The environment of aromas affects our perception of them.
A GLASS OF VINEGAR IS UNDRINKABLE, but it becomes palatable if one adds a large amount of sugar to it. Yet the pH—the acidity of the vinegar measured in terms of the concentration of hydrogen atoms—is unchanged. Why is the sensation of acidity weakened? Because the perception of tastes depends on the environment in which taste receptors operate. The same interactions take place in the vicinity of the olfactory receptors. Chantal Castelain and her colleagues at the Institut National de la Recherche Agronomique station in Nantes sought to identify these synergies in order to lay the basis for a rational theory of the aromatization of foods.
Most foods consist of water and fats, which are insoluble in water. When foods are put in the mouth, the taste molecules reach the taste receptors after having first diffused through the saliva (a watery solution). Simultaneously the odorant molecules pass into a vapor phase, in the air contained in the mouth, and from there migrate toward the olfactory receptors in the nose. This simple description is complicated by the fact that taste and odorant molecules are never completely soluble or insoluble in water and that, depending on their chemical composition, they are variably distributed in liquids and air. It is clear, then, that the aromatization of foods can be mastered only if the diffusion of molecules between liquid phases (either water or oil) and gaseous phases is understood.
To analyze these various migrations, the Nantes biochemists constructed a device in which a neutral oil can be placed in contact with water and one can measure the distribution of molecules among four compartments: water, oil, the air above the water, and the air above the oil. They began by dissolving the molecules, in either the water or the oil, and then measured their concentration in the three other compartments. The aromatic molecules were found to follow different trajectories depending on experimental conditions. When dissolved initially in oil, for example, they pass into the air above the oil while diffusing in the water and passing from there into the air above.
Several molecular mixtures were tested: esters, aldehydes, alcohols, and ketones. Paradoxically, transfer was observed to be more rapid from oil to water than from water to oil in the case of the esters and ketones but more rapid from water to oil than from oil to water in the case of the alcohols and aldehydes. Because many alcohols are soluble in water, for example, one would have expected them to have trouble migrating toward oil, in which case the transfer ought to be slower from water to oil than from oil to water.
In the Nose
To better understand how foods are aromatized, these studies must be supplemented by an analysis of the effects of odorant molecules on the receptors in the nose once they have penetrated the mucus layer covering these receptors and dissolved in the hydrophobic phases of the cellular membranes. The Nantes researchers are investigating the perception of molecules above the water and oil phases. Having dissolved a single molecule in two liquids—oil and water—in concentrations such that a sensor registers the same partial pressure of the molecule above each liquid, they analyzed the perceptions of people who breathed the air above the two.
Paradoxically, again, the human nose sometimes perceives a difference. For certain molecules, the nose registers a very similar result to that of the detectors (in the case of linalol, for example, which has an odor of lavender and bergamot, depending on the concentration), but no general law has been deduced because other odorants such as 1-octane 3-ol (which has a smell of mossy decomposing undergrowth and mushroom), benzaldehyde (a smell of almond), and acetophenone (a smell of beeswax) give rise to different perceptions. Obviously the presence of water vapor can affect the perception of the aroma.
Such phenomena are likely to be found in connection with taste as well. The texture of foods determines flavors by affecting the length of time during which taste and odorant molecules remain in the mouth and the rates of diffusion of these molecules from foods toward the olfactory and gustatory receptors. Thus a mayonnaise that is too acid becomes less so when it is beaten because its firmer texture slows down these transfers.