The oil emerges from the mill in a dark and impure state, and must be thoroughly refined before it is fit for use. The refined product is light, flavourless when fresh, and high in polyunsaturates. It goes rancid quickly on exposure to air. For this reason it is mostly used in the manufacture of vegetable margarine and cooking fats; but it can also be bought as a salad or cooking oil and, if used promptly, is suitable for these purposes. It was the preferred medium for frying fish and chips in large parts of Lancashire in the early 20th century.