Taste physiology and perception

We now jump from smell to taste. On the human tongue lie about 10,000 taste buds, which each contains a group of about $50$ to $150$ taste receptors [300]. The receptors live for an average of $10$ days, with regeneration constantly occurring. Five basic types of taste receptors have been identified:

• Umami: This one is sensitive to amino acids, such as monosodium glutamate (MSG), and is responsible for an overall sense of tastiness. This enables food manufacturers to cheaply add chemicals that made food seem to taste better. The biological motivation is likely to be that amino acids are important building blocks for proteins.
• Sweet: This is useful for identifying a food source in terms of its valuable sugar content.
• Salty: This is useful for determining whether a food source has sufficient salt content, which is required for normal neural functions.
• Sour: This is useful for determining the amount of acidity in a food, which could imply useful vitamins, unripeness, or even bacteria in spoiled food.
• Bitter: This is often associated with toxic plants, which may trigger a natural aversion to them.

All of these work by dissolving food and generating a response based on chemical decomposition. The sensory pathways connect to through the thalamus to the gustatory cortex and to the amygdala, which affects emotional responses.

Taste perception is closely related to the taste receptor types. One of the most widely known models is Henning's tetrahedron from 1927, which is a 3D space of tastes that is generated using barycentric coordinates (Section 7.2) over four extreme vertices that each represent pure sweet, salty, sour, or bitter. Thus, each taste is a linear interpolation the four components. This of course, neglects umami, which was added to the list of receptor types very recently [43,232]. Adaptation occurs for taste, including an aversion to foods that might have been coincident with sickness. The concept of flavor is a perceptual experience that combines cues from taste, smell, temperature, touch, vision, and sound. Therefore, it is challenging to understand the mechanisms that create a flavor experience [58].