Thermodynamic and structural insights into sodium caseinate - Quinine binding interactions and their effect on bitterness perception.

Bitter-tasting phytochemicals and pharmaceuticals often trigger aversive responses, limiting intake of nutritious foods and compliance with oral medications. This study investigated quinine binding by sodium caseinate (NaCas), testing the hypothesis that favorable protein-quinine binding can reduce bitterness perception. Unbound quinine in 0-5% w/w NaCas solutions was quantified using fluorescence spectroscopy, revealing a logarithmic decrease in free quinine with increasing protein (to 3.4% at 5% NaCas) and Stern-Volmer analysis indicated a strong binding affinity (quenching rate constant = 3.75 × 1011 M-1 s-1). Isothermal Titration Calorimetry showed the binding interaction was spontaneous (∆G = -5.20 kcal/ mol) and fitted a biphasic model suggesting multiple binding sites on the protein while far-UV circular dichroism analysis revealed increased structural order in NaCas upon binding. Psychophysical testing with adults (n = 44) indicated quinine solutions containing 2% NaCas were perceived as significantly less bitter on a 0-100 visual analogue scale (mean bitterness = 35.7) than aqueous quinine solutions (mean bitterness = 49.5), but the reduction was smaller than expected based on the estimated protein-unbound quinine concentration. Taken alongside other reported data, this appears to be a general rule for the effect of protein binding on taste perception.