The sense of taste (gustation) is a chemical sense that allows us to evaluate the nutrients and potential dangers in food. Taste begins when dissolved chemical compounds interact with sensory organs in the mouth, initiating a signal to the brain. Current biological science recognizes five basic tastes that form the foundation of our perception of food: sweet, sour, salty, bitter, and umami.
How Taste Perception Works
Taste perception begins on the tongue within small structures called papillae, which house the taste buds. Each taste bud contains 50 to 150 specialized taste receptor cells. For a substance to be tasted, it must first dissolve in saliva and then bind to or enter these receptor cells.
The mechanism of detection differs based on the taste. Salty and sour tastes are detected when their constituent ions pass through or interact with ion channels on the receptor cell membrane. Conversely, sweet, bitter, and umami tastes are detected by complex G-protein coupled receptors (GPCRs) that bind to specific molecular shapes. This binding triggers a cascade of internal signals within the cell, leading to the transmission of a message to the brain.
The idea of a “tongue map,” where different areas are exclusively responsible for specific tastes, is a misconception. All areas of the tongue containing taste buds are capable of detecting all five basic tastes. The receptors for all five tastes are distributed across the entire surface, though minor variations in sensitivity may exist.
The Familiar Trio: Sweet, Sour, and Salty
The three familiar tastes are sweet, sour, and salty, each serving a distinct purpose for survival. Sweet taste is triggered by sugars (like glucose and sucrose), certain proteins, and artificial sweeteners. This sensation evolved to signal the presence of high-calorie, energy-rich foods, rewarding the consumption of ripe fruits and other caloric sources.
Salty taste is the detection of sodium ions (\(\text{Na}^+\)), most commonly from sodium chloride. The attraction to salt remained a necessity for survival as vertebrates moved to land, reflecting the marine environments where early organisms evolved. The ability to perceive salt is tied to the regulation of electrolyte balance and fluid homeostasis in the body.
Sour taste is created by the presence of acids, specifically the concentration of free hydrogen ions (\(\text{H}^+\)). This taste often serves as a warning sign, as high acidity is associated with unripe fruit, spoiled food, or substances that could damage tissue. However, the enjoyment of mild sourness in fermented foods and citrus demonstrates a complex relationship with this signal.
The Essential Duo: Bitter and Umami
The two remaining basic tastes, bitter and umami, possess important roles. Bitter taste is a sensitive aversive signal, acting as a protective mechanism against toxins. A significant number of poisonous compounds, particularly plant alkaloids, are bitter.
Humans possess approximately 25 functional genes for bitter taste receptors (TAS2Rs), reflecting the vast chemical diversity of potential toxins. This system encourages an immediate rejection response, such as spitting out the substance, before it can be fully ingested. While high concentrations are aversive, low concentrations of bitter compounds can be tolerated or enjoyed, such as in coffee or dark chocolate.
Umami, often described as savory or meaty, was identified by Kikunae Ikeda in Japan in 1908. This taste is triggered by the amino acid L-glutamate, as well as certain nucleotides like inosinate and guanylate. Umami signals the presence of protein and amino acids, which are the building blocks for the body’s tissues.
Sensing umami encourages the secretion of saliva and digestive juices, aiding in protein digestion. The taste is commonly found in foods where protein has been broken down, such as aged cheeses, cured meats, fermented products like soy sauce, tomatoes, and mushrooms. The presence of umami in breast milk suggests it is one of the first tastes humans experience.
Taste Versus Flavor: The Role of Olfaction
It is important to distinguish between the five basic tastes and the overall perception of flavor. Taste is limited to the five sensations detected on the tongue, while flavor is the comprehensive sensory experience created by the brain. Olfaction (the sense of smell) plays a dominant role in constructing flavor, contributing the nuanced notes that distinguish one food from another.
When food is chewed, volatile chemical molecules are released and travel through a passage at the back of the throat into the nasal cavity (retro-nasal olfaction). These molecules stimulate the olfactory receptors. The resulting signal is integrated with the basic taste signals from the tongue to create a unified perception of flavor. This explains why food seems bland or tasteless when a person has a cold, as congestion blocks the retro-nasal pathway, severely limiting the olfactory component of flavor.