Taste and Its Function
The sense of taste is one of the traditional five senses (sight, hearing, taste, smell and touch) and refers to the sensation derived when non-volatile chemical molecules stimulate receptors sited on taste cells in the surface areas of the tongue, soft palate and the oropharyngeal region [18]. Taste is stimulated through the activation of taste receptor cells (TRC) found in the surface regions throughout the oral cavity [18]. Once the TRC are activated, electrical impulses are transmitted via the sensory afferent fibres to the brain areas involved in the cortical processing of taste, and a taste perception associated with the chemical will be experienced [19]. TRC are housed within the taste buds, which are distributed across three different types of tongue papillae (i.e., circumvallate, foliate and fungiform papillae) [18].
The human taste system is now widely accepted to include five taste qualities: sweet, salty, bitter, sour and umami [20]. From an evolutionary perspective, it is postulated that the human taste system functions as a gatekeeper of the digestive system to ensure that we consume essential nutrients for survival and functioning, while rejecting potentially harmful or toxic foods [20]. For example, a salty taste quality signals the presence of either sodium or minerals; umami indicates the presence of proteins; excessive sour taste signals spoiled food; bitter taste quality often indicates the presence of poisons; and sweet taste indicates the presence of carbohydrates or energy in the food [20,21]. However, from an evolutionary standpoint, it is maladaptive to recognize sweet taste from HIS, as the sweeteners do not contain energy for physiological function.
A taste perception for a particular taste quality is experienced when the concentration of that particular solvent in the oral cavity reaches a level that activates a taste receptor [19] (Figure 1). For illustration, when only a small amount of sucrose is diluted in an aqueous solution, an individual may not be able to differentiate the aqueous solution from water [19]. As the concentration of sucrose increases, the aqueous solution can be differentiated from water, and a detection (absolute) threshold is reached [19]. However, at this stage, the individual will not be able to recognize the taste quality. As the concentration increases further, the recognition threshold will be reached when the taste quality is correctly identified (sweet in this case) [19]. As the concentration of sucrose increases even further, the perceived intensity of sweetness jointly increases to a hypothetical asymptote, where an additional increase in the concentration of sucrose no longer causes consequential increases in perceived taste intensity [19,21,22]. This dynamic phase is defined as the suprathreshold intensity perception range