Onion oil is, in most circumstance, one of two related things: either the essential oil extracted from an onion or basic oil, such as olive or vegetable, that is flavored and augmented with onion, usually during cooking. Essential oils are most common in homeopathic and alternative medicine. In cooking, onion oil is almost always intended for flavor more than any health benefit.
Medicinal onion oil usually comes packaged in dropper bottles or capsules. The oil is highly concentrated and has generally been procured either through boiling the onion down slowly over time, or steaming it dry. Onions naturally contain a lot of moisture, which means that the water must either be pressed or boiled out of the vegetable before the oils can be isolated. The process is often arduous, but not particularly complicated.
Homeopathic practitioners sometimes recommend natural onion oil as a treatment for upper respiratory problems, particularly colds and seasonal allergies. It is believed that the oil can aid in decongestion while helping rid the body of toxins. The oils are also sometimes used to treat ear infections. Some traditional medicine practices prescribe the attachment of entire onion segments to the body to achieve the oil’s healing properties, but more modern practitioners usually find the ingestion or direct application of concentrated oil to be more effective.
A number of different pharmaceutical studies also look at the oil from onions. Most varieties of onions are rich in disease-fighting vitamins and nutrients. Researchers tend to look for ways of harnessing these benefits and channeling them into specific treatment regimens for human ailments. Some scientists believe that onion oil is capable of anything from lowering cholesterol to negating the harmful effects of nicotine and stymieing the growth of some cancers. The results are largely inconclusive.
In culinary settings, onion oil is usually more flavorful, but also less concentrated. Chefs can often extract onion essential oils simply by heating thin slices of the vegetable, then reserving the resulting liquid. Most of the time, taste is more important than purity. Cooks usually add salt and other seasonings and may or may not leave onion pieces to fry as the oil is extracted.
Home-pressed onion oil is often used as a base for any number of dishes. Oil is a common facet of many recipes. Cooking with onion oil instead of commercially-available vegetable or corn versions often adds a new layer of flavor to a dish and can enhance a range of both savory and sweet tastes.
Sometimes, cooks will also create a sort of onion-flavored oil by simmering onion slices in other oils. Olive oil is a popular choice, as its nutty, smooth flavor can serve as a good base for the more pungent onion. This sort of oil is popular as a dressing and can also be used for dipping breads and vegetables.