unaffected. In particular, Green and Frankmann (1987, 1988) showed that the perceived sweetness of sucrose (Figure 9-1), fructose, and glucose increased in intensity when the temperature of the solution was increased between 20° and 36°C, but to a degree that was inversely related to the concentration of the taste stimulus; the stronger the taste stimulus, the smaller was the effect of temperature. The same result had been observed earlier for sucrose alone (Bartoshuk et al., 1982; Calvino, 1986). Green and Frankmann (1987) also reported that the bitterness of caffeine grew stronger at warmer temperatures, whereas the sourness of citric acid and the saltiness of NaCl were not significantly altered (Figure 9-2). Overall, therefore, as temperature rises, perceptions of sweetness and bitterness tend to intensify, and perceptions of sourness and saltiness tend to remain the same. Because the effect of temperature is not uniform across compounds, it can be expected that the taste "profile" of a food will change as its temperature changes. If all else is equal, at hot temperatures bitter and sweet tastes should dominate salty and sour ones.
From a practical standpoint, these thermal effects are not particularly large. Although Green and Frankmann (1987) noted changes in the perceived intensity of sweetness as great as 100 percent, these perceptions only occurred when the temperature of the tongue—not just the temperature of the solution—had been changed by 16°C (from 36° to 20°C). Large changes in tongue temperature are difficult to produce under normal circumstances because of the tongue's abundant vascularization. Furthermore, these effects were obtained by cooling the tongue. Although other studies have shown that the trends observed for sweetness and bitterness between 20° and 36°C persist at solution temperatures above normal oral temperature (for example, Bartoshuk et al., 1982; Paulus and Reisch, 1980), the relationship between temperature and perceived intensity at very hot temperatures (for example, greater than 45°C or 113°F) has not been clearly worked out.
The "feel" of a food or beverage, produced by mechanical stimulation and mediated by the tactile sense, is an important but often overlooked aspect of flavor. The perception of food devoid of its tactile properties is difficult to imagine; foods would literally be intangible substances, and flavor would be rendered a disembodied sensory quality. It is consequently of interest to know if the temperature of a food affects its perceived tactile characteristics. Although no data exist that address this issue directly, it has been established that, in a manner similar to taste, the tactile sensitivity of the tongue changes as its temperature changes. In general, the sensitivity to so-called high-frequency vibration (greater than 100 Hz) varies directly with temperature between 20° and 36°C (Green, 1987). In contrast, the sensitiv-