The texture of vinaigrettes determines their odor.
COOKS WELL KNOW THAT ADDING too much flour to a sauce makes it tasteless. The reason is that the flavor of foods does not depend solely on the odorant and taste molecules they contain but also on interactions between these molecules. Molecules with no odor, such as proteins and starch molecules, bind with certain odorant molecules and prevent them from acting on our senses. One would like to know exactly which aromas are masked in this way.
Identifying all the chemical bonds between the various molecules in a food is not enough, for its physical structure plays a role as well. Even in homogeneous phases such as solutions, the release of odorant molecules depends on the viscosity of the system. However, foods are not solutions but dispersed systems. In foams, air bubbles are trapped in liquids or solids; in emulsions, oil droplets are dispersed in water; in suspensions, solid particles are distributed in liquids; and so on. The odorant molecules that are present within the dispersed or continuous phases of such physicochemical systems are not released in the same fashion as if they are simply dissolved.
Complicating matters still further is the fact that in emulsions, for example, dispersed droplets of oil are held in place by layers of tensioactive molecules. Because of their solubility in both oil and water, these molecules almost necessarily bind with odorants, which then become inaccessible to the olfactory system. To identify the phenomena that determine the release of odorant molecules, Marielle Charles, Élisabeth Guichard, and their colleagues at the Institut National de la Recherche Agronomique and École Nationale Supérieure de Biologie Appliquée à la Nutrition et l’Alimentation stations in Dijon studied salad dressings in collaboration with researchers at Amora-Maille, the well-known producer of mustards and condiments.
Fragrant Salad Dressings
In the vinegar-based sauces studied, the watery phase of these emulsions consisted of a mixture of wine vinegar, lemon juice, and salt. Drops of sunflower oil were then emulsified in it (that is, kept in a dispersed state) thanks to the action of whey proteins. Finally, a mixture of xanthan (a polymer obtained by microbial fermentation of glucose) and starch was incorporated to stabilize the sauce. To this basic salad dressing the physical chemists added fixed quantities of odorant molecules: allyl isothiocyanate, in the oil phase, which yielded a hint of mustard; and, in the water phase, phenyl-2-ethanol and ethyl hexanoate, which gave rose and fruit notes, respectively. When several emulsions were compared each one was found to contain oil droplets of a particular size. A jury of trained tasters evaluated the sauces, noting the intensity of the various accents—lemon, vinegar, mustard, and so on.
The results of these taste tests were difficult to analyze: Although the acid taste was preponderant, the tasters struggled to describe the other sensations. Their training nonetheless enabled them to perceive that the intensity of the overall odor, as well as that of the taste and odor of egg, the mustard odor, and the butter taste, increased with the size of the oil droplets and, conversely, that the intensity of the citrus fruit odors diminished as the oil droplets got larger.
Migrations of Odorant Molecules
To interpret these observations more precisely, the researchers analyzed the concentrations of volatile molecules in the air above the sauces. They found that the water-soluble components were present in lower concentrations as the oil droplets became smaller and, conversely, that in this case the oil-soluble molecules were more abundant.
What accounts for these differences? Consider first the oil-soluble odorant molecules. When emulsions containing fixed proportions of water and oil are vigorously whisked, the oil droplets become smaller and more numerous. The lipophilic molecules therefore have a shorter distance to cover in order to reach the surface of the droplets. Moreover, because the total surface area of the oil–water interface has increased, the tensioactive molecules form a thinner layer above the surface of the oil droplets, with the result that the lipophilic odorant molecules diffuse more easily outside the droplets. Finally, the extent of the water layer (in which these odorant molecules bind with the thickening agents) that must be traversed before they reach the air above the emulsion is smaller.
For the odorant molecules that are more soluble in water, the effect is opposite: The smaller the oil droplets, the harder it is for these molecules to diffuse in a thinner layer of water, and so they are less readily perceived.
Are these general laws? Because foods contain various kinds of molecules, many studies will be necessary to characterize all the relevant mechanisms and then to elucidate their relative importance. “Life is short,” said Hippocrates, “the art is long, opportunity fleeting, experience misleading, judgment difficult.”