Vegetable oils are pressed from various seeds, fruits and nuts. Nutrients are best retained by cold pressing. Among the oil sources are corn, cottonseed, olive, soybean, sesame, safflower, sunflower and peanut. There are also such nut oils as walnut, hickory and beechnut, which are better used for salad dressings than for cooking, as they break down under high heat.
After being pressed, the oils are refined, bleached and deodorized so thoroughly that, except for olive, the end products are rarely distinguishable one from the other by flavor or odor; but in cooking they differ greatly in their smoking points. Safflower, soybean, cottonseed and corn oil have higher smoking points than peanut and sesame oils. Soybean oil is not recommended for deep-fat frying, as it foams. Olive oil, the lowest, ranges around 400 degrees.
Most oils for salad are further treated to remove cloudiness at refrigerator temperatures. Oils should not be held too long at 70 degrees even if tightly closed and in dark bottles. Most of them remain in a liquid state under refrigeration. Olive oil, which becomes semisolid when refrigerated, should be allowed to stand at 70 degrees to return to a liquid condition before using.
Olive oils are like wines in the way their flavors are affected by the soils in which they are grown. Greek, Spanish, Italian--try them all to find a favorite. Olive oil is less expensive by the gallon, but, as it is susceptible to rancidity--especially the cold-press type--when exposed to light and air, decant it into smaller containers. Use one and keep the other resealed bottles in a cool, dark place.