Taste interaction is a well-known phenomenon in sensory studies, but how to quantify the magnitude of the effect of one tastant on other taste attribute(s) is still largely unsolved, let alone further explorations from a salivary biochemistry perspective. Upon acquiring the individual taste threshold and evaluating the just noticeable difference (JND), this study firstly established a quantitative method to assess the magnitude of taste interaction in a binary taste mixture by evaluating the concentration difference upon psychologically-comparable stimulation. The change of salty taste intensity (CSI) was therefore defined as the subtraction of JND using sour-salty taste stimulant from JND using pure salty taste stimulant, with a dimension of concentration in mmol/L. CSI was then used to demonstrate how citric acid affected salty taste perception in a salty - sour binary taste mixture in 16 young and healthy participants. Concentrations of citric acid used in the binary taste mixture were 0.05, 0.09, 0.14, 0.24 and 0.40 mmol/L, respectively, and results showed that salty taste perception was enhanced (average CSI of 0.93 mmol/L) when the citric acid concentration was low (at 0.05 and 0.09 mmol/L), but with an increasing concentration of citric acid ranging from 0.14 to 0.40 mmol/L, this effect gradually turned from enhancement to suppression of salty taste perception (correspondingly a continuously decreasing CSI all the way down to an average of –2.94 mmol/L). It was also found that cystatin SN concentration in participants’ unstimulated saliva samples was significantly negatively correlated with salty taste threshold (with and without the presence of citric acid), and it was one of the most significant factors affecting CSI, as shown in multiple regression analysis. Carbonic anhydrase VI concentration in participants’ unstimulated saliva samples was also found to be significantly negatively correlated with salty taste recognition (with and without presence of citric acid), but it did not pose significant effect on CSI. From these results, this study had not only demonstrated a citric acid concentration-dependent salty taste perception phenomenon based on a proposed methodology to quantitively assess the taste interaction in binary taste mixtures, but also showed how salivary biochemical properties (cystatin SN and carbonic anhydrase VI) might have been associated with salty taste perception during food oral processing.