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 (similar to 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.