A taste sensor with lipid/polymer membranes is one of the devices that can evaluate taste objectively. However, the conventional taste sensor cannot measure non-charged bitter substances, such as caffeine contained in coffee, because the taste sensor uses the potentiometric measurement based mainly on change in surface electric charge density of the membrane. In this study, we aimed at the detection of typical non-charged bitter substances such as caffeine, theophylline and theobromine included in beverages and pharmaceutical products. The developed sensor is designed to detect the change in the membrane potential by using a kind of allosteric mechanism of breaking an intramolecular hydrogen bond between the carboxy group and hydroxy group of aromatic carboxylic acid (i.e., hydroxy-, dihydroxy-, and trihydroxybenzoic acids) when non-charged bitter substances are bound to the hydroxy group. As a result of surface modification by immersing the sensor electrode in a modification solution in which 2,6-dihydroxybenzoic acid was dissolved, it was confirmed that the sensor response increased with the concentration of caffeine as well as allied substances. The threshold and increase tendency were consistent with those of human senses. The detection mechanism is discussed by taking into account intramolecular and intermolecular hydrogen bonds, which cause allostery. These findings suggest that it is possible to evaluate bitterness caused by non-charged bitter substances objectively by using the taste sensor with allosteric mechanism.
A taste sensor is composed of several kinds of lipid/polymer membranes as transducers which convert taste information to electric signal. Thus, the role of membranes is very important to detect various taste components. In this paper, we developed novel membranes which specifically respond to quinine that is typical bitter substances. These membranes were composed of hydrophobic ionic liquid such as N, N, N-trimethyl-N-propylammonium bis(trifluoromethansulfonyl)imide, 1-butyl-3-methylimidazolium hexafluorophosphate and 1-butylpyridinium hexafluorophosphate, a plasticizer, 2-nitrophenyl octyl ether and a polymer, polyvinyl chloride. In addition to quinine, they also showed response to both several kinds of alkaloids such as caffeine and strychnine, and non-alkaloid such as phenylthiocarbamide. The order of these responses was equal to that of the tongue glossopharyngeal nerve of flog. Furthermore, there were the other alkaloids which response to these membranes. Especially in these alkaloids, they showed high response to denatonium benzoate and berberin chloride which have a strong bitter taste.
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.
The bitterness sensor with negatively charged lipid polymer membrane has been reported to perform high sensitivity and selectivity to bitterness of medicines. However, the conventional cleaning solution cannot completely remove residual substances after measuring high concentration bitter samples. Surfactant is an important cleaning agent used for membrane materials. This paper reported the effects of four kinds of surfactants in cleaning the lipid polymer membrane of the bitterness sensor. Among these surfactants, anionic surfactant linear alkylbenzene sulfonate (LAS) showed no detectable impact on membrane integrity and a good cleaning effect for the bitterness sensor.
This chapter reviews the state-of-the-art technology of electronic tongues, mainly by focusing on a taste sensor, that is, an electronic tongue with global selectivity. It first explains the physiological knowledge of taste. The features of electronic tongues based on sensor arrays to measure liquid are low selectivity and high cross-selectivity instead of high selectivity and the capability of statistically analyzing the outputs from multiple sensors. There are two types of commercialized electronic tongue in the world. One is the taste sensing systems SA402B and TS-5000Z, which is usually called the taste sensor, and another is the Astree II e-tongue. In the future, an integrated biosensor system for the five senses will be developed to enable us to quantify the quality (palatability and safety), construct a quality information database, develop a quality description tool of foods, and visualize the five senses.
A multichannel taste sensor using lipid/polymer membranes has higher sensitivities to electrolytes such as sour and salty substances than nonelectrolytes such as sweet substances. The purpose of this study is to improve sensitivity to sweet taste substances. The response to sweetness is assumed to have a mechanism different from electrolytic taste substances due to its nonelectrolyte. We could make large improvement of the response to sweetness by mixing positively and negatively charged lipid/polymer membranes and utilizing their hydrophobic property.
