Saturated fatty acids. In a saturated fatty acid, each carbon atom has bonded with two hydrogen atoms. In other words, it's "saturated" with hydrogen. This saturation makes the fatty acid very stable, which means it can withstand more heat before it becomes rancid. Common examples of saturated fats are butter and coconut oil. An easy way to know if a fat is saturated is if it's solid at room temperature. Saturated fats are ideal for cooking because of their natural ability to withstand heat without being damaged.
Monounsaturated fatty acids. In a monounsaturated fatty acid, one pair of carbon atoms forms a double bond with each other that replaces the bond each would have with one hydrogen atom. So it is unsaturated, but only by one bond. This means the fatty acid is less stable than a saturated fatty acid molecule. The classic example of monounsaturated fat is olive oil. The oil in almonds, hazelnuts, and avocados is also monounsaturated. Monounsaturated fats are liquid at room temperature, but they solidify in the refrigerator. These are okay for cooking, but only at very low temperatures.
Polyunsaturated fatty acids. A polyunsaturated fatty acid has two or more carbon pairs that have bonded together rather than with a hydrogen atom. This means the fatty acid is quite unstable. Examples of polyunsaturated fats include most vegetable oils, most seed oils, soybean oil, flaxseed oil, and sunflower oil. These fats are liquid both at room temperature and in the refrigerator, and they should never be used for cooking. This might be surprising, given that so many of our standard cooking oils are exactly these unstable vegetable oils.
There is a fourth type of fatty acid, but I haven't included it in the list above because it's not found in nature. It's the trans fat, which is an unsaturated fatty acid that has been either partially or fully hydrogenated.
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