Hence, sensory evaluations have several problems, for example, low objectivity, low reproducibility, the stress possibly imposed on panelists and the significant cost of selecting and training panelists. Additionally, in the medical and pharmaceutical industries, it is difficult to carry out sensory evaluations because of the potential for medication side effects. Although quantitative analysis can be conducted, it cannot be used to estimate the intensities of each basic taste. Against these backgrounds, objective methods of evaluating tastes without using human sensory systems have attracted attention. The development of objective methods of evaluating taste should contribute greatly to the compliance of drug products and the qualities of foods and beverages.

Examinations of objective methods of taste evaluation, such as electronic tongues (e-tongues), have been performed worldwide [16�C23]. Since e-tongues are potentiometric multisensor systems mostly using metal and ion-selective electrodes, principal component analysis (PCA) and partial least squares (PLS) analysis are generally carried out to analyze taste information obtained by sensor outputs with low selectivity. Objective evaluations of the taste of unknown samples are difficult, because PCA needs to define the meanings of each principal component. In spite of this issue, e-tongues are suitable for comparing and distinguishing known samples, such as for quality control. There have been reports on the assessment of the bitterness of drugs and bitterness-masking formulations using e-tongues [16,18,20,23].

Rundnitskaya et al. [23] mentioned the quantification of bitterness of structurally various active pharmaceutical ingredients using an e-tongue under parameter-limited conditions.Our research team has developed a taste sensor, which is an e-tongue with global selectivity, using some electrodes with lipid/polymer membranes comprising a lipid, polyvinyl chloride (PVC), and a plasticizer as sensing parts [24�C29]. Global selectivity is one of the unique characteristics of our taste sensor. This means that the taste sensor must respond consistently to the same taste similarly to the human tongue, despite Drug_discovery the various chemical structures and sizes of tastants. The taste sensor has been commercialized by Intelligent Sensor Technology, Inc., (Kanagawa, Japan) as a taste sensing system and is the first e-tongue system commercialized in the world.

Each taste sensor electrode in the sensor system has global selectivity, responding to only one taste. The taste-sensing system is a potentiometric measurement system, which determines the membrane potential of lipid/polymer membranes. The change in membrane potential is used as the sensor output. It is caused by electrical and hydrophobic interactions between the lipid/polymer membrane and tastants in a sample solution.