Effects of chitosan molecular weight and mass ratio with natural blue phycocyanin on physiochemical and structural stability of protein

Phycocyanin (PC), an algae-extracted colorant, has extensive applications for its water-solubility and fresh blue shade. When PC is added to acidified media, dispersions are prone to aggregate and decolorize into cloudy systems. For palliating this matter, chitosan with high, medium, and low molecular weights (HMC, MMC, and LMC) were adopted in PC dispersions, and their protective effects were compared based on physiochemical stabilities. The optimal mass ratio between chitosan and PC was identified as 1:5 based on preliminary evaluations and was supported by the higher zeta-potential (31.0-32.1 mV), lower turbidity (39.6-43.6 NTU), and polyacrylamide gel electrophoresis results. Through interfacial and antioxidant capacity analyses, LMC was found to display a higher affinity to PC, which was also confirmed by SEM images and the maximum increase in transition temperature of their complex (155.70 degrees C) in DSC measurements. The mechanism of electrostatic interaction reinforced by hydrophobic effects and hydrogen bonding was elucidated by FT-IR and Raman spectroscopy. Further comprehensive stability evaluations revealed that, without light exposure, LMC kept PC from internal secondary structure to external blueness luster to the maximum extent. While with light exposure, LMC was not so flexible as HMC, to protect chromophores from attack of free radicals.