When I start out with 100% fresh, never-used oil to fry chicken, for instance, it takes forever and then some more forever to get any kind of deeper color than just the lightest golden. But if I re-use oil, it’s not an issue.
Why is that?
New oil is highly hydrophobic so the water content of the food repels contact with the oil so efficiently that it takes longer for the Maillard reaction to occur for it to brown. As oil degrades, this becomes less of an issue, and even problematic.
Some tempura restaurants here in Japan mix some old oil into new just to get it to take on some colour. If they used all new oil, the food would get overcooked before it reached the same colour.
Basically the little bit of old oil has enough of the hydrophilic compounds to more or less kickstart the development of the same in the new oil.
This allows the new oil to more intimately contact the food, thereby producing the Maillard reactions that cause the food to be brown. (the oil doesn’t somehow dye the food).
Yeah, you’ll never have consistent results with frying unless you re-use a portion of the old oil to “kick start” it. Learned that one from Justin Wilson.