Chapter 13: Gustatory Senses

Introduction to Gustatory Senses

The chemical senses of taste and smell are vital for our survival and provide immense pleasure through the enjoyment of food. This chapter explores taste and the significant role it plays in our lives. From celebrity chefs to the pleasure of fine dining, taste has become a major industry, showcasing our profound connection to the culinary arts.

Basic Physiology of Taste Receptors

Our journey begins with the taste receptors, which are housed within taste buds located on specialized structures called papillae on the tongue. There are four major types of papillae: filiform, fungiform, foliate, and circumvallate, each with a distinct appearance, location, and function.

Fungiform papillae

These are small, mushroom-shaped structures scattered across the front two-thirds of the tongue, especially near the tip, and each contains a small number of taste buds.

Foliate papillae

These are folded ridges located along the sides of the back of the tongue and contain numerous taste buds, particularly during childhood.

Circumvallate papillae

These are the largest papillae and form a V-shaped row across the back of the tongue. Although there are only about 8–12 circumvallate papillae, each contains hundreds of taste buds, making them an important site for taste perception.

Filiform papillae

These are the most numerous and cover most of the central surface of the tongue, but these do not contain taste buds. Instead, they are covered with keratin, giving the tongue a rough texture that helps grip, manipulate, and move food during chewing and swallowing. In many animals, filiform papillae are extremely rough. A cat's tongue, for example, feels like sandpaper because its filiform papillae form stiff, backward-pointing spines that are used for grooming fur, removing debris, and scraping meat from bones.

Figure 13.1

Diagram of the different types of papillae on the tongue.

"The Tongue" by OpenStax is licensed under CC BY 4.0

Variability in Taste Perception

Supertasters and non-tasters

Taste perception is not the same for everyone. Some individuals experience tastes much more intensely than others, a phenomenon largely influenced by genetics (Kim et al., 2003; Reed et al., 2006). People who are especially sensitive to certain bitter compounds are often called supertasters, whereas non-tasters experience these compounds as only weakly bitter or not bitter at all (Bartoshuk et al., 1994). Most people fall somewhere between these extremes. Supertasters tend to have a higher density of fungiform papillae on the tip and front of the tongue that contain taste buds, although genetic differences in bitter taste receptors are also an important contributor (Bartoshuk et al., 1994; Hayes et al., 2008).

The discovery of individual differences in taste sensitivity began by chance in 1932. Chemist Arthur Fox was synthesizing a compound called phenylthiocarbamide (PTC) when some of the powder accidentally became airborne. A nearby colleague complained that the dust tasted intensely bitter, while Fox himself could taste nothing at all (Fox, 1932). Intrigued by this striking difference, Fox tested PTC on many other people and found that some experienced an intensely bitter taste whereas others detected little or no bitterness. This simple observation led researchers to discover that sensitivity to certain bitter compounds is strongly influenced by inherited genetic variation, providing one of the earliest demonstrations that genes can shape sensory perception (Kim et al., 2003).

Today, researchers usually study this phenomenon using a related compound called 6-n-propylthiouracil (PROP or 6-PROP) rather than PTC. PROP produces a similar bitter taste but is considered more suitable for research because it has a well-established safety record for use in taste testing (Bartoshuk et al., 1994). Participants typically place a paper strip that has been soaked in 6-PROP on their tongue and rate how bitter it tastes. Individuals who perceive PROP as extremely bitter are often classified as supertasters (sometimes needing to run for a drink to wash out the mouth), whereas non-tasters report little or no bitterness. PROP testing remains one of the most widely used methods for studying genetic differences in taste perception (Tepper, 2008).

Why do these differences exist? One possibility is that heightened sensitivity to bitter compounds evolved as a protective mechanism. Many naturally occurring plant toxins taste bitter, so individuals who were more sensitive to bitterness may have been less likely to consume poisonous plants. However, this heightened sensitivity also has costs. Supertasters may find many nutritious vegetables, such as broccoli, Brussels sprouts, kale, and cabbage, unusually bitter because these foods contain naturally occurring bitter compounds. As a result, genetic differences in taste sensitivity may influence food preferences and dietary choices (Drewnowski & Rock, 1995; Tepper, 2008).

Taste perception also changes across the lifespan. Children are generally more sensitive to bitter tastes than adults, which may help protect them from ingesting harmful substances before they learn which foods are safe (Mennella et al., 2005). Sensitivity to taste, including bitterness, gradually declines with age as taste buds become less responsive and other sensory changes occur (Mojet et al., 2001). Consequently, older adults often prefer foods with stronger flavors or more seasoning than they did when they were younger.

Although biology contributes to food preferences, it is only part of the story. Culture, repeated exposure, family traditions, and personal experiences play an even larger role in determining what people enjoy eating (Pliner, 1982; Birch, 1999). Foods that initially taste unpleasant often become liked through repeated exposure, and cuisines around the world demonstrate that people can learn to appreciate foods with strong bitter, sour, spicy, or fermented flavors. Thus, our food preferences arise from a complex interaction between genetic differences in taste perception and lifelong learning and cultural experience.

Counting the number of fungiform papillae

One of the simplest ways to demonstrate individual differences in taste anatomy was popularized by sensory scientist Linda Bartoshuk (Bartoshuk et al., 1994). In this procedure, the tongue is painted with blue food coloring, which stains most of the surface of the tongue blue but leaves the fungiform papillae relatively unstained because their thicker surface does not absorb the dye as readily. As a result, the fungiform papillae appear as small pink dots standing out against a blue background. A small hole (typically about 6 mm in diameter) is cut in a piece of wax paper or a reinforcement label and placed on the tip of the tongue. Students or researchers then count the number of fungiform papillae visible within this circle. The counts vary widely from person to person, reflecting natural anatomical differences. Individuals with a higher density of fungiform papillae are, on average, more likely to experience PROP as intensely bitter and to perceive many tastes more strongly, although papilla density is only one factor influencing taste sensitivity. Genetic differences in bitter taste receptors also contribute substantially to these individual differences.

Taste Pathway to the Brain

To understand how taste is processed, we can follow the pathway from the tongue to the brain. Taste information from the anterior two-thirds of the tongue is carried by the chorda tympani branch of the facial nerve, while the posterior one-third is served by the glossopharyngeal nerve. A smaller amount of taste information from the throat and epiglottis is carried by the vagus nerve. These nerves project to the nucleus of the solitary tract (NST) in the brainstem, where taste signals are first processed. Research suggests that the NST exhibits a chemotopic organization, meaning that neurons responding most strongly to different taste qualities (such as sweet, salty, etc.) tend to be located in different regions (Chen et al., 2011; Yarmolinsky et al., 2009). This organization is conceptually similar to the sensory maps encountered in other systems. In the visual system, neighboring locations on the retina are represented in neighboring regions of the cortex (retinotopic maps); in the auditory system, neurons are organized according to the sound frequencies to which they respond (tonotopic maps); and in the somatosensory system, adjacent body regions are represented in adjacent cortical locations (somatotopic maps). In the gustatory system, the organizing principle is chemical quality rather than physical location or sound frequency. However, unlike these other sensory maps, the chemotopic organization of taste appears to be relatively coarse, with considerable overlap between taste qualities (Carleton et al., 2010; Yarmolinsky et al., 2009). From the NST, taste information is relayed to the thalamus before reaching the primary gustatory cortex, located primarily in the anterior insula and frontal operculum (Small, 2010; Rolls, 2015). Taste information is then transmitted to the orbitofrontal cortex, where it is integrated with information from smell, texture, temperature, and vision to create the rich perception we recognize as flavor (Rolls, 2015; Small & Prescott, 2005). The orbitofrontal cortex also contributes to evaluating the reward value and pleasantness of foods, helping determine whether we find a particular food appealing. Although we often use the terms taste and flavor interchangeably, they are not the same. Taste refers only to the sensations produced by taste receptors (sweet, sour, salty, bitter, umami, and possibly fat), whereas flavor is a multisensory experience that depends heavily on smell, as well as texture, and temperature (Small & Prescott, 2005; Shepherd, 2012).

Figure 13.2

Diagram of the brain with the orbitofrontal cortex highlighted

"Neural systems proposed to process emotion" by Barger N, Hanson KL, Teffer K, Schenker-Ahmed NM and Semendeferi K is licensed under CC BY 3.0

The Five Basic Tastes and Beyond

Traditionally, humans have been thought to possess five basic taste qualities: sweet, sour, salty, bitter, and umami (Chandrashekar et al., 2006; Yarmolinsky et al., 2009). Each represents a distinct sensory modality produced by the activation of specialized taste receptor cells by particular classes of chemicals. Sweet signals the presence of sugars and other energy-rich compounds, salty detects sodium and other mineral salts, sour responds primarily to acids, bitter serves as a warning signal for many potentially harmful compounds, and umami, which many describe as a savory, meaty, or brothy, signals the presence of protein-rich foods (Lindemann, 2001; Yarmolinsky et al., 2009). Together, these five taste qualities provide important information about the nutritional value and potential safety of what we eat (Breslin & Spector, 2008). Over the years researchers have debated whether additional basic tastes should be recognized. One leading candidate is metallic, the distinctive sensation produced by metals such as iron and copper or by certain medications (Riera et al., 2007).

Influences on Flavor Perception

Flavor perception is not limited to taste alone. It is significantly influenced by olfaction, as smell and taste work together to create the rich tapestry of flavors we enjoy(Small & Prescott, 2005; Shepherd, 2012). Additionally, factors like texture, appearance, and recent food experiences contribute to how we perceive flavor, with the orbital frontal cortex playing a crucial role in integrating these sensory inputs (Rolls, 2015; Spence, 2015).

Modifiers of Taste Perception

Certain substances can dramatically alter taste perception by temporarily changing how taste receptors respond to food. Two of the best-known examples are miracle berries (Synsepalum dulcificum) and Gymnema sylvestre. Both are readily available from online retailers and specialty food stores, making them fun to try at home (or in a Sensation and Perception lab).

Miracle berries

Miracle berries contain a protein called miraculin, which binds to sweet taste receptors on the tongue. Under normal conditions, miraculin does not activate these receptors, so the berry itself tastes only mildly sweet. However, when an acidic (sour) food is eaten, the low pH changes the shape of the bound miraculin, causing it to activate the sweet receptors (Kurihara & Beidler, 1968; Theerasilp & Kurihara, 1988). As a result, sour foods like lemons, limes, grapefruit, or vinegar taste both sour and sweet. For example, biting into a lemon after eating a miracle berry often produces a flavor resembling sweet lemonade. Importantly, the sour receptors are still responding normally—the perception of sweetness is simply added because the sweet receptors are now activated. The effect typically lasts around 20-30 minutes and will wear off as you eat sour foods.

Gymnema sylvestre

Gymnema sylvestre is a plant native to India that produces almost the opposite effect. Compounds in its leaves temporarily bind to and block the sweet taste receptors on the tongue, preventing sugars from activating them (Kurihara, 1969; Sanematsu et al., 2014). As a result, foods that normally taste sweet become surprisingly bland. If the tongue is thoroughly coated with a strong Gymnema tea, even a spoonful of sugar loses all of its sweetness. Instead of tasting sweet, the sugar crystals simply feel like gritty grains that gradually dissolve in the mouth. Depending on the preparation, this effect may also last for roughly 30 minutes.

These natural taste modifiers provide striking demonstrations that taste perception depends not only on the chemicals present in food, but also on the way those chemicals interact with receptors on the tongue. By selectively enhancing or blocking receptor activity, miracle berries and Gymnema sylvestre reveal that our perception of taste is affected by activity at the very first stage of sensory processing.

Figure 13.3

Picture of "Miracle berries".

"Synsepalum Dulcificum - Miracle Fruit Farm" by Miracle Fruit Farm is licensed under CC BY-SA 3.0

Figure 13.4

Picture of Gymnema sylvestre "Gymnema sylvestre leaves and flowers" by Vinayaraj is licensed under CC BY-SA 3.0

Conclusions

In this chapter, we discovered that taste is far more than the simple detection of sweet, sour, salty, bitter, and umami. It is a complex sensory system that begins with specialized receptors on the tongue and culminates in the brain's integration of taste with smell, texture, temperature, and other sensory cues to create the rich experience of flavor. Individual differences in genetics, age, and experience shape how we perceive foods, while substances such as miracle berries and Gymnema sylvestre illustrate how easily taste perception can be modified by altering receptor activity. Together, these findings demonstrate that gustation is not only essential for guiding nutrition and avoiding harmful substances but also provides a fascinating example of how sensory systems transform chemical signals into meaningful and highly personal perceptual experiences.

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