Aftertaste

Aftertaste is the taste intensity of a food or beverage that is perceived immediately after that food or beverage is removed from the mouth. The aftertastes of different foods and beverages can vary by intensity and over time, but the unifying feature of aftertaste is that it is perceived after a food or beverage is either swallowed or spat out. The neurobiological mechanisms of taste (and aftertaste) signal transduction from the taste receptors in the mouth to the brain have not yet been fully understood. However, the primary taste processing area located in the insula has been observed to be involved in aftertaste perception. Aftertaste is the taste intensity of a food or beverage that is perceived immediately after that food or beverage is removed from the mouth. The aftertastes of different foods and beverages can vary by intensity and over time, but the unifying feature of aftertaste is that it is perceived after a food or beverage is either swallowed or spat out. The neurobiological mechanisms of taste (and aftertaste) signal transduction from the taste receptors in the mouth to the brain have not yet been fully understood. However, the primary taste processing area located in the insula has been observed to be involved in aftertaste perception. Characteristics of a food's aftertaste are quality, intensity, and duration. Quality describes the actual taste of a food and intensity conveys the magnitude of that taste. Duration describes how long a food's aftertaste sensation lasts. Foods that have lingering aftertastes typically have long sensation durations. Because taste perception is unique to every person, descriptors for taste quality and intensity have been standardized, particularly for use in scientific studies. For taste quality, foods can be described by the commonly used terms 'sweet', 'sour', 'salty', 'bitter', 'umami', or 'no taste'. Description of aftertaste perception relies heavily upon the use of these words to convey the taste that is being sensed after a food has been removed from the mouth. The description of taste intensity is also subject to variability among individuals. Variations of the Borg Category Ratio Scale or other similar metrics are often used to assess the intensities of foods. The scales typically have categories that range from either zero or one through ten (or sometimes beyond ten) that describe the taste intensity of a food. A score of zero or one would correspond to unnoticeable or weak taste intensities, while a higher score would correspond to moderate or strong taste intensities. It is the prolonged moderate or strong taste intensities that persist even after a food is no longer present in the mouth that describe aftertaste sensation. Foods that have distinct aftertastes are distinguished by their temporal profiles, or how long their tastes are perceived during and after consumption. A sample testing procedure to measure a food's temporal profile would entail first recording the time of onset for initial taste perception when the food is consumed, and then recording the time at which there is no longer any perceived taste. The difference between these two values yields the total time of taste perception. Match this with intensity assessments over the same time interval and a representation of the food's taste intensity over time can be obtained. With respect to aftertaste, this type of testing would have to measure the onset of taste perception from the point after which the food was removed from the mouth. The categorization of people into 'tasters' or 'nontasters' based on their sensitivity to the bitterness of propylthiouracil and the expression of fungiform papillae on their tongues has suggested that the variations from person-to-person observed in taste perception are genetically based. If so, then sensations of aftertaste could also be affected by the activities of specific genes that affect an individual's perception of different foods. For example, the intensity of the aftertaste sensations 'nontasters' experienced after caffeine consumption was found to diminish faster than the sensations 'tasters' experienced. This may imply that because of their taste bud profiles, 'tasters' may be more sensitive to the tastes of different foods, and thus experience a more persistent sensation of those foods' tastes. Because a lingering taste sensation is intrinsic to aftertaste, the molecular mechanisms that underlie aftertaste are presumed to be linked to either the continued or delayed activation of receptors and signaling pathways in the mouth that are involved in taste processing. The current understanding of how a food's taste is communicated to the brain is as follows: In the context of aftertaste, the combination of both receptor-dependent and receptor-independent processes have been proposed to explain the signal transduction mechanisms for foods with distinct aftertastes, particularly those that are bitter. The receptor-dependent process is the same as what was described above. However, the receptor-independent process involves the diffusion of bitter, amphiphilic chemicals like quinine across the taste receptor cell membranes. Once inside the taste receptor cell, these compounds have been observed to activate intracellular G-proteins and other proteins that are involved in signaling pathways routed to the brain. The bitter compounds thus activate both the taste receptors on the cell surface, as well as the signaling pathway proteins in the intracellular space. Intracellular signaling may be slower than taste cell receptor activation since more time is necessary for the bitter compounds to diffuse across the cell membrane and interact with intracellular proteins. This delayed activation of intracellular signaling proteins in response to the bitter compounds, in addition to the extracellular receptor signaling is proposed to be related to the lingering aftertaste associated with bitter foods. The combination of both mechanisms leads to an overall longer response of the taste receptor cells to the bitter foods, and aftertaste perception subsequently occurs. The primary taste perception areas in the cerebral cortex are located in the insula and regions of the somatosensory cortex; the nucleus of the solitary tract located in the brainstem also plays a major role in taste perception. These regions were identified when human subjects were exposed to a taste stimulus and their cerebral blood flow measured with magnetic resonance imaging. Although these regions have been identified as the primary zones for taste processing in the brain, other cortical areas are also activated during eating, as other sensory inputs are being signaled to the cortex.