Perception of citric acid and citrate salt mixtures in humans

With the recent identification of a sour taste receptor Otopetrin1 in mice, interest in the perception of sourness has increased. However, research is limited with regard to the human response to organic acids in combination with organic salts. Additionally, the role of counterions in sourness perception remains underexplored. Here we report on several psychophysical experiments of citric acid and citrate mixtures. A total of 206 prescreened frequent consumers of sour foods and/or beverages (i.e. at least once a month) were recruited for this study. Participants rated attribute intensities on general Labeled Magnitude Scales for aqueous equimolar mixtures of varying citric acid and citrate ratios (monosodium citrate [n = 51 participants], disodium citrate [n = 50], trisodium citrate [n = 55], and tripotassium citrate [n = 50]), each at 3 different concentrations. Results revealed that variations in total concentration, amount of citric acid in solution, and type of citrate affected sourness perception and related attributes. Specifically, the number of counterions (i.e. 1, 2, 3 sodium ions) significantly affected sourness, puckering, and drying. The effect of the counterion type (i.e. sodium and potassium) also affected sourness and saltiness. These differences can be attributed to the chemical structures and the protonation states of the mixtures. Additionally, pH measurements revealed the same sourness levels were achieved at different pH levels. From this finding, solutions can be formulated to provide the same perceived sourness at a higher pH level.