But not too long ago, food was fried in solid fats like beef tallow, according to Blumenthal. The switch helped satisfy demands for more healthful food. At the same time, however, it added some messy problems to the process. Oils break down during frying, and liquid fats break down faster than do solids. As it fries food, more oil makes its way into the food, creating a greasier, fattier product.
Flavors suffer as well.
Water and oil combine to make soap. The less often they have to change the oil, the less money they have to spend. So industry and fast-food chains process their oils with a technique called hydrogenization. By inserting hydrogen into a liquid oil's chemical structure, they make the grease partially solid and more stable. Some restaurant chains use semi-solid oils.