Mallory's Camera

Of fatty acids are what's in that oil. There are any number of factors that cause rancidity, but in restaurants, it's just that oil is costly and thus marginal places choose not to change it as frequently as is required for it to stay tasty. Like so much else in restaurant economics, the race is to the bottom on such matters. Bitter fries and other items lead to less business, making changing oil harder to pay for.