A certain amount of processing of the oil source is necessary to produce oil--there are no offshore olive oil reserves! (Ha, ha). The processing techniques range from fairly harmless to rather harmful to the consumer. So, how an oil is processed becomes as important as the source itself. The following are the key processes to know, and in some cases, avoid.

• Hydrogenation is something you probably want to avoid. It's the process of adding hydrogen to a polyunsaturated oil to make it solid and therefore more stable. Partially-hydrogenated oils result in a semi-solid fat: margarine and vegetable shortening (e.g. Crisco) are perhaps the best known examples. While the fats in these products contain no cholesterol, they are indeed saturated, and as a result, may increase your cholesterol levels. What's more, hydrogenation creates trans fatty acid chains, also called trans fats, which are difficult to digest and are now associated with both heart disease and cancer. (Used regularly in processed foods, hydrogenated oils are a food ingredient that many health professionals advocate avoiding completely).

Oil Extraction

All oils start from seeds, nuts, or some other plant source. This source is generally heated to some degree and then extracted through mechanical or chemical means.

• Many inexpensive oils are chemically extracted: the oil source is soaked in chemicals like hexane (a six octane gasoline), which is unfit for human consumption. The result is a low-quality oil that requires intense refining processes (see below) to remove the dangerous solvents used for extraction. Since the yield from chemical-extraction is much higher than other processes (over 99% of the available oil), the oil is far less expensive to produce and purchase.

• Expeller-pressed oils are those derived from squeezing the oil source in a mechanical press--a process which doesn't use any toxic solvents. Yet, considerable heat is used in expeller-pressed oils, and even so-called cold-pressed oils are processed at up to 160F (70C). But cold-pressing is a big improvement--it not only avoids toxic solvents, its relatively low temperature preserves the subtle flavors in the oil, as well as the natural antioxidants (which slow oxidation). Note: you can assume that an oil that lacks the expeller- or cold-pressed claim on the label is chemically extracted.

• Refined oils undergo processes as simple as filtration (to remove physical impurities) or as complex as bleaching and deodorizing. For example, those products labeled "vegetable oils" are usually highly processed to the point of being odorless and tasteless. In many cases, refined oils are heated to temperatures upwards of 450F (about 230C) which reduces an oil's nutritional value. In some cases the extreme heat creates serious toxins (unsaturated fatty acids become trans fatty acids). As a result, refined oils (in the opinion of certain nutritionists) are joining the ranks of refined sugar and flour products as unwholesome foods. Unfortunately, most oils you'll see on the market are indeed refined, despite claims like "cold-pressed without the use of solvents". These claims don't refer to the refining process, and so all of the ill-effects of refining may still be present. But if you use small amounts of these oils, you're playing it safe.