The role of amide groups in the mechanism of acid-induced pectin gelation: A potential pH-sensitive hydrogel based on hydrogen bond interactions

Gelation of amidated pectin in the acid environment with the absence of calcium has been found, but the mechanism and gel properties are still not fully understood, especially the role of amide groups. In this study, a series of pectins with different degrees of amidation (DA) were prepared by ammonia reaction, and their structural characteristics including neutral sugar contents, molecular weight contribution, and molecular morphology were fully identified. Acid-induced hydrogels with various DA were then obtained to investigate the impact of amino groups on pectin gelation. Results showed that the higher the DA, the faster the gelation process, and the more stable the gel network. Dynamic rheological and pH stability tests confirmed the essential roles of low pH value (<3.5) and high DA (>49.76%) on the amidated pectin gelation without calcium. Hydrogen bonds were identified as the main interaction for the amidated gel mechanism and enabled the thermo-reversible properties of amidated gels. High content of amide groups allowed the formation of a denser gel network structure with greater mechanical strength, stronger thermo-reversibility, and higher pH stability.