential Oils and Aromatics, I read her personal experiences and her unfortunate long term consequences for having used lavender essential oil neat:
"One of my mentors reminds me 'sensitization is forever.' And I know she is right. Years ago I read the books saying that lavender oil could be used neat (undiluted). I very unwisely used undiluted lavender on broken skin, and consequently set up a sensitivity reaction. Today, almost two decades later, if I come in contact with lavender in any form, I will immediately start a new round of contact dermatitis that can take months to heal." [Marge Clark, Essential Oils and Aromatics (Sandy, UT: Silverleaf Press, 2008), 32.]
Not even lavender or tea tree.
What is Sensitization?
Treat Essential Oils With Respect
Vegetable and carrier oils have a plethora of fine uses which include acting as a carrier for therapeutic applications, as an excellent addition to your culinary creations, as a lathering agent for soap, and a binding medium for cosmetics. It is hard to imagine where we would be with our creations, if it were not for the goodness and substance that pure oil provides.
Expeller Pressed - A method of natural, mechanical extraction and processing of oils where a small amount of heat is produced simply through the frictional heat created by hydraulic presses. This is usually around 120-200 degrees Fahrenheit and makes the oil suitable and economical as a base for cosmetics because of its fairly undisturbed molecular state. It also makes a fine food grade oil.
Refined - A fully processed oil where it has been exposed to all methods of refinement including a flash fluctuation in temperature as high as 450 degrees and winterization as low as -30 degrees, deodorization, which removes the heavy and often unsettling odor in oil, and finally bleaching, where natural clays and other mediums are used to alter or remove an oils color, and scent. This makes for an economical oil in cosmetics and body care products, but it is not the healthiest as a food grade oil.
Partially Refined - A process where only some of the methods available are employed to produce a manufactured oil. Only one or two of the three available methods are used in a partially refined oil. These include, but are not limited to; deodorization, winterization and natural bleaching. These methods are used for oils which have been known historically to go rancid quickly, and they are also used to further stabilize an oil or remove its heavy odor and deep color.
Unrefined - A process of mechanical extraction and screen filtering where no additional refining process has taken place. This ensures the finest quality product and makes the oil the most exquisite for food and cosmetic preparation. The unrefined process helps oil retain a rich, strong flavor and color that is true to its natural state. Unrefined oils are always darker in color and richer in scent.