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and bitter; many Japanese researchers also argue that the taste of monosodium glutamate (MSG), which they refer to as umami, is unique and "basic" (for example, Nakamura and Kurihara, 1991; Rogers and Blundell, 1990). All other sensations associated with the ingestion of foods derive from other sensory systems that innervate the oral and nasal cavities. In particular, the qualities that we frequently use to describe how something tastes—such as chocolate, vanilla, strawberry, and orange—are actually odors detected retronasally via the opening between the oropharynx and the nasal cavity. Qualities such as creaminess and crunchiness derive from mechanical stimulation and thus are mediated by the sense of touch. The "burn" or ''heat" of chili pepper, mustard, alcohol, and other irritants is mediated at least in part by the pain and thermal senses (Green, 1991; Green and Lawless, 1991). Thus, the term taste should be reserved for the limited range of gustatory sensations, and the term flavor should be used to describe the totality of oral sensations—taste, smell, touch, temperature, and chemical irritation (pain)—that accompany eating.

To evaluate thermal effects on flavor therefore requires more than merely measuring the modulation of sweet, sour, salty, and bitter tastes under conditions of changing stimulus temperature. The present chapter reviews the current literature on thermal effects in all four of the above-mentioned modalities and suggests future research.

SENSORY EFFECTS OF TEMPERATURE

Taste

The effect of temperature on the perception of taste has been studied scientifically for over a century (for review see Green and Frankmann, 1987; Pangborn et al., 1970). However, because temperature-taste effects were usually measured in piecemeal fashion (that is, testing only one or two taste stimuli at a time) in different laboratories using different experimental methods, few generalizations could be gleaned from the early experiments. The only reliable finding seemed to be that the threshold for detecting the four basic tastes tended to vary in a U-shaped manner as a function of temperature, having a minimum somewhere in the range between 20° and 30°C. The temperature at which the minimum occurred varied across taste stimuli (see McBurney et al., 1973 for example), which means that, in general, when foods or beverages are heated to temperatures above 30°C (about 86°F), detecting weak tastes becomes more difficult.

Interestingly, the pattern of thermal effects at threshold does not extend to suprathreshold concentrations, when tastes are unambiguously present. At these higher concentrations, the perception of some taste stimuli continues to be affected by temperature while the perception of others is relatively



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