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.
Others, like Popeye's and Church's, use solid shortenings. McDonald's has worked hard to find an oil that maximizes frying efficiency. Its oil is substantially different from the oil used by other chain restaurants, according to Blumenthal, and french fry connoisseurs consistently rate McDonald's fries as the gold standard. Though the chain guards its recipe closely, McDonald's admits to changing the oil in its fryers every 10 days. But as processed oil gets old, not only does food start tasting bad and containing more fat, it may harbor more frightening problems.