In writing his book How to Read a French Fry, the writer Russ Parsons discovered that oil that has previously been used to cook something else in it actually cooks the potatoes better. Cooking food in oil breaks down the oil (which is why you can’t use it indefinitely). One of the breakdown components is a surfactant, Russ explains, that lowers the surface tension between the oil and the vapor (water) bubbling out of the fry. If there are too many bubbles, the surface tension is such that the oil has a hard time contacting the potato itself and browning it. This is why you may notice that if you save a batch of oil (as you should) and use it for the next batch, the second batch browns even better.
