Preparation, characterization and gel properties of tryptophan-amidated pectin

Native pectin has weak gelation properties and requires strict reaction conditions, limiting its application in the food industry. In this study, food-grade L-tryptophan was used as the amidating modifier to investigate the influence of reaction pH on the degree of amidation (DA), molecular structure, and gelation behavior of pectin. As the reaction pH increased from 5.0 to 9.0, the molecular weight (Mw) decreased significantly and the conformation became more dispersed. Meanwhile, the DA increased and the degree of esterification (DE) decreased. The tryptophan-amidated pectin prepared at pH 9.0 showed the highest DA (29.50% ± 0.89%) and lowest DE (37.48 ± 0.87%), together with the highest apparent viscosity and strongest gelation. Alkaline conditions partially depolymerized the pectin backbone and promoted de-esterification, thereby exposing additional reactive sites on pectin and facilitating amide-bond formation. Scanning electron microscopy revealed the densest gel network in the tryptophan-amidated pectin prepared at pH 9.0, corroborating that a higher pH during preparation favors the formation of robust pectin networks. Overall, tryptophan-mediated amidation substantially improves the structural and gelation properties of pectin, providing a practical route to high-performance pectin gels for applications in the food industry.