Effects of polysaccharide charge density on the structure and stability of carboxymethylcellulose-casein nanocomplexes at pH 4.5 prepared with and without a pH-cycle

Carboxymethylcellulose (CMC) can stabilize acidic casein dispersions but the CMC charge density effect has not been studied. The objective of the present work was to study the structure and stability of CMC-casein nanocomplexes at pH 4.5 using CMC with the degree of substitution (DS) of 0.7, 0.9, and 1.2. Dispersions were prepared at 1%w/v casein and casein:CMC mass ratios from 5:1 to 1:1 with and without a pH-cycle. The calcium chelating capacity of CMC was strengthened as the DS increased, which enhanced the partial disassociation of casein micelles and therefore improved the dispersion clarity. The lowest dispersion turbidity was observed at a casein:CMC mass ratio of 3:1 for all three kinds of CMC, and the pH-cycle, by alkalizing dispersion pH to 11.3 followed by acidifying to pH 4.5, further lowered the dispersion turbidity. Electrostatic attraction was the mechanism of complex formation at pH 4.5, and the CMC on the complex surface was critical to the stability against aggregation. Furthermore, increasing the DS of CMC resulted in smaller complexes and clearer dispersions. Particularly, the dispersions with a CMC:casein mass ratio of 3:1 after the pH-cycle had the absorbance at 600 nm of 1.25, 0.85, and 0.57, and the mean hydrodynamic diameter of 379, 392, and 188 nm at the DS of 0.7, 0.9, and 1.2 respectively; the increased DS also increased the stability of complexes against aggregation and precipitation at elevated ionic strengths, from 50 mM to 200 mM NaCl. The present findings may broaden the application of caseins in acidic systems.

Automated summary

This study found that as the DS of CMC increased, the clarity of casein dispersions improved, the stability of complexes enhanced, and the size of complexes reduced. The dispersions with a CMC:casein mass ratio of 3:1 after the pH-cycle showed good stability under different DS levels.