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Sourness is the taste that detects acidity. The sourness of substances is rated relative to dilute hydrochloric acid, which has a sourness index of 1. By comparison, tartaric acid has a sourness index of 0.7, citric acid an index of 0.46, and carbonic acid an index of 0.06.[23][24]
Sour taste is detected by a small subset of cells that are distributed across all taste buds in the tongue. Sour taste cells can be identified by expression of the protein PKD2L1,[25] although this gene is not required for sour responses. There is evidence that the protons that are abundant in sour substances can directly enter the sour taste cells through apically located ion channels.[26] This transfer of positive charge into the cell can itself trigger an electrical response. It has also been proposed that weak acids such as acetic acid, which is not fully dissociated at physiological pH values, can penetrate taste cells and thereby elicit an electrical response. According to this mechanism, intracellular hydrogen ions inhibit potassium channels, which normally function to hyperpolarize the cell. By a combination of direct intake of hydrogen ions (which itself depolarizes the cell) and the inhibition of the hyperpolarizing channel, sourness causes the taste cell to fire action potentials and release neurotransmitter.[27]
The most common food group that contains naturally sour foods is fruit, such as lemon, grape, orange, tamarind, and sometimes melon. Wine also usually has a sour tinge to its flavor, and if not kept correctly, milk can spoil and develop a sour taste. Children in the US and UK show a greater enjoyment of sour flavors than adults,[28] and sour candy is popular in North America[29] including Cry Babies, Warheads, Lemon drops, Shock Tarts and sour versions of Skittles and Starburst. Many of these candies contain citric acid or malic acid.

Sour Taste in Mammals
Sour is detected by a subset of taste receptor cells in the tongue and palate epithelium that respond to acidic pH and weak organic acids with electrical activity (Huang et al., 2006, Huang et al., 2008). Over the years, a number of candidates for sour receptors or components of sour signaling have been proposed, including ASICs, HCNs, K+ channels, and most recently the TRP channels PKD2L1 and PKD1L3 (Roper, 2007). However, presently there is no evidence that any of these proteins mediate sour taste, and knockouts of mouse PKD2L1 or PKD1L3 only slightly attenuate nerve responses to acid stimulation (Horio et al., 2011). Nonetheless, PKD2L1-expressing cells respond to, and are required for, sensory response to acids (Chang et al., 2010, Huang et al., 2006, Oka et al., 2013). The response of PKD2L1-expressing cells to acids is mediated by an unusual proton-selective ion channel (Chang et al., 2010). Proton selectivity allows the cells to respond to acids without interference from Na+, which may vary independently in concentration. The molecular identity of the proton channel is presently unknown.

Entry of protons into sour cells produces cellular acidification, which may affect cell signaling. Notably, taste cells express several resting two-pore K+ channels (Lin et al., 2004, Richter et al., 2004), which may be blocked by intracellular acidification to produce further depolarization of the cell (Figure 3). The idea that intracellular acidification activates sour cells is attractive, as it could explain why, at the same pH, acetic acid and other weak acids that penetrate the cell membrane taste more sour than strong acids, such as HCl, that do not penetrate the cell membrane (Lyall et al., 2001, Roper, 2007).

In addition to sour stimuli, PKD2L1-expressing cells are required for the gustatory response to carbonation (CO2). This response is dependent on a membrane-anchored carbonic anhydrase isoform 4, Car4 (Chandrashekar et al., 2009), which interconverts CO2 + H2O to H+ + HCO3−. The mechanism by which Car4 contributes to the activation of sour cells is not known. One possibility is that protons generated apically by this enzymatic reaction enter through the proton channel to depolarize the cell.

It should be noted that the response to acids and carbonation is complicated by the fact that the trigeminal system, which heavily innervates the mouth and oral cavity, is also sensitive to these stimuli (Bryant and Silver, 2000). TRPA1 is expressed by nociceptors and can be activated by CO2 and acetic acid (Wang et al., 2010), and the capsaicin receptor, TRPV1, is activated by extracellular protons (Tominaga et al., 1998). Moreover, afferent nerve fibers that innervate the oral cavity retain sensitivity to acids in otherwise taste-blind mice (Ohkuri et al., 2012). Thus, somatosensory afferents undoubtedly contribute to the burning sensation experienced when ingesting sodas and organic acids.

Taste of Sour and Carbonation in Drosophila
Fruit flies prefer slightly acidic foods, such as carbonated water, while they reject foods that are too acidic. Carbonated water triggers Ca2+ influx in the region of the SOG innervated by taste peg GRNs, suggesting that these neurons are involved in CO2 detection (Fischler et al., 2007). Fruit flies avoid many carboxylic acids with a low pH. Behavioral and physiological analysis reveals that the avoidance to carboxylic acid is mainly mediated by a subset of bitter GRNs (Charlu et al., 2013). In addition, acids also inhibit the activity of sugar GRNs. However, the molecular identities of the taste sensors for both weak carboxylic acids and strong metallic acids are unknown. Nevertheless, since only a subset of bitter GRNs sense acids, this localized expression pattern may contribute to the discrimination of sour versus bitter tastants (Charlu et al., 2013).

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