How vegetable oils are made
People have been extracting vegetable oil from plant materials for thousands of years. The very earliest methods of pressing vegetable matter probably yielded at best 10% of the available oil. More modern methods involving chemical solvent extraction can extract all but 1% to 2% of the oil.
Most cooking oil contains 100% vegetable oil, with no additives or preservatives. The oil most commonly comes from the seeds of the plant (in the case of sunflower, palm kernel, grapeseed, sesame, soybean, cottonseed, and safflower oils) or the nuts (in the case of walnut, peanut, hazelnut, and almond oils). Several cooking oils are derived from the flesh of the fruit of the plant. For example, coconut oil comes from the white meat of the coconut, palm oil from the pulp of the palm fruit, and olive oil from the flesh of fresh olives.
Some cooking oils, including olive, peanut, and some coconut and sunflower oils, are expeller-pressed, meaning that the plant part from which oil is derived is crushed and pressed to produce the oil. The oil is then left crude or refined. Crude oils may have a cloudy appearance and are more flavorful, but they have a shorter shelf life than refined products. Some manufacturers label their products cold-pressed to indicate that the plant materials were not heated during oil extraction. However, this label term is largely unregulated and cannot always be relied on to mean that the oil was never exposed to high heat.
Most cooking oils are extracted from ground plant material with a chemical solvent. After extraction, the oil is cleaned, refined, and filtered and/or distilled. This process produces a bland, clear oil.
Types of cooking oils
All fats, including cooking oils, are a mixture of saturated, monounsaturated, and polyunsaturated fatty acids in varying proportions. There is no such thing as a saturated-fat-free oil or one that contains only polyunsaturated or monounsaturated fat.
Fatty acids, the units that make up fats, differ primarily in the amount of hydrogen they contain. Saturated fatty acids contain the most hydrogen, while monounsaturated and polyunsaturated fatty acids contain less hydrogen. A fat’s consistency at room temperature is a clue to its fatty acid makeup: Vegetable oils that are liquid at room temperature contain high amounts of unsaturated fats and little saturated fat. Coconut, palm, and palm kernel oil, which have more saturated fat, are solid at room temperature. Stick margarine that is made with hydrogenated fats and butter, which is high in saturated fat, are solid at room temperature.
All cooking oils will oxidize and therefore hydrogenate to a small degree if repeatedly heated to a very high temperature, as is done in commercial frying. This means the fats become more saturated and start to contain some trans fat over time. However, it is almost impossible to oxidize fat to this degree in home cooking.
Cooking oils differ in their “smoke point,” or the temperature to which they can be heated before they smoke, discolor, and break down. At the smoke point, the oil not only smokes and begins to emit unpleasant odors, but it also imparts unsavory flavors onto the food. Reaching the smoke point means you are approaching the “flash point,” which is when the oil can erupt into flames.
Factors that will lower the smoke point of an oil include the number of times an oil has been used, the length of time it has been heated, any exposure to light, oxygen, and high temperatures during storage, and the presence of substances such as salt or food particles in the oil. Cooking oils with a high smoke point can be heated to high temperatures before burning. Knowing a cooking oil’s smoke point gives you some indication of whether it is good for cooking and frying. In general, the best oils for high-temperature cooking methods such as deep-fat frying are safflower, sunflower, peanut, soy, and canola oils.