Grafting of edible colorants onto O-carboxymethyl chitosan: preparation, characterization and anti-reduction property evaluation

Abstract

Two polymer dyes were prepared by grafting two edible colorants, Sunset Yellow and Allura Red, onto O-carboxymethyl chitosan through a chlorosulfonylation reaction in a SOCl₂/N,N-dimethylformamide chlorination system and a grafting reaction in a mixed solvent system of water and tetrahydrofuran (Schotten–Baumann reaction). The analysis results from Fourier transform-infrared spectroscopy, ¹H-NMR spectroscopy, mass spectrometry, differential scanning calorimetry and gel permeation chromatography showed that the edible colorants were successfully grafted onto the O-carboxymethyl chitosan. The grafting degrees were determined by UV-vis absorption spectroscopy and elemental analysis, and the analysis results showed that the highest values the two edible colorants could reach were 29.1% and 33.3%, respectively, by the adjustment of the reaction pH, time, temperature and mass ratio of the edible colorant sulfonyl chloride to OMCS. Finally, the anti-reduction properties of the polymer dyes were estimated and compared with the edible colorants by a nicotinamide adenine dinucleotide reduction experiment, and the results showed that the prepared polymers showed an obvious improvement in the anti-reduction properties.