Whey protein isolate–low methoxyl pectin nanocomplexes improve physicochemical and stability properties of quercetin in a model fat-free beverage

In this study, whey protein isolate (WPI)-low methoxyl pectin (LMP) electrostatic complexes were used to encapsulate quercetin (Q) in a model fat-free beverage system. The effect of the pH and WPI : LMP ratio was first studied to form soluble complexes with optimal physical properties, in terms of the hydrodynamic diameter, surface charge, and yield. Based on the results, pH 5.0 and a 2 : 1 (w/w) ratio of WPI : LMP were selected for encapsulation of Q. The stoichiometry of the binding (n) and the binding constant (Kb) of WPI:Q were evaluated at pH values of 5.0 and 7.0 at room temperature. The Q-loaded WPI:LMP nanocomplexes were produced by mixing WPI with Q at two loading concentrations corresponding to 5 : 1 and 1 : 1 WPI : Q molar mixing ratios, followed by the addition of LMP and pH adjustment to 5.0. The microstructure of Q-loaded WPI:LMP complexes was investigated by cryo-SEM imaging. Q was efficiently entrapped at two loading concentrations with an efficiency of about 97%. Q-loaded WPI:LMP complexes showed physical stability during storage and high temperature processing, as well as in the presence of challenging formulation conditions such as a high sugar concentration or salt addition (at a limited concentration). The stability of encapsulated Q against UV irradiation was approximately 4 times better than that of free Q. Moreover, Q-loaded WPI:LMP complexes were also lyophilized into dry powder, which can be useful for practical application in food products.