For centuries, the human palate was thought to perceive only four basic tastes: sweet, sour, salty, and bitter. In the 20th century, umami—the savory taste from glutamate—was officially recognized, bringing the count to five. Today, researchers are pushing the boundaries of taste science, with strong evidence supporting multiple contenders for the title of the sixth basic taste. This complex discussion involves understanding specific cellular receptors, physiological functions, and the nuanced perception of flavor.
The Newest Candidate: Ammonium Chloride
In late 2023, a study from the USC Dornsife College of Letters, Arts and Sciences, published in Nature Communications, provided compelling new evidence for ammonium chloride as a basic taste. While known for its bitter, salty, and somewhat sour notes, researchers identified a specific protein channel, OTOP1, that is activated by ammonium chloride.
The Science Behind the Sensation
- Receptor: The OTOP1 protein channel, which also plays a role in detecting sour tastes, is directly responsible for perceiving ammonium chloride.
- Survival Instinct: Scientists hypothesize that the ability to taste and avoid ammonium—a substance present in waste products and decaying matter—is an evolutionary survival mechanism to help organisms avoid harmful substances.
- Human Perception: Although its consumption is typically aversive, the taste is part of some regional delicacies, such as Scandinavian salty licorice, proving that cultural factors can influence the palatability of even inherently unpleasant tastes.
The Long-Standing Contender: Oleogustus (Fat)
Before the ammonium chloride discovery, fat was a leading candidate for the sixth taste. While fat has long been appreciated for its textural qualities, research from Purdue University in 2015 identified and named its distinct taste component, oleogustus.
The Mechanisms of Fat Perception
- Distinguishing from Texture: Scientists confirmed that oleogustus is a true taste sensation, perceived independently of the mouthfeel or aroma of fat. It involves detecting free fatty acids at low concentrations.
- The Receptors: The mechanisms for detecting fat taste are still being fully elucidated but are believed to involve receptors such as CD36 and GPCR120, which are found on taste bud cells.
- Health Implications: An individual's sensitivity to fat taste has been linked to dietary behaviors and BMI, suggesting a physiological connection between fat taste and obesity. People less sensitive to oleogustus may overconsume fatty foods.
The Flavor Enhancer: Kokumi
Another contender, though not a basic taste in the traditional sense, is kokumi. Discovered by Japanese scientists in the 1980s, the term means 'rich taste' or 'mouthfulness' in Japanese. Kokumi substances are often tasteless on their own but enhance and prolong the flavor of other tastes, such as sweet, salty, and umami.
How Kokumi Works
- Taste Modifier: Kokumi acts as a flavor amplifier, adding a sense of richness, continuity, and complexity to a dish. It is often described as creating a 'full-bodied' sensation.
- Calcium-Sensing Receptor: Kokumi compounds, like gamma-glutamyl peptides found in fermented and aged foods, activate the calcium-sensing receptor (CaSR). This receptor enhances the signals of other tastes without having a distinct taste of its own.
- Culinary Applications: Kokumi-rich ingredients, such as aged cheese, garlic, miso, and yeast extracts, are used to create deeper, more satisfying flavor profiles, including in low-salt or low-fat dishes.
The Role of Multisensory Perception
The perception of taste is not an isolated function but a complex interplay involving the tongue, nose, and brain. Other senses like olfaction (smell), temperature, and touch combine with basic tastes to create the full experience of flavor. The identification of new taste components like ammonium chloride, oleogustus, and kokumi adds to this complexity, providing new targets for understanding food preference and human health. This article by the Journal of Biological Chemistry explains the role of the calcium-sensing receptor in human taste perception.
Candidate Comparison Table: 6th Taste Contenders
| Feature | Ammonium Chloride | Oleogustus (Fat) | Kokumi |
|---|---|---|---|
| Primary Sensation | Distinct, often aversive (salty, bitter, sour) | Lingering, fatty (not textural) | Mouthfeel enhancer (rich, full-bodied) |
| Key Receptor | OTOP1 protein channel | CD36, GPR120, GPR40 | Calcium-sensing receptor (CaSR) |
| Typical Source | Waste products, decaying matter, salty licorice | Free fatty acids in foods | Aged cheeses, yeast extracts, garlic |
| Evolutionary Role | Survival mechanism to avoid toxic substances | Indicator of energy-dense foods | Potentially signals nutritious, fermented foods |
| Status as 'Taste' | Strong recent evidence, but still under review | Well-supported as a distinct perception | Functions as a taste enhancer, not a basic taste itself |
Conclusion
While the five basic tastes of sweet, sour, salty, bitter, and umami remain the standard, the question of what is the 6th type of taste is far from settled. The recent discovery of a specific receptor for ammonium chloride provides powerful support for its inclusion. At the same time, the solid scientific backing for oleogustus (fat) and the unique flavor-amplifying properties of kokumi present their own compelling cases. Ultimately, the 'sixth taste' may not be a single sensation but rather a new category of perception that fundamentally changes how we understand and appreciate the food we eat.
