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    Mapping of beta-lactoglobulin - mucin interactions in an in vitro astringency model: Phase compatibility, adsorption mechanism and thermodynamic analysis

    Year: 2022

    Journal: Food Hydrocolloids, Volume 129, AUG

    Authors: Ahmad, Mehraj; Ritzoulis, Christos; Bushra, Rani; Meigui, Huang; Zhang, Xinyu; Chen, Jianshe; Song, Junlong; Jin, Yongcan; Xiao, Huining

    Organizations: College of Light Industry and Food, Nanjing Forestry University [163105084, 163105102]; Ministry of Science and Technology, People's Republic of China, The Foreign Youth Talent Program [QN2021014015L]

    Keywords: beta-Lactoglobulin; Mucin; Astringency; QCM-D; Molecular interactions; Thermodynamics

    This study investigates the astringency-related interfacial behaviour, mechanistic parameters, and interaction mechanisms of a typical milk protein, beta-lactoglobulin (beta-LG), with mucin acting as model salivary protein. For a series of beta-LG - mucin compositions, the most intense phase separation is observed for a 75:25 w/w mixture of mucin and beta-LG at pH 3. QCM-D shows that, at pH 3, beta-LG adsorption on a mucinous interface yields a heavily-hydrated viscoelastic layer with increased energy dissipation [Delta D < 9.92 +/- 0.01 ( x 10(-6)] thickness (Delta t = 25.51 +/- 0.01 nm), and wet mass values (Delta m = 2623.30 +/- 1.27 ng cm(-2)). At pH 7, beta-LG (Delta in = 855.86 +/- 0.45 ng cm(-2)) forms highly-ordered (more rigid), relatively less dissipative, but thinner interfacial layer, as indicated by the decreased Delta t (7.13 +/- 0.01 nm) and Delta D values [<= 2.52 +/- 0.01 ( x 10(-6))]. Furthermore, Delta D vs.Delta f plots point to the existence of greater conformational and molecular reordering upon progressive coupling of beta-LG at pH 3, as compared to pH 7. beta-LG exhibits a two-step binding with mucin at pH 3; whilst a single-step (low affinity) mechanism exists at pH 7, as elucidated by fluorimetry. Thermodynamic analysis suggests two macromolecular populations interact spontaneously (Delta G < 0) regardless of pH. Interestingly, Delta S values are substantially greater at pH 3 (>= 182.76-292.30 J K-1M-1), in comparison with pH 7 (<= 17.07 J K-1M-1); suggesting that entropydriven flocculation of beta-LG and mucin arise from well-defined electrostatic and/or hydrophobic forces. The above results highlight the inherent complexity of pH-induced beta-LG - mucin interactions, and the resulting complexity of the physicochemical basis of astringency.
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