There are just too many uncontrolled variables when cooking.
The cruel fact of life is that when a recipe tells you to cook for “x hours at y degrees,” it’s only a guideline, an educated guess, a ballpark estimate. It’s what worked, most of the time, for the elves who tested the recipe, but there’s no guarantee it will work for you. So, sorry, Virginia, but there is no Santa Claus. (I’ve been wanting to straighten that kid out for years.)
Except perhaps in a food research laboratory, there is no such thing as a standard roast on a standard rack in a standard pan at a standard position in a standard oven at a carefully regulated oven temperature. Each one of these factors can vary, producing different results even if all other things were equal. But as Joe’s Law of Pervasive Perversity says, “All other things are never equal.”
You can’t just go around saying that a beef or pork roast or a chicken or turkey should be cooked for so many minutes per pound at a certain temperature. Even if Wolke’s Law didn’t apply and you could magically control everything else, the one variable that you have no control over is the most important one: the shape of the roast. Not its weight but its shape: how much surface area it presents to the oven’s heat. Heat can enter the meat only through its surface, so the more surface area a roast has for its weight, the faster it will cook.
Here’s an example.
If we had two roasts of the same weight, that is, the same volume, one shaped like a cube and the other shaped like a sphere, the cubic roast would have 24 percent more surface area than the spherical one. That’s just geometry. Work it out yourself if you get your kicks that way. For my part,
I never saw a cubic cow
I never hope to see one.
But I can tell you anyhow,
It’ll roast about 24 percent faster than a spherical one.
Another example: Suppose we cut that cubic roast in half parallel to one face. Its surface area will then be increased by 33 percent. The two halves, then, should cook in roughly 33 percent less time than the whole one.
So again, dear, naïve little Virginia, no Santa Claus, or even a reasonably good fairy, exists who can weigh your irregularly shaped rib roasts or turkeys and tell you exactly how many minutes per pound to cook it, even if Joe’s Law were repealed.