A measurement was carried out for membranes with lipid concentrations of 0.001–10 wt%. The amount of adsorbed iso-α acid increased with the lipid concentration to its maximum when the lipid concentration was 0.715 wt%, then decreased when the lipid concentration further increased, as expected. The maximum amount of adsorbed iso-α acid was approximately 1 μg/cm2 (Fig. 10). The relationship between the amount of adsorbed iso-α acid and the CPA value was evaluated using a lipid/polymer membrane with a lipid concentration of 0.715 wt%. For an amount of adsorbed iso-α acid of 2 μg/cm2 or more, the CPA value saturated, meaning that the taste substances were adsorbed onto the membrane surface or into the membrane interior, as illustrated in Fig. 4. This adsorption causes the suppression of dissociation and the screening effect by paired ions, resulting in the reduced change in the surface charge density of the membrane.