Hydroxypropylmethylcellulose–β-lactoglobulin mixtures at the oil–water interface. Bulk, interfacial and emulsification behavior as affected by pH

In this work we have studied the bulk, interfacial and emulsification behavior of a surface-active polysaccharide, hydroxypropylmethylcelulose (HPMC) and β-lactoglobulin (βlg) mixtures at two pHs, 3 and 6. The absence of an increase of particle size or a modification of zeta potential in HPMC/βlg mixtures indicated that both biopolymers would not form complexes in the bulk at pH 6. The protein presented a positive charge at pH 3, which makes possible the formation of a weak complex with the HPMC. The interface was dominated by the protein, indicating the displacement of the less surface-active HPMC at pH 6. The interfacial pressure of the HPMC/βlg mixture at pH 3 failed in between the curves corresponding to the single components. βlg Complexation may decrease its availability to adsorb at the O/W interface, lowering the mixture interfacial pressure. A bimodal distribution was obtained in emulsions at pH 6 and their confocal images evidenced a strong flocculation, non-adsorbed HPMC would remain in the continuous phase, thus promoting depletion flocculation of oil droplets. At pH 3 the emulsion droplet size distribution curve was bimodal, indicating polidispersity. Nevertheless, a high stability to coalescence was observed. The mixed emulsion zeta potential is reduced in comparison with the protein alone, indicating complexation between βlg and HPMC. When analyzing the confocal microcopy images of recently prepared emulsions, no flocculation was observed.