Electrostatic Stabilization of β-lactoglobulin Fibrils at Increased pH with Cationic Polymers

In order to improve the stability of β-lactoglobulin fibrils formed in acidic conditions to increased pH values (pH 3-7), formation of electrostatic complexes between fibrils and cationic polymers chitosan (CH), amine-terminated poly(ethylene glycol) (APEG), low molecular weight poly(ethylenimine) (LPEI), and high molecular weight poly(ethylenimine) (HPEI) was investigated by electrophoretic mobility, turbidimetry, and atomic force microscopy. Except for suspensions with APEG, addition of polycations increased ζ-potential values of the fibrils at pH 5, 6, and 7, verifying their interactions with fibrils. Maximal increase in ζ-potential at pH 7, indicating optimal electrostatic interactivity, occurred at concentrations (w/w) of 0.05, 0.01, and 0.01% (corresponding to 6.9, 50, and 4 μmol·kg(-1)) for CH, LPEI, and HPEI, respectively. Turbidity of fibril solutions at pH 5, indicating isoelectric instability, was decreased significantly with increasing concentration of CH, LPEI, and BPEI, but not with added APEG. Turbidity was increased at pH 7 with added polycation, except for suspensions containing ≥0.02% HPEI. Fibril length and resistance to aggregation, as observed by atomic force microscopy, were increased at pH 5 with increasing concentration of CH and LPEI, yet only HPEI was capable of maintaining the morphology of fibrils at pH 7. Calculated persistence lengths of the fibrils, as compared to pure fibrils at pH 3 (∼4 μm), were only slightly reduced at pH 5 with CH and at pH 7 with HPEI, but increased at pH 5 with LPEI and HPEI. Improvement in the stability of β-lactoglobulin fibrils at higher pH conditions with the addition of polycations will contribute to their potential utilization in packaging, food, and pharmaceutical applications.