The saltiness enhancement effect can be produced by adding specific substances to dietary salt (sodium chloride). This effect has been used in salt-reduced food to help people forge healthy eating habits. Therefore, it is necessary to objectively evaluate the saltiness of food based on this effect. In a previous study, sensor electrodes based on lipid/polymer membrane with Na⁺ ionophore have been proposed to quantify the saltiness enhanced by branched-chain amino acids (BCAAs), citric acid, and tartaric acid. In this study, we developed a new saltiness sensor with the lipid/polymer membrane to quantify the saltiness enhancement effect of quinine by replacing a lipid that caused an unexpected initial drop in the previous study with another new lipid. As a result, the concentrations of lipid and ionophore were optimized to produce an expected response. Logarithmic responses have been found on both NaCl samples and quinine-added NaCl samples. The findings indicate the usage of lipid/polymer membranes on novel taste sensors to evaluate the saltiness enhancement effect accurately.

Currently, lipid/polymer membranes are used in taste sensors to quantify food taste. This research aims to improve sweetness sensors by more selectively detecting uncharged sweetening substances, which have difficulty obtaining a potentiometric response. Lipid/polymer membranes with varying amounts of tetradodecylammonium bromide (TDAB) and 1,2,4-benzene tricarboxylic acid (trimellitic acid) were prepared. The carboxyl groups of trimellitic acid bind metal cations, and the sweetness intensity is estimated by measuring the potential change, as a sensor response, when these cations are complexed with sugars. This research showed that the potential of a sensor using the membrane with enough trimellitic acid in a sucrose solution remained constant, regardless of TDAB amounts, but the potential in the tasteless, so-called reference solution, depended on TDAB. By optimizing the content of TDAB and trimellitic acid, a sensor response of −100 mV was achieved, which is over 20% more sensitive than a previous sensor. This sensor also demonstrated increased selectivity to sweetness, with similar interference from other tastes (saltiness, sourness, umami, and bitterness) compared to previous sensors. As a result, the sensitivity to sweetness was successfully improved. This result contributes to the development of novel sensors, further reducing the burden on humans in quality control and product development.

Currently, taste sensors utilizing lipid polymer membranes are utilized to assess the taste of food products quantitatively. During this process, it is crucial to identify and quantify basic tastes, e.g., sourness and sweetness, while ensuring that there is no response to tasteless substances. For instance, suppression of responses to anions, like tasteless NO₃⁻ ions contained in vegetables, is essential. However, systematic electrochemical investigations have not been made to achieve this goal. In this study, we fabricated three positively charged lipid polymer membranes containing oleylamine (OAm), trioctylemethylammonium chloride (TOMACl), or tetradodecylammonium bromide (TDAB) as lipids, and sensors that consist of these membranes to investigate the potential change characteristics of these sensors in solutions containing different anions (F⁻, Cl⁻, Br⁻, NO₃⁻, I⁻). The ability of each anion solution to reduce the positive charge on membranes and shift the membrane potential in the negative direction was in the following order: I⁻ > NO₃⁻ > Br⁻ > Cl⁻ > F⁻. This order well reflected the order of size of the hydrated ions, related to their hydration energy. Additionally, the OAm sensor displayed low ion selectivity, whereas the TOMACl and TDAB sensors showed high ion selectivity related to the OAm sensor. Such features in ion selectivity are suggested to be due to the variation in positive charge with the pH of the environment and packing density of the OAm molecule in the case of the OAm sensor and due to the strong and constant positive charge created by complete ionization of lipids in the case of TOMACl and TDAB sensors. Furthermore, it was revealed that the ion selectivity varies by changing the lipid concentration in each membrane. These results contribute to developing sensor membranes that respond to different anion species selectively and creating taste sensors capable of suppressing responses to tasteless anions.