Sour taste is usually due to the presence of acids such as acetic acid, citric acid, benzoic acid. Upon dissociation, the acids give H ions which impart acidity. More the H ion concentration higher is the acidity and more is the intensity of sourness. Fruits and vegetables are sour due to the acids present in them for example, tartaric acid in grapes and citric acid in lemon. Acids may be strong or weak depending upon their dissociation ability. Stronger the acid more will be the dissociation and higher will be the acidity, which imparts sourness to a particular substance. Of the two acids, HCl and acetic acid (CH3COOH), acetic acid is sourer than HCl inspite of the fact that HCl is stronger. This indicates that there are other factors also along with the dissociation governing the intensity of sourness. All these factors influence the reaction time which is defined as the time interval between tasting of a substance and identification of the taste by the brain.
The solution is first tasted, the impulse is transmitted to the brain and then identification takes place or in other words, reaction time interval between initial stimulation of the receptors and the final response.
The reaction time to perceive the acidic taste is indirectly proportional to the concentration of the acid. More the concentration, lesser is the time required. Individual variations are there in perception due to difference in the pH of the saliva. Low pH saliva will give a better perception of sour taste.