Improvement of chitosan water solubility by fumaric acid modification

•The modification of native chitosan by fumaric acid was successful.•The C-6 position of chitosan polymer was the main site of acylation.•The modified chitosan was soluble in a wide pH range.•Water solubility of modified chitosan was higher than that of native chitosan. Chitosan is a natural biopolymer with widespread availability. Its absorption, biodegradability, and biocompatibility make it ideal for extensive application. However, the high molecular weight and the high degree of polymerization limit its water solubility at neutral pH. Water solubility can be improved through the transformation in chitosan properties. We aimed to enhance the physicochemical properties through the modification of low, medium, and high molecular weight chitosan by using fumaric acid. With increasing molecular weight of native chitosan, the modified chitosan showed a decrease in water solubility and thermal properties. According to the pH dependence study, the modified products showed higher water solubility than native chitosan in a wide pH range. Fourier transform infrared (FTIR) spectroscopy results proved the effectiveness of modification by fumaric acid. The enhanced water solubility of modified chitosan can maximize its application to the food, biomedical, and cosmetic industry.