With every application, frying fat loses quality, it spoils. This rate of degradation depends on the temperature of the frying bath and on what has been fried. Sight and smell are not reliable: "Clean", normal smelling fat can be spoiled. Fat that does not inspire confidence can often be used without hesitation and without any particular taste at consumption.
The condition of cooking oil is measured based on the percentage exposure to polar components. Specified tolerance values are checked regularly in restaurants' kitchens, where testing the quality of frying oil or fat on a regular basis is essential.