Tyramine modification of high and low methoxyl pectin: Physicochemical properties, antioxidant activity, and gelation behavior

Pectin modification is an effective approach for changing its functional properties and discovering new applications. To provide more insight into the modified pectin structure-function relationships, tyramine (Tyr) modified high (HMP) and low methoxylated pectin (LMP) were prepared to construct novel pectin gel systems. FTIR, 1H NMR, and UV–vis analyses confirmed that Tyr groups were successfully grafted onto pectin chain and obtained the unreacted galacturonic acid (GalA) content. Physiochemical characterization revealed that Tyr modified pectin exhibited enhanced solubility, amphiphilicity and antioxidant activity. Tyr modified pectin gels possessed good viscoelastic property, but obviously decreased elasticity and strength compared to native pectin gel. Entanglement network number (ENN) and scanning electron microscopy (SEM) microstructure demonstrated the decreased internal cross-linking networks density which contributed to the altered gelling properties of Tyr modified pectin gels. The proposed gelling mechanism indicated that bulky Tyr involved physical interactions instead of the ones of carboxyl groups related changed gelling behavior and gelation process in nature. Similarly, the textural properties and water holding capacity (WHC) of Tyr modified pectin gels were closely related to their physical interaction categories and gelling network microstructure. Sucrose induced Tyr modified LMP gel with strong carboxyl groups related hydrogen bonding showed higher hardness. Calcium ions induced Tyr modified LMP gel formed a larger porous gel matrix which resulted in a poor WHC. The findings provide theoretical basis on physicochemical properties and gelling behaviors of Tyr modified pectin for expanding the pectin application in the fields of food preservation, emulsifiers, stabilizers and tunable gel-type foods.