I knew that in deep frying, the later batches went quicker– browning faster, particularly. I wondered if the food was really cooking through quicker, or whether things browned quicker in darker oil. I had learned by experience to bring the heat down as cooking continued without understanding a reason for it.
It turns out they really do cook more quickly and that there’s a reason for it. As with all things involving science and cooking, I’ll turn to Harold McGee, whose recent Keys to Good Cooking: A Guide to Making the Best of Foods and Recipes is book four in my book-a-week resolution:
The early cooking of a given batch requires more heat to maintain the temperature than later stages. Later batches fry faster than early batches cooked at the same temperature, accumulating impurities transfer heat to the food more efficiently.
It’s nice to learn the underlying explanation for something I’d observed!