Sensing technologies for objective evaluation such as the discrimination and quantification of tastes have been developed since around 1990, before the discovery of taste receptors. Electronic tongues aim to discriminate and analyze foods and beverages and are well known as sensing technologies that greatly contribute to quality management. A taste sensor, i.e., an electronic tongue with global selectivity, is developed to realize a sensor that responds to taste chemical substances and can be used to quantify the type of taste focusing on the fact that humans discriminate the taste of foods and beverages on the tongue with the five basic tastes. In this paper, we focus on the taste sensor and describe its sensing principle, its difference from general electronic tongues that do not aim to quantify tastes, examples of its use, and the recent trend of research of electronic tongues.

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The bitterness of bitter substances can be measured by the change in the membrane electric potential caused by adsorption (CPA) using a taste sensor (electronic tongue). In this study, we examined the relationship between the CPA value due to an acidic bitter substance and the amount of the bitter substance adsorbed onto lipid/polymer membranes, which contain different lipid contents, used in the taste sensor. We used iso-α-acid which is an acidic bitter substance found in several foods and beverages. The amount of adsorbed iso-α-acid, which was determined by spectroscopy, showed a maximum at the lipid concentration 0.1 wt % of the membrane, and the same phenomenon was observed for the CPA value. At the higher lipid concentration, however, the amount adsorbed decreased and then remained constant, while the CPA value decreased monotonically to zero. This constant adsorption amount was observed when the membrane potential in the reference solution did not change with increasing lipid concentration. The decrease in CPA value in spite of the constant adsorption amount is caused by a decrease in the sensitivity of the membrane as the surface charge density increases. The reason why the peaks appeared in both the CPA value and adsorption amount is based on the contradictory adsorption properties of iso-α-acid. The increasing charged lipid concentration of the membrane causes an increasing electrostatic attractive interaction between iso-α-acid and the membrane, but simultaneously causes a decreasing hydrophobic interaction that results in decreasing adsorption of iso-α-acid, which also has hydrophobic properties, onto the membrane. Estimates of the amount of adsorption suggest that iso-α-acid molecules are adsorbed onto both the surface and interior of the membrane.