Why Food Cooks Slower in Oil than in Water
There's more to cooking speed than just temperature.
One of the niftiest things we learned from our Poached Fish Fillets recipe is that the fish cooks more gently (and slowly) in oil than in water—even when both liquids are exactly the same temperature. This is true, it turns out, not just for fish but for any other food, including eggs (see photos below). But how can this be? Isn’t temperature what determines speed of cooking? As it happens, equally critical is the liquid’s thermal capacity, or how much energy is needed to change its temperature by 1 degree centigrade. Oil has roughly half the thermal capacity of water, which means it requires half the amount of energy to reach the same temperature as an equal volume of water. This, in turn, means it has less energy to transfer to food and will cook it more slowly.
While we understood the concept, it seemed to defy common sense. So we decided to check it out using our own senses. We heated equal amounts of water and oil to 135 degrees and then asked test cooks to stick a finger in each liquid simultaneously. Testers immediately snatched their fingers out of the water, which felt very hot, but left them in the oil, which felt merely warm. Point proved.
EGG IN WATER
After 6 minutes at 165 degrees, this egg is well on its way to being poached.
EGG IN OIL
After 6 minutes at the same temperature, this egg is still raw.