Threshold
Japanese: 閾値（しきい値）
The minimum concentration at which humans can perceive a specific taste or aroma. Not just content level, but the relationship to threshold determines 'perceived flavor.'
A threshold is the minimum concentration at which humans detect a specific taste or aroma. No matter how much is present, if it doesn't exceed the threshold, it goes unnoticed. Conversely, trace amounts with extremely low thresholds are strongly perceived.
Glutamic acid's umami threshold is about 0.03%; salt (NaCl) is about 0.2%—meaning umami is detected at lower concentrations than saltiness. Some volatile aroma compounds have extraordinarily low thresholds at the ppb level, significantly affecting aroma even in traces.
Understanding miso flavor requires considering not just component quantities but their ratio to thresholds. This ratio (OAV: Odor Activity Value) indicates each component's actual contribution to flavor.