Is There a Sixth Taste? Scientists Discover New Taste Receptor
The so-called quintessential flavors of ancient China, namely sour, sweet, bitter, spicy, and salty, have been unveiled as an imprecise depiction in light of contemporary knowledge. A remarkable sum of approximately 80,000 taste buds grace the human palate, each equipped with distinct taste receptors that discern various flavors and transmit taste signals. Presently, the scientific community has acknowledged sourness, sweetness, bitterness, saltiness, and umami as the quintessential flavors.
Nevertheless, the relentless pursuit of scientific inquiry has perpetuated the exploration into a sixth taste sensation. In 2015, scholars from Purdue University in the United States postulated the existence of a ‘fat taste.’ During their experimentation, numerous research subjects demonstrated a remarkable ability to accurately identify the elusive ‘fat taste’ amidst a plethora of flavors. It exuded a subtle greasiness and a tantalizing aroma, perhaps even bearing hints of fat. In 2016, Lin Juyun, an associate professor of food science and technology at Oregon State University, speculated that the sixth taste might manifest as a ‘starch taste.’ Alas, Lin Juyun’s team encountered disappointment as they failed to discern taste receptors on the tongue with the specific capability to detect the taste of starch. Consequently, the designation of ‘starch taste’ as the sixth taste remains elusive.
March of 2023 witnessed the publication of a study in the esteemed journal Nature Metabolism, unveiling the existence of a novel taste receptor in Drosophila responsible for perceiving alkaline sustenance. Scientists theorize that if this ‘alkaline taste receptor’ is also present on the human tongue, it would substantiate the existence of a sixth taste.
In stark contrast to the relatively unfamiliar ‘alkaline taste,’ the sour taste enjoys familiarity among the masses. In light of acid-base equilibrium, given the presence of taste receptors for sourness, one might ponder whether alkaline taste receptors also grace the human tongue, thus enabling the perception of alkaline flavors.
In their investigation into fruit flies, Zhang Yali’s team at the Monell Chemical Senses Center in the United States endeavored to provide neutral and alkaline sustenance (pH=12) to the flies. The outcome revealed that the fruit flies predominantly favored neutral fare and exhibited minimal interest in alkaline sustenance. This observation indicates the fruit flies’ ability to discern alkalinity and potentially assess the nutritional properties of food; sourness may connote food spoilage, while alkalinity could signify toxicity.
However, a small subset of fruit flies did not actively evade alkaline sustenance. Zhang Yali’s team postulates that these anomalies may be attributed to genetic mutations, resulting in a diminished capacity to perceive alkalinity. From these flies, they isolated a basophilic gene and discovered that it encodes a basophilic chloride channel protein. Successful expression of this basophilic chloride channel protein in the taste receptor neurons of the flies’ labial flap facilitated their avoidance of alkaline fare. Conversely, when the expression of the basophilic chloride channel protein encountered obstacles in a limited number of fruit flies, they exhibited an inability to actively avoid alkaline sustenance.
Through electrophysiological analysis, Zhang Yali’s team further ascertained that basophilic chloride channel proteins can be activated by hydroxide ions. When the concentration of chloride ions within the Drosophila taste receptor neurons surpasses that outside the cells, indicating exposure to an environment with a high pH value or alkaline milieu, chloride ions surge from the interior to the exterior of the cells. Subsequently, these neurons activate taste receptors, which transmit a signal to the brain, prompting aversion towards alkaline sustenance. Remarkably, when researchers stimulated taste receptor neurons with light, the flies lost interest in their customary preference, sucrose. Since sucrose qualifies as an alkaline food, this phenomenon underscores the Drosophila brain’s reception of the ‘avoid alkaline food’ signal, leading to a decisive choice. Conversely, when these neurons remained dormant, the flies exhibited no significant aversion towards alkaline sustenance.
Collectively, this groundbreaking study substantiates the presence of an entirely novel category of taste receptors, namely, the alkaline taste receptors. Admittedly, the confirmation of alkaline taste receptors pertains solely to Drosophila. The inquiry into the existence of alkaline taste receptors in humans and the veracity of a sixth taste sensation necessitates further exploration.
