Cooking Oils

The available vegetable oils are derived primarily from soybeans, rapeseed (canola oil), sunflower seed, corn, cottonseed, and safflower seed. Fruit oil sources include the avocado, coconut, palm kernel, palm, and olive. Oil differ dramatically in their taste, color, and texture, depending on their source and method of extraction.

Extracting and Refining Oils

Oils are obtained from their plant sources through a process that involves both extraction and refining. Extraction of oils is done by either mechanically pressing the seeds against a press, called cold pressing, or chemically removing the oil from the seeds with solvents. Specialty, full-flavored, cold-pressed oils are often consumed unrefined, but most commercially produced oils are extracted by the use of heat and solvents.

When the goal is a neutral, clean-flavored oil, the oil is purified or refined after extraction to remove impurities such as water, resins, gums, color compounds, soil, and free fatty acids. If these compounds are not removed, they adversely affect the oil’s flavor, color, clarity, smoke point, and shelf life. For example, free fatty acids detract from the oil’s flavor and reduce the temperature to which an oil can be heated without smoking. Refining, which results in oil that is 99.5 percent pure, consists of five steps: degumming, neutralizing, washing and drying, bleaching, and deodorizing.

• Degumming - Certain impurities in oil form gums when combined with water. These are removed by adding hot water to the oil and spinning it at high speeds to separate the oil from the gums.
• Neutralizing - Free fatty acids are removed by adding an alkaline medium to convert the fatty acid to an insoluble soap, which settles to the bottom of the neutralizing tank. Newer methods use a centrifuge to separate the major layers according to specific gravity.
• Washing / Drying - Traces of soap created by the neutralizing process are removed by washing the oil with water. The water is drained, and the oil is dried under a vacuum.
• Bleaching - Colored matter in the oil is removed by adding absorbent materials, such as fuller’s earth or activated carbon. The absorbed colored matter is then filtered out.
• Deodorizing - Volatile compounds - aldehydes, free fatty acids, hydrocarbons, ketones and peroxides - which contribute off-odors are removed by passing steam through the heated oil.