You might have noticed by my profile that I am interested in cooking and eating too. I have not had a blog on cooking for a while so I thought that I owed you all one. This is on prime rib roast. When you get a good one, you really appreciate it. The problem is why are some so bad? Most of us have tried to cook one at home for a special occasion and were very disappointed. As both a cook and an engineer, I look at the cooking process from a little different angle. He goes what I think.
First you have to understand basic heat transfer. From the simplest stand point, heat transfer depends of temperature gradients. That is, the difference in temperature from point "A" to point "B". Heat will not travel between "A" and "B" if they are both at the same temperature. You need a difference in temperature to get heat to travel. The second point is that the rate of travel of the heat from point "A" to point "B" is directly proportional to the difference in temperature. As the temperature difference gets greater and greater, the faster the heat travels. That is why your heating bill for you home is higher in winter than summer. The heat loss thru your walls is greater because the temperature on the inside and outside is very large. It can be just the reverse in the summer when you are trying to keep cool. Now that you have the basic idea about heat transfer, let's see how it applies to cooking a prime rib roast.
I am going to explain a couple of cooking examples to show how the above applies. First, let's try cooking in a hot oven of about 400f. The roast starts out at about 60f all the way thru. When it goes into the oven the outer most surface of the roast is quickly heated upto 400f while the center is still at 60f. As time goes by heat is gradually transfer ed to the roast from the outside toward the center. After a short time the outside is still at 400f and a short distance inside the roast, the temperature starts to rise. Let's say that the roast is now at about 300f a half inch inside and still at 60f in the center. It takes a lot of time before the center temperature starts to rise. By the time the center temperature starts to rise, the area a half inch in that was at 300f has gone up to 325f. By the time that the center is upto 140f, the outside layers of the roast are much higher than 140f and climb gradually from 140f at the center to the original 400f on the outside.
What we have here is a roast that has a small rare spot in the center and the rest is all well done. This is the kind of prime rib roast that all know we don't want. The problem is how to get the roast just right?
We all like to have a prime rib roast that is nice and brown on the outside yet rare in the middle. By using our understanding of heat transfer, we can get just what we want and not the first roast above. Notice that soon after we put the roast into the oven, the out side temperature of the roast went to 400f or well done and by the time the center was 140fs, we had over cooked the roast most of the way thru. The only things that we can control in the cooking process are the oven temperature and the time that the roast is in the oven. We can't change the laws of heat transfer, they stay the same no matter what we do. What if we put the roast into an oven that was only 140f? Since the roast started at 60f, we would have heat transfer, but at a much slower rate than with the first roast. Now it is going to take a lot longer to get the center of the roast upto 140f. The fact is that theoretically, the center of the roast will never actually get to 140f. It will gradually get very close, but not theoretically to the 140f temperature. Now I think you are starting to get the idea. Remember, we can only control oven temperature and time in the oven.
After all this, just how do we get the perfect prime rib roast? First, put the roast into a 500f oven until it is well browned on the outside. This will not take too much time and by not taking too much time, the center of the roast will get very little heat. Only the very outside will be well done while most of the center is still raw. Now how do we cook the center? We reduce the oven temperature to about 250f and put the browned roast back into the oven. Now with the outside at 250f and the center at 60f, it is going to take a lot longer to gradually get the center temperature upto 140f. Let's look at the math. In the first case we had an oven temperature of 400f and a roast temperature of 60f. That is a difference of 340f. In this last example we had an oven temperature of only 250f and the same inside temperature of 60f for a difference of 190f. Recall that the rate of heat transfer is proportional to the temperature difference between inside and outside. In this case it is going to take twice as long to cook the roast, but when it is all finished, most of the inside of the roast will be rare. Theoretically, if you used second oven temperature of only 200f and more time in the oven, you could increase the amount of rare roast in the middle.
I think that from this, you should understand why cooking a perfect prime rib roasts takes multiple oven temperatures and different amounts of time at each. Now a second point is the fact that cooking continues after the roast comes out of the oven. I think we all knew this, but did not fully understand why. Now when the roast comes out of the oven, the temperature is "not" the same through out the roast. It is hotter on the out side than at the center. When you take the roast out of the oven, the temperature inside the roast has to even out. The hotter areas get cooler and the cooler areas get hotter. That is why when your thermometer reads 140f on the inside when you take it out that the center turns out to be over cooked at 155f when the roast is served. As a cook, you have to compensate for this fact by when you take the roast out. It you cooked the roast at 400f, you would have to take the roast out earlier, while if you cooked at 200f, you could take the roast out later. It might start to seem complicated, but is not. All you have to do is to follow the laws of heat transfer.
Simple and good eating.
