AFM study of casein micelles cross-linked by genipin: effects of acid pH and citrate

Food-Grade Gelatin Nanoparticles: Preparation, Characterization, and Preliminary Application for Stabilizing Pickering Emulsions

In this paper, the food-grade gelatin nanoparticles (GNPs) were prepared by a two-step desolvation method and using genipin as a cross-linker. The GNPs with narrow size distribution and good dispersion could be obtained only at pH 12. The effect of the genipin dosage (8-12 wt%) on the GNPs was systematically investigated. The results showed that the cross-linking degree of the GNPs increased with the increasing dosage of genipin, thus leading to a more obvious cross-linking morphology observed from scanning electron microscope (SEM). The obtained GNPs showed a good dispersibility with a size range of 386-438 nm. However, the GNPs cross-linked by 8 wt% genipin dosage revealed a relatively higher size because of the aggregation induced by hydrogen bond. The 10 wt% group had good thermal stability and storage stability. The optical microscopy results showed that the Pickering emulsions (30-50 vol% internal phase) stabilized by the GNPs had good uniformity and stability, even after 30 days of storage time, suggesting that the stable GNPs had great potential in food-grade Pickering emulsions.

Insights into interaction of chlorophylls with sodium caseinate in aqueous nanometre-scale dispersion: color stability, spectroscopic, electrostatic, and morphological properties

Chlorophylls are the major pigments present in photosynthetic plants but their application in foods is limited due to their lack of solubility in aqueous media and their susceptibility to degradation during processing and storage. These problems might be overcome by the addition of sodium caseinate (NaCas) whose hydrophobic and hydrophilic groups may result in electrostatic and steric stabilization. In the present work, 1% and 3% (w/w) chlorophylls in NaCas dispersion or in ethanol (control) were prepared and their color stability under light treatment for 5 h and light-shed storage for 10 days was studied along with their interaction mechanism, using electrostatic, spectroscopic and morphological methodologies. Chlorophylls remained 58.72% and 53.84% in NaCas dispersions compared with the control (41.29% and 45.93%) after light treatment for low- and high-dose treatment, respectively, suggesting that NaCas improved the solubility and stability of the pigments. Additionally, 1% and 3% (w/w) chlorophylls in NaCas dispersion remained stable at 60.49% and 57.62% compared with the control (44.81% and 48.17%) for the 10 day storage during which the zeta-potential of the dispersions changed from -31.7 to -52.2 mV and from -36.7 to -56.2 mV with a well-defined diameter (∼221-245 nm). The data obtained from electron microscopy, together with the results of fluorescence spectroscopy, suggests that chlorophylls were entrapped in NaCas dispersions mainly via hydrogen bonds.

Atomic Force Microscopy Study of the Topography and Nanomechanics of Casein Micelles Captured by an Antibody

Casein micelles (CMs) are colloidal phospho-protein-mineral complexes naturally present in milk. This study used atomic force microscopy (AFM) in a liquid environment to evaluate the topography and nanomechanics of single native CMs immobilized by a novel capture method. The proposed immobilization method involves weak interactions with the antiphospho-Ser/Thr/Tyr monoclonal antibody covalently bound to a carboxylic acid self-assembled monolayer (SAM) on a gold surface. This capture strategy was compared to the commonly used covalent immobilization method of CMs via carbodiimide chemistry. With this conventional method, CMs remained mainly mobile during AFM measurements in liquid, disturbing the evaluation of their average size and elastic properties. Conversely, when captured by the specific antibody, they were successfully immobilized and their integrity was preserved during the AFM measurement. The characterization of both CM topography and elastic properties was carried out in a liquid ionic environment at native pH 6.6. The CMs' capture efficiency via antibody was concurrently proved by surface plasmon resonance. The calculation of casein micelles' width, height, and contact angle was carried out from the recorded 2D AFM images. CMs were characterized by a mean width of 148 ± 8 nm and a mean height of 42 ± 1 nm. Weak forces were applied to single captured CMs. The obtained force versus indentation curves were fitted using the Hertz model in order to evaluate their elastic properties. The elasticity distribution of native CMs exhibited a unimodal trend with a peak centered at 269 ± 14 kPa.