Dear A Moment of Science: My cookbook says when you take a roast out of the oven and set it on the counter, the inner temperature will continue to rise by five degrees. How is it possible for the temperature to keep going up? Sincerely, Curious Cook.
In a conventional oven, not a microwave, when you heat something up, like a piece of meat, you aren’t increasing the temperature of the entire thing all at one go. The first part of the roast to be affected is, naturally enough, the outside. Because the outside comes in most direct contact with the oven, it will grow hotter fastest.
The heat will now slowly diffuse through the inner portions of the roast, which stay cooler longer than the outside. That’s why you can have a roast that looks cooked on the outside, but when you cut into it you find its not done all the way through.
So, take that roast out of the oven and what happens? The hot outside does indeed begin cooling off. But residual heat is still being diffused through the cooler inside for several minutes. You could say that for a little while, the cool inside of the roast is being cooked by the hot outside!