sourness

Sourness is a basic taste distinct from bitterness and is produced by acids. This entry distinguishes the taste component from somatosensory responses or avoidance behavior that may also occur at high concentrations. A mouse study examined OTOP1, a protein that passes acid-derived hydrogen ions into taste cells. In mice genetically altered so OTOP1 did not function, electrical responses of taste cells to acid disappeared and taste signals sent toward the brain were greatly reduced.

Even in mice without functional OTOP1, however, avoidance of concentrated acids did not disappear. The persistence of avoidance despite reduced taste signaling suggests that mechanisms other than sour-taste perception, such as pain-like stimulation, may also contribute to responses to acids. These findings were obtained in mice and cannot be directly applied to how humans perceive sourness or how sourness functions in cooking.

In a taste-mixture experiment presenting sucrose, sodium chloride, citric acid, and quinine sulfate (QSO4) at approximately equal perceived intensities, mixing all 4 components suppressed citric-acid sourness by up to 83.2% and QSO4 bitterness by up to 86.2%, while sucrose sweetness was suppressed by no more than 42.7%. In the binary mixture of citric acid and QSO4, the dominant-component hypothesis—predicting overall intensity equal to that of QSO4 alone—underestimated the measured intensity. A model adding the perceived intensities of sourness and bitterness was reported to fit better.

Sourness can also suppress a drug’s bitterness. In a test of orally disintegrating tablets containing olopatadine hydrochloride and 0–10% citric acid, the VAS bitterness score during disintegration declined from 47 with no citric acid as its concentration rose. In the post-spit evaluation, all citric-acid tablets (0.1–10%) scored below the no-acid tablet (40), with 2.5% giving the lowest score.