What is a sign that the oil in the fryers is too old?

When oil in fryers begins to smoke, it is a clear indicator that the oil has degraded and is too old for safe cooking. This smoking occurs because the oil has broken down, leading to the release of free fatty acids and other compounds that produce smoke when heated. Fresh oil should have a relatively high smoke point and should not produce smoke during regular frying temperatures. If the oil is smoking, it affects not only the taste and quality of the food being prepared but also may indicate that harmful compounds could be forming, which could be detrimental to health.

The other options do not indicate aging oil effectively. Clear oil does suggest freshness, but it is not a direct sign of being overly aged, since oil can remain clear even when it's still good to use. Enhanced flavors can occur with new oil as it can contribute to better frying results, making this a misleading indicator. Lastly, a lack of bubbling can also occur for various reasons unrelated to the oil's age, such as the temperature not being sufficient for frying. Therefore, smoking oil provides the clearest sign of degradation and indicates that it should be replaced.