Allowing meat to rest after cooking causes the juices that have been retained in its center to flow outward to the dry periphery.
MANY HOME COOKS TODAY are pressed for time. Their haste prevents them from eating good roasts of beef, for example, for they often neglect an indispensable step after cooking: letting the meat rest, with the door of the oven open. Omitting this step means that the meat will be tough and dry. Professional cooks are well aware that letting meat rest is essential if it is to be tender, but they believe this is because, in the process of cooking, the juices want to “escape” the heat of the oven and consequently flow back into the center of the meat. Letting meat rest afterward therefore is seen as a way of ensuring that its fluids will be thoroughly redistributed. Why should this be true? What really happens when we roast a piece of meat?
Let’s examine the matter with the cold and clinical eye of a physical chemist. Chemists know that meat is composed of cells, or muscle fibers: sacs filled mainly with water that contain molecules responsible for metabolism and contraction. These cells are sheathed with collagen and grouped together in bundles, which themselves are grouped together in larger bundles. Naturally this is a simplified description; animal muscle also contains fatty matter, blood, and so on.
How does the muscle structure react when it is heated? Because heat is introduced into a roast by conduction, air being heated in an oven to a temperature of about 200°C (392°F), the water evaporates from the outer layer inward to a point where the temperature is 100°C (212°F). This crusty, desiccated outer layer is thin. Closer to the center, the temperature slowly rises during cooking, and the structure of the meat is transformed by degrees because the various proteins in the meat coagulate at different temperatures. From 70°C (158°F), for example, myoglobin, which transports oxygen in the blood, is oxidized: The ferrous iron it contains is transformed into ferric iron, with the result that the meat turns pink. At 80°C (176°F) the cell walls begin to break down, bringing the myoglobin into contact with oxidant compounds and causing the meat’s color to change to brown.
Can blood accumulate in the heart of the roast? When a temperature of 50°C (122°F) is reached in the outside layer, the collagen contracts, compressing the juices inside (although the degree of compressibility is small because the juices are mainly water) and expelling the juices of the periphery outward. The center of the roast, composed of liquids and largely incompressible solids, cannot receive these juices. Anyone who is not convinced of this has only to roast a few pieces of beef, weigh them, and determine their density before and after cooking.
Good Advice, Bad Reasoning
These steps are instructive. First of all, one notices that a roast shrinks when cooked in the usual manner, losing almost a sixth of its weight. This loss results from the elimination of the meat’s juices, which are expelled by both contraction of the collagen and evaporation of peripheral water. Note that this observation fatally undermines the theory of cauterization, which holds that the coagulated surface of the meat seals in its internal juices. Near the turn of the twentieth century, for example, Mme. E. H. Gabrielle, author of La cuisinière modèle, remarked, “Put the roast on the spit before a very hot fire, in order to sear and tighten the pores of the meat, which thus conserves its juices.” Similarly, the great French chef August Escoffier (1846–1935) wrote in his book for home cooks, Ma cuisine (1934), that the purpose of browning is “to form around the piece a sort of armature that prevents the internal juices from escaping too soon, which would cause the meat to be boiled rather than braised.” Both views are mistaken. Not only does the notion of “pores” have no anatomical basis, but measurement shows that the loss of juices actually increases with cooking.
Empirical analysis also establishes that juices do not flow back to the center of the roast; the density of the cooked center does not differ significantly from that of raw meat. This means that the center has undergone little or no modification during cooking (which is not surprising in the case of French-style roasts, which remain almost raw in the center) and also that the center is full of juices compressed by the shrinking of the collagen.
Why, then, is it a good idea to let meat rest after cooking? Weighing the center and outside portions of roasts, we find that the cooked center loses more juice while resting than do the peripheral parts (which, having already been dried out, are less likely to lose any). Letting the meat rest therefore does redistribute juices from the center outward so that the outer parts regain their tenderness. But this is not because cooking had previously forced fluids to flee to the center.
Given that the juiciness of the meat depends on the amount of juice it has, why not use a syringe to reinject the juices that have drained out from the roast during cooking? Seasoned with salt and pepper, these juices would give the meat a taste it never had—except in the old days, when cooks used to lard meats before cooking with seasoned pieces of bacon.