Common bitter stimuli show differences in their temporal profiles before and after swallowing

Abstract

Prior data suggest humans can distinguish between isointense bitter stimuli (i.e., bitterness may not be unitary). Cues for such discrimination remain unclear but temporal and regional differences have both been implicated. Here, ten bitterants – caffeine, quinine, l-phenylalanine, l-tryptophan, urea, naringin, SOA (sucrose octaacetate), and 3 hop extracts –were assessed in water using time-intensity scaling. Trained assessors (n = 14) rated overall intensity of each bitterant continuously for 90 s in triplicate using line scales. During tasting, solutions were swished in the mouth for 10 s and then swallowed. Temporal curves using normalized intensity ratings for each bitterant and replicate were obtained for each assessor. From these curves, various parameters were extracted using a Python script (provided in Supplementary Materials). For each parameter, differences between bitterants were tested in repeated measures ANOVAs that accounted for sample and replicate (fixed) and panelist (random) effects. Relationships between bitterants and parameters were explored further via Principal Component and Cluster Analysis. Collectively, these analyses revealed two distinct groups. Group 1 (caffeine, quinine, l-phenylalanine, l-tryptophan, urea) was characterized by its faster initial (in-mouth) rate of onset and faster rate of decay. Group 2 (naringin, SOA, hop extracts) was characterized by a slower initial rate of onset, an increase in intensity after swallowing, and a slower rate of decay. These data indicate bitter stimuli found in foods show substantial differences in their temporal profiles. Additional work is needed to determine causes of these temporal differences, and whether these properties may be systematically related to differential liking and/or intake of bitter food products.