Odorant molecules are trapped so that they may be better perceived.
SOME CLASSIC RECIPES APPEAR PARADOXICAL. To make a salmis de canard, for example, one removes the breasts and thighs of the duck and roasts them. The cook then makes a sauce by cooking the scraps and bones of the duck in water together with vegetables and aromatic herbs. Isn’t the second step redundant? No, because in the final dish the odorant and taste molecules contributed by the duck are retained for different lengths of time by the meat and the sauce before being released. As a result, the flavor of the dish lingers in the mouth longer.
The problem of trapping odors has received additional attention with the widespread use of extrusion cooking technology by food processing companies. This technology, borrowed from the polymer industry, involves a screw that turns inside a cylindrical barrel, as in a meat grinder, only here the screw is tapered (so the distance between the edges of its blades decreases from the top of the screw shaft to the bottom). A mixture of solids and liquids placed in this apparatus is forcefully compressed—so much so that the water that is suddenly released evaporates, causing the food to be “puffed.” Cocktail crackers typically are made using this method.
The puff is achieved at the expense of flavor, however, for the food’s odorant molecules are carried away with the evaporated water. Commercial producers often spray aromas on the extruded crackers afterwards. But because this is a costly expedient, they are interested in finding ways to trigger chemical reactions during the extrusion process that will produce odorant molecules or otherwise to trap the volatile molecules.
How can molecules be trapped? In the sauce that accompanies the roast duck, for example, the long cooking of the scraps and bones in water with vegetables has the effect of extracting the gelatin present in the skin, tendons, and bones, producing a decoction that, as chemists very well know, extracts the odorant and taste molecules present in animal and vegetable matter. Odorant molecules therefore are found in two distinct physicochemical environments: in the meat, where they are dissolved for the most part in fats, which are themselves dispersed between the muscle cells; and in the sauce, where they are in a liquid solution. In the mouth these molecules are released in different ways, so that the flavor of the duck lasts longer.
Retention in Solution
Let’s try to figure out why this is so. If the duration of a dish’s flavor in the mouth has to do with the release of odorant and taste molecules, how can this release be controlled? By controlling the environment of the odorant molecules.
In their pure state these molecules are highly volatile, and the pressure of saturated vapor increases with temperature. The cook is able to vary the degree of volatility by redistributing the odorant molecules into environments of different temperatures. The molecules can also be placed in solution. In this case their volatility depends on the solvent used (whether water, alcohol, or oil) because the molecules bind to a greater or lesser degree with the molecules of the solvent (thus saturated and unsaturated oils differentially retain odorant molecules in solution).
Playing with Molecular Interactions
Cooks can slow down the evaporation of volatile molecules further by putting them in the presence of larger molecules, with which they bond. For example, we know that iodine turns food containing starch blue because the amylose molecules in the starch (long chains composed of linked glucose molecules) wrap themselves around the iodine in a helix. More generally, in water solution, the amylose wraps around hydrophobic molecules, many of which are odorant molecules. Amylose is by no means the only molecule that wraps around in this fashion: Gelatin does the same thing, hence its usefulness in sauces. More generally, well-chosen flexible polymers bond with odorant molecules to retard their release.
Compartmentalization is another, more radical means of retaining molecules. For example, fines herbes (a mixture of fresh chopped herbs used in cooking that typically includes parsley, chervil, tarragon, and chives) release their odorant molecules only when their cells are ruptured by chewing. Emulsions, foams, gels, and pasta are systems of the same type. Similarly, cooks may soon be able to use liposomes, which are sorts of artificial cells created by the assembly of molecules analogous to those of cell membranes. This question is being studied as part of a European Union project devoted to the innovative transfer of technology for culinary purposes.
In addition to microcompartmentalization at the cellular level, there are various forms of macrocompartmentalization. Consider the farces, or stuffings, used in classic cooking, in which aromatic meat and vegetable mixtures are placed beneath the skin or inside the bodily cavities of fowl and other animals. Odors can also be retained if they are made to penetrate foods: The flavor of a meat that is marinated or of a meat that cooks in a fragrant broth is enriched by the odorant molecules of the marinade or broth. Related techniques include decoction, infusion, and maceration.
There are many methods, then, for retaining aromas to one degree or another so that they are released at different moments as one eats a dish. The greatest cooks are able to create waves of flavor that call to mind the peacock-tail effect of the great wines of Bordeaux.