Cooking a Roast

For many types of meat beef, pork, lamb, etc. one can buy a roast from the butcher with or without the bone inside. Suppose we have two beef roasts of the same weight of 4.4 pounds (2 kg) and cook them in identical ovens at the same temperature. One roast has the bone in and the other does not. Which roast cooks faster? Why?

Answer

The bone-in roast cooks faster because the bone, even though porous, rapidly conducts thermal energy to the inside faster than the meat itself does. So the bone-in roast cooks from both directions outside in and inside out. There will be some minor speedup effect from the difference in specific heats of bone and meat, and there will be slightly less meat if both roasts weigh the same, but in the simplified, ideal case we ignore these differences. One could use computer modeling with the appropriate physics equations to determine the temperature distribution in various parts of the meat and bone in the two cases, showing that some parts of the meat are cooked more than others in both types of pieces.

Any general physics text discussing thermal conductivity and specific heat contains the pertinent information for analyzing this problem, but the actual temperature profile as a function of the elapsed time is difficult without some idealizations about the shape, the uniformity, and other things. We have considered the idealized case above that ignored the change in bone properties with temperature change, such as the specific heat of the bone and its thermal conductivity. Somehow Nature has figured out all these things without special computer modeling!

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