Cooking for engineers
E&T provides technological tips for cooking the perfect prime rib.
There's nothing quite like serving a perfectly cooked seven-rib standing rib roast to a large room full of guests. It's an extravagant gesture, but the cost and the time are well worth the smiles and compliments.
How does one prepare a rib roast properly? It's easy once we understand how to cook large uniform pieces of meat, so let's start by taking a quick look at the cow.
American beef is sectioned into eight primal cuts. The shoulder cut, called the chuck, includes the first five ribs. The rest of the ribs, six to 12, are actually called the rib. It is the muscles sitting on top of these seven ribs that make up the meat of a rib roast.
Different names are given to the different ways this piece can be cut. Slices parallel to the ribs and with the bones removed are called rib steaks or rib-eye steaks. If left whole with the bones on, a slice is referred to as a standing rib roast ("standing" because the cut can be roasted with the ribs standing up vertically).
Both of these rib roasts can be cut into smaller sections, which are usually named by the number of ribs they contain. The rib meat closer to the chuck or shoulder (sometimes called the "large-end rib roast") is considered tougher, while the rib meat closer to the loin (the "small-end rib roast") can be more tender. When roasting, the preparation of these cuts is similar.
Now, let's leave our imaginary cow to graze on her no-less-imaginary lawn and have a look at the rib roast as such. It is basically a large cylinder of meat and fat, with or without a row of rib bones attached to one side. Thinking about it as a cylinder makes it easy to grasp why recipes which provide cooking times in minutes per pound are not going to work. For example, if the weight of the roast doubles, the length of the roast doubles accordingly - as well as its total surface. Since cooking in an oven transfers heat through radiation and conduction affecting the exposed surface of the roast, the time required to cook a cylindrical roast will be similar regardless of its weight. Factoring the end caps back in, we find that the surface area doesn't quite double when the length doubles, but the ratio doesn't decrease linearly either - the end caps matter less the longer the roast is. The smaller the diameter of the roast, the less the caps affect cooking time. In short, because of the shape of the roast (length and size of end caps), it is impossible to provide accurate cooking times for something that needs to be roasted as precisely as that. Also, if the ribs are still attached, they act as an insulating barrier, slowing down the penetration of heat from one direction.
The perfect rib roast has a thin outer crust of well-browned meat. Under the crust, the meat should be of the desired temperature that extends evenly through to the centre of the roast. The difficulty lies in getting the entire roast to be of a uniform temperature, and there is only one way to accomplish this - low heat over a long period of time.
If a large piece of meat is cooked at high temperatures, the outside gets extremely hot when the interior finally reaches the desired heat. Because of this temperature gradient, an effect called ‘carryover cooking' occurs. After the roast has been removed from the oven, the exterior meat, which is much hotter than the interior, continues to conduct heat inside. This raises the temperature by as much as 6°C. The exact amount depends on how quickly the meat was cooked. The faster the cooking, the larger the temperature gradient in the meat and the greater the temperature increase in the centre after removal from the heat source. Cooking at a lower temperature allows the entire roast to heat up almost uniformly.
Preparation of a perfect prime rib is straightforward and simple. Rub salt and pepper over the exterior of the roast. Brown the exterior by: (a) searing all the sides in a hot pan (no oil is needed - the roast will provide the fat necessary for even browning); (b) browning over a hot grill; or, (c) roasting in an oven for 15 minutes at a high temperature (260°C).
Once the roast has been browned, the probe of a remote thermometer should be inserted into its centre (the tip should be away from any bones). Unfortunately, because the final texture of the roast depends heavily on how hot it is, it's important to monitor the temperature to stop the cooking at the right time. Due to the length and size differences of the roasts, time estimates are simply not accurate. The probe thermometer enables you to monitor the temperature increase over time, allowing to regulate the oven heat to match a specific time window.
Roast for approximately four to five hours at 95°C, adjusting the temperature slightly up or down during cooking. When the desired temperature is achieved (52°C for rare, 54°C for medium-rare; little or no "carryover cooking" will occur when roasting at these temperatures), remove the roast from the oven and leave it for at least 45 minutes to allow the juices to distribute evenly. A large roast should be able to hold its warmth for at least an hour after resting.
And, finally, enjoy your delicious meal!