Sour sensation, the detection of acidity, is one of the five basic tastes. Sourness is an aversive taste in that it is generally regarded as unpleasant, though there is some evidence that sour is less aversive in children compared to adults. It has likely been advantageous through human evolution by facilitating the detection of spoiled foods, as bacterial growth and fermentation produce acidic compounds. The standard substance used for the definition of sour taste is diluted hydrochloric acid (HCl), which produces hydrogen ions (H+) in solution. Many foods contain organic compounds that are weak acids, including citric acid in fruits and acetic acid in vinegar. Sour sensation is specifically the detection of H+, as opposed to the molecules from which the H+ has dissociated.
The cells responsible for the detection of sour have been identified as taste receptor cells (TRCs) that express the PKD2L1 receptor, which is part of the transient receptor potential (TRP) family. While PKD2L1 is a marker of sour-responsive TRCs, it is not conclusively the sour receptor. The mechanism of sour detection has yet to be fully elucidated, and it is still unclear whether the taste is the sensation of extracellular or intracellular H+ levels. The ambiguity stems from the fact that HCl and acetic acid sensation are discordant with their acidity. It has been proposed that since acetic acid can diffuse into cells, it is detected more sensitively despite being a weaker acid.
The detection of carbonation is also mediated through sour TRCs, but is dependent on the presence of carbonic anhydrase 4 (Car4), an extracellular carbonic anhydrase that reversibly converts carbon dioxide (CO2) gas and water (H2O) into bicarbonate (HCO3−) and H+. This receptor is expressed on sour TRCs, suggesting that its activity and the local production of H+ allows carbon dioxide and acidic compounds to be sensed as the same taste.
Only TRCs that express the PKD2L1 sense sourness, and TRCs that detect sourness do not detect other tastes. Different TRCs are responsible for all five of the primary tastes. TRCs are organized into taste buds, which contain 50—150 TRCs each. Taste buds throughout the tongue contain TRCs for all five basic tastes; therefore, there is no topographic taste map. The specificity of the TRC to one taste alone is essential for the way that the different taste sensations are encoded.
Depolarization of the sour TRCs following stimulation of the unidentified receptor transmits a signal to afferent neurons. The signal is relayed by the facial nerve (cranial nerve VII) in the anterior two-thirds of the tongue and glossopharyngeal nerve (cranial nerve IX) in the posterior third of the tongue. Sensory afferents synapse in the rostral portion of the nucleus of the solitary tract in the brainstem and are relayed to the thalamus with projections to the primary gustatory cortex.
Despite this delineation of taste reception, sour receptors may mediate the aversive taste of high-salt solutions. At high concentrations, salt switches from appetitive to aversive, and it has been shown that high salt concentrations may stimulate sour receptors as well as bitter receptors.
Michael S. Harper
Foods and beverages provide calories and nutrients that are essential for humans to survive. Sour foods generally have an aversive taste that is determined by the amount of acidity (pH) they contain. This sidebar focuses on the perception of sour foods and how this perception changes when a natural agonist—miraculin—binds to sweet taste receptors when the pH of the mouth changes.
Miraculin is a glycoprotein dimer that can be isolated from the red berries of the West African plant Richadella dulcifica. These berries are often called Miracle Berry or Magic Fruit. Miraculin on its own has no taste at a neutral pH. However, it can elicit an intense and persistent sensation of sweet when the tongue is exposed to an acidic pH, such as that of a lemon. Thus, miraculin converts sour-tasting foods (e.g., lemons, vinegar) to be perceived as sweet.
1 Miracle Berry/Magic Fruit tablet (this can be purchased online from Amazon.com)
1 small paper cup
2 to 4 tablespoons of soda (such as a cola type)
2 pieces of lemon (cut the lemon into 6 to 8 wedges)
Pour the soda into the paper cup and set it aside. As a control for this experiment, bite into or eat one of the lemon wedges. This should taste sour. Next, drink some of the soda. This should taste sweet. Next, for about 1 minute, chew/dissolve, swish, and coat your mouth with the Miracle Berry/Magic Fruit tablet. You want to ensure that the miraculin binds to your sweet receptors, which are located on the tongue, cheeks, and the roof of the mouth. Gargling the chewed tablet with your saliva will also work. For best results, ensure you take your time with this step. Swallow the chewed Miracle Berry/Magic Fruit tablet with your saliva. You may take a small drink of water (about 1 to 2 tablespoons) to wash down any remaining liquid in your mouth.
Next, bite into or eat the last piece of lemon. This should now taste sweet and not sour! In fact, it should taste like lemonade. It is because the miraculin has bound to sweet taste receptors because of the acidity of the lemon, and everything will now taste sweet. Next, drink some of the soda. For most persons, this should taste even sweeter than before. If the experiment was done right, food will have a sweet(er) perception for about an hour.
Jennifer L. Hellier
See also: Facial Nerve; Glossopharyngeal Nerve; Supertaster; Taste Aversion; Taste Bud; Taste System; Type III Taste Cells
Kataoka, Shinji, Ruibiao Yang, Yoshiro Ishimaru, Hiroaki Matsunami, John C. Kinnamon, & Thomas E. Finger. (2008). The candidate sour taste receptor, PKD2L1, is expressed by type III taste cells in the mouse. Chemical Senses, 33(3), 243—254. http://dx.doi.org/10.1093/chemse/bjm083
McLaughlin, Susan K., & Robert F. Margolskee. (1994). The sense of taste. American Scientist, 82(6), 538—545.