By Mark Sabini, Justin Yu, & James Lee
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| Fig. 1. Types of oils with their precursors displayed inside |
Oils often get a bad rap. People frequently view them as trademarks of obesity and unhealthiness, where some people go so far as to nearly eliminate them from their diets. However, the set of oils contains a plethora of different compounds, each with its own unique qualities. One subset of oils is the cooking oils. Found in virtually every household and type of world cuisine, cooking oils are used to give food flavor and aid the cooking process. Every oil is different, and with different molecular structures comes different chemical properties.
Background on Oils
The introduction of vegetable oils into the American food industry began in the early 1900’s. Before then, oil was primarily derived from animal fats. In the mid 1800’s, farmers realized that vegetables such as corn held higher nutritional value than their animal counterparts and also were cheaper to grow. This led to a sharp increase in the production of corn and other vegetables, and later on, the production of vegetable-based oils.
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| Fig. 2. Formation of oil products from raw materials |
Vegetable oils were favored because they were less saturated than those derived from animals due to the amount of double bonds in their structure (See Fig. 7. below). This meant that they were much more fluid and much less likely to clog blood vessels, making them healthier for the body and easier to process. A process called fractionation is used to purify oils by cooling the oil until some of it crystallizes into fat and filtering that fat out so only pure oil is left, removing any impurities in the oil. This process is widely used in the manufacturing of cooking oils in order to ensure the oil is safe to consume.
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| Fig. 3. Oil consumption trends |
Before chemical properties of oils can be examined though, physical properties must be set straight. A common belief is that pure fats and oils have color, taste, and odor. However, this is not true! Pure fats and oils have none of the three aforementioned characteristics. For example, the bitter taste of extra-virgin olive oil is produced by several polyphenols, including oleocanthal and oleuropein. In addition, the color of olive oil depends on the color of the olives themselves, as black olive oil comes from olives that fall off the branches of trees. It is important to distinguish between pure fats and oils, and “mixed” oils such as olive and canola oil.
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| Fig. 4. Left: Structure of oleocanthal. Right: Structure of oleuropein |
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| Fig. 5. Classification of Fats and Oils |
Cooking oils vary in the components that make them up. Cooking oils are generally split up into three different categories, vegetable oils, animal-derived oils, and synthetic oils. These oils differ in their chemical composition because they come from different sources. Vegetable oils, for example, are derived from the triglycerides in plants, primarily extracted from plant seeds. Some common vegetable oils include sesame oil, olive oil, peanut oil, and coconut oil.
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| Fig. 6. Comparison of Average Fatty Acid Values of Dietary Fats |
Smoke Point of Oils
Oils, as well as other chemical compounds, decompose when heated because at a certain temperature, enough heat is absorbed to break the chemical bonds that hold the compound together. The amount of energy needed to break the bonds in an oil and therefore decompose it is determined by the oil’s structure. Thus, the smoke point of an oil is the temperature at which it begins to give off smoke, due to the thermal decomposition of the oil into glycerol and free fatty acids.
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| Fig. 7. Oil beginning to smoke |
In addition, the smoke point marks the start of nutritional and flavor degradation. Temperatures above the smoke point of an oil are undesirable, as the acrolein starts to be produced, and the oil goes rancid. Several factors determine the smoke point of an oil. Saturated fatty acids provide stability and are more resistant to high heat, while polyunsaturated fats are more sensitive to both light and heat. This means that excessive heat can cause production of heat free radicals and harm the body if consumed.
The smoke point for a single batch of oil does not stay constant. As an oil is used more and at higher temperatures, the smoke point decreases. In addition, foreign entities (such as batter and bread crumbs) in the oil can cause the smoke point of an oil to decrease more rapidly. That being said, there are several ways to stop the smoke point of an oil from lowering too quickly and therefore prolong the oil’s life. The first is to use a more refined oil. Oils that are more refined (such as safflower oil) tend to have higher smoke points than say, lards. According to Ohio State University food scientist Sam Vance, another way to prolong the life of an oil is to mix it with oil that has a higher smoke point. Finally, reducing the amount of salt in deep-fried food as well as ensuring that excess bread crumbs and batter do not enter the oil help stop the oil from going rancid.
Health Effects of Cooking Oils
Cooking oils, as evidence shows, come in many types. Thus, naturally, their nutritive values and health effects should be just as diverse. Oils are comprised of fats, which fall into two main groups, saturated and unsaturated. Saturated fats have as many hydrogens bonded to carbons as possible, and thus result in straight molecules. These straight molecules can be packed tighter, so solid fats like butter are composed mostly of saturated fats. Cooking oils contain a mix of saturated and unsaturated fats.
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| Fig. 8. Structure of fatty acids |
Fats such as palm oil, which contains 52% saturated fat, are not heart healthy due to the possible adverse health effects. Since saturated fats clump more easily, they can form deposits in blood vessels and cause atherosclerosis. In fact, unsaturated fats with trans bonds are much straighter than those with cis bonds, and therefore less healthy for the reasons mentioned.
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| Fig. 9. Examples of types of fatty acids |
However, there are alternatives that can in fact be healthy. Omega-3 fatty acids have been getting a lot of good press lately, and for good reason. Omega-3’s can help prevent heart diseases and strokes, according to Cleveland Clinic. Oils like walnut and hemp oil are good sources of this type of polyunsaturated fatty acid (a fatty acid with more than one double bond).
Conclusion
Cooking oils have many properties that differentiate them, and the huge variety can make choosing one difficult. However, each oil has its advantage, whether it be its higher smoke point or high level of Omega-3 fatty acids. Compromises must be made, since there is no perfect oil. However, with a better understanding of each oil and the chemistry behind it, one can make more informed decisions when cooking, taking into consideration the multi-faceted characteristics of cooking oils.
BCA Chemistry 