2.3. Taste Threshold Test

The taste threshold test was conducted for the five basic tastes (sweet, salty, bitter, sour, and umami) based on the “whole mouth sip and spit” method, as described by Hazelhof et al. [26]. The solutes were sucrose (CJ Cheiljedang, Seoul, Korea) for sweet, sodium chloride (Hanju, Seoul, Korea) for salty, caffeine (Samchun Chemicals, Seoul, Korea) for bitter, citric acid (Sigma-Aldrich, Darmstadt, Germany) for sour, and monosodium glutamate (MSG) “Miwon” (Daesang, Seoul, Korea) for umami taste. Each compound was dissolved in deionized water to six consecutive dilutions: the solute for sweet taste was diluted at concentrations in the range 5–400 mM, that for salty at 0.9–300 mM, that for bitter at 0.2–26.8 mM, that for sour at 0.005–9.89 mM [27], and that for umami at 0.3–76.3 mM [28]. Two pilot surveys were conducted to determine the ranges of the concentrations of each solute. Prior to testing, the solutions were freshly prepared every week and stored at 4 °C in individual bottles. The participants were provided with one cup of distilled water to rinse their mouth, one empty cup for expectoration, and disposable cups with stimulant solutions in ascending concentrations. The participants rinsed their mouths before each session and trial and then spit into the expectorating cup. The samples were held and swirled around in the mouth to cover the entire tongue and then spit out. The participants indicated which cup tasted different from the blank and which taste quality it was. The recognition threshold (RT) was evaluated as the minimum concentration of the taste at which participants answered the correct taste. The participants who did not respond to the taste stimuli (non-responder) or responded to an incorrect taste (response error) in the taste threshold test were excluded in the statistical analysis.