Effects of acidification by glucono-delta-lactone or hydrochloric acid on structures of zein-caseinate nanocomplexes self-assembled during a pH cycle

Zein is a hydrophobic protein and is conventionally prepared as dispersible nanoparticles with the anti-solvent precipitation method. The dispersion stability of zein nanoparticles can be enhanced after adsorbing with a biopolymer such as sodium caseinate (NaCas). In our previous work, self-assembled nanocomplexes of zein and NaCas were formed after a pH-cycle from neutral to pH 11.5 and then to pH 7.0 using HCl. This pH cycle method eliminates the use of alcohol but is time-consuming during titration. The objective of the present work was to compare properties of dispersions with different mass ratios of zein: NaCas at pH 7.5 after the pH cycle and acidification with D-glucono-delta-lactone (GDL) or HCl. Compared with the HCl method, zein nanoparticles prepared with GDL had a smaller mean hydrodynamic diameter (64.3 vs. 134.8 nm) and a narrower distribution measured in dynamic light scattering, which was further confirmed in scanning electron microscopy. The different particle size distributions resulted in stable and unstable zein dispersions of the GDL and HCl treatments, respectively, after storage at 4 degrees C for 30 days. The incorporation of NaCas greatly reduced the turbidity of zein nanoparticle dispersions and zein-NaCas nanocomplex dispersions prepared with GDL exhibited better storage stability than those prepared with HCl. Based on fluorescence and FTIR spectroscopy, similar nanoparticle formation mechanisms were observed for GDL and HCl treatments. Hydrolysis of GDL resulted in uniform distribution of hydroniums to induce more nuclei for growth into more abundant and homogeneous zein nanoparticles than the HCl treatment. Therefore, GDL can be used to further improve the scalability of producing nanoparticles to make use of zein for various applications. (c) 2018 Elsevier Ltd. All rights reserved.