Chewing slowly deepens the perception of odorant molecules in cooked food.
WHICH AROMAS DO WE PERCEIVE when we eat? For a long time this question could not be answered, for chemical analysis was unable by itself to determine the concentrations of odorant molecules in the vicinity of the receptor cells in the nose. Andrew Taylor, Rob Linforth, and their colleagues at the University of Nottingham, working in association with Firmenich (an international perfume and flavor research group), have been conducting experiments since 1996 with a device that shows how aromatic compounds are released during the mastication of food. The same food, it turns out, smells different to different people.
Odorants—volatile molecules that stimulate the nasal receptors in passing upward from the mouth through the rear nasal fossae as food is chewed—are important components of flavor. Nonetheless, their sensory action is difficult to analyze because these molecules interact with saliva and with various other compounds present in foods. Accordingly, the odorant profile of a particular food cannot be reduced to its chemical composition.
Because molecules can be detected by smell only if they pass into the vapor phrase, physiologists have sought to measure the concentration of odorants in the air above foods. But given that the chewing of food, breathing, and salivation all affect the release of aromas, one cannot rely on this measurement alone. To identify the active aromas of a food, it is necessary also to measure the release of odorant molecules while it is being consumed.
The new method of mass spectrometry devised by Taylor and his colleagues directly measures the concentrations of odorant molecules in the breath of subjects as they are chewing food. A stream of gas containing the volatile molecules to be analyzed is pumped into a chamber equipped with an electrically charged needle that ionizes water molecules. The hydrogen ions that are formed in this way then transmit their electrical charge to the odorant molecules, which are attracted by a series of electrically charged plates in a focalization chamber. From here they are channeled into another chamber for analysis.
The Wisdom of Chewing Slowly
The British chemists first examined how a gel composed of gelatin and saccharose releases the volatile components—ethylbutyrate, found in fruits such as strawberries, and ethanol—that are trapped in it. A tube was placed in a nostril of each of the subjects (who were nonetheless able to breathe without difficulty) to capture a sample of the air present in the naval cavity.
The first observation was not surprising. Because molecular concentrations in the air in the nose vary periodically with the rhythm of respiration, what one wants to know, for each breath taken and expelled, is the maximum measured concentration. In the case of acetone, the maximum concentration was the same for each respiratory cycle because this molecule, which is released by the metabolism of fatty acids in the liver, is naturally found in the breath.
By contrast, the ethylbutyrate and ethanol detected in the breath came from the gel alone: The ethylbutyrate was released only during mastication, for about a minute, whereas the ethanol was released for a longer period of time. Because ethanol is soluble in water, it dissolves in saliva after having been released by the rupture of the gel, and only afterward does a part of it pass into the air. This slow exchange between water and air is stimulated by chewing and continues even after chewing is finished.
These studies also confirmed what the makers of chewing gum have long suspected, namely that the release of odorant molecules depends on both the speed of mastication and the softness of the gum. A comparison of the reactions of three people to the same food (a gel made from gelatin containing ethanol, butanol, and hexanol) revealed what might be called odorant inequality: The maximum concentration of molecules in the breath and the time it took for this concentration to appear varied according to the rate of mastication for each person tested. The maximum odorant concentrations were lowest in the case of the most rapid eaters, presumably because they broke up the gel the least. Brillat-Savarin therefore was right to say, “Men who eat quickly and without thought do not perceive the [succession of] taste impressions, which are the exclusive perquisite of a small number of the chosen few; and it is by means of these impressions that gastronomers can classify, in the order of their excellence, the various substances submitted to their approval” (Meditation 2, The Physiology of Taste).