Scientists have discovered what exactly leaves a sour taste in the mouth.
The tongue has a complex network of sensors to detect taste
A US team have found a protein in taste receptor cells on the tongue that detects sourness - warning mammals the food might be spoiled or unripe.
The protein is also present in spinal fluid, and the researchers suggest it might be useful for detecting acidity in other parts of the body.
The research is published in the journal Nature.
The researchers from Howard Hughes Medical Institute and the University of California San Diego found that the protein, PKD2L1, was the key.
Tests showed that mice who were genetically engineered to lack these cells cannot respond to sour-tasting stimuli, such as citric acid.
Responses to other tastes were unaffected.
The team has already identified cell types that mediate sweet, bitter and savoury tastes.
The PKD2L1 receptor is also active in certain neurons in the spinal cord and may, the authors propose, be involved in detecting the acidity of cerebrospinal fluid, suggesting a common mechanism for detection of acidity in different parts of the body.
The sourness sensor was found using a search of DNA and protein sequence databases to find the PKD2L1 protein from a potential list of 30,000.
Dr Charles Zuker, who led the research, said: "Killing these cells and showing that the mice now are totally unable to detect sour proved that these cells are the sensors for sour taste, and that indeed no other taste cells detect sour."
This finding that the protein was present in the central nervous system could help explain how the body monitors the quality of critical body fluids, Dr Zuker said.
For example, the body controls respiration in part by monitoring the acidity of the blood, since an increase in carbon dioxide dissolved in the blood increases acidity.
Dr Zuker said defects in these blood, spinal or brain fluid-sensing systems might underlie a wide range of disorders.
Professor Tim Jacob, of Cardiff University, said the research contradicted the idea that there are cells which can respond to a variety of tastes.
"Neurophysiologists will have to go back to the lab."