Free Molecular Gastronomy: Exploring the Science of Flavor by Hervé This

good roasts of beef, for example, for they often neglect an in-
    dispensable 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. Profes-
    sional 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 chem-
    ist. 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 tempera-
    ture 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
    | 47
    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, compress-
    ing 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.
    48 | secrets of the kitchen
    Empirical analysis also establishes that juices do not flow back to the