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Sourness is illustrated by this yucky icon for taste sensations that are lemon flavoured.

Any flavor or gustatory perception that could be roughly described as acidic or caustic is called a sour taste sensation. We use words like soapy, tart, corrosive, sharp, astringent, tangy, acerbic, rancid, vitriolic, biting, vinegary etc. to identify these flavors. To make a binary description of a sour sensation compare it to tasting a lemon. Report the result using one of the following algebraic statements. If the two experiences are not comparable, then say that the sensation is not a sour sensation and express this as \delta_{\mathit{H}}=0. If the sensation is like tasting a lemon, then say that it is tart. Write this as \delta_{\mathit{H}}=+1. If the sensation is not like tasting a lemon, then say that it is soapy and that \delta_{\mathit{H}}=-1. The number \delta_{\mathit{H}} is called the sourness. These relations are mathematically expressed by

\delta_{\mathit{H}} \equiv \begin{cases} +1 &{\text{if a sour sensation is tart }} \\ \; \; 0 &{\text{if a  sensation is not sour}} \\ -1 &{\text{if a sour sensation is soapy }} \end{cases}

Reference Constant for Sourness

The numerical constant associated with tasting a lemon has not yet been determined.