Bidirectional modulation of psychophysical thresholds in sucrose-bitterant mixtures: sugar reduction and bitterness masking

Abstract

Reducing sugar content while maintaining palatability remains challenging, particularly when functional ingredients introduce bitterness. However, sweet-bitter interactions are rarely quantified at detection and discrimination thresholds. In this study, we applied a threshold-based psychophysical approach to characterise the bidirectional modulation between sucrose and three bitterants: iso-α-acid, epigallocatechin gallate, and quinine hydrochloride. Absolute and difference thresholds were determined under reciprocal background concentrations, concentration-difference strength functions were established, and a computable bidirectional interaction model was developed using quinine hydrochloride as a representative bitter compound. The absolute thresholds were 0.173% w/v for sucrose, 0.00018% w/v for iso-α-acid, 0.0093% w/v for epigallocatechin gallate, and 0.00025% w/v for quinine hydrochloride, indicating the following bitterness potency order: iso-α-acid > quinine hydrochloride > epigallocatechin gallate. The Weber fraction was 18.43% for sucrose and 11.09%, 21.40%, and 21.24% for iso-α-acid, epigallocatechin gallate, and quinine hydrochloride, respectively. Under sucrose backgrounds ranging from 1% to 8%, bitter absolute thresholds increased by 188.9% for iso-α-acid, 201.1% for epigallocatechin gallate, and 28.0% for quinine hydrochloride, while the corresponding Weber fractions increased by 148.9%, 51.2%, and 45.0%, respectively. Conversely, bitter backgrounds increased the sucrose absolute threshold by 74.1% to 116.2%. Despite these threshold shifts, the concentration–difference strength functions remained consistent with Weber–Fechner behaviour, with R 2 values greater than 0.97. These findings provide a quantitative, compound-specific framework for preserving perceived sweetness and mitigating bitterness in sugar-reduced food formulations.