Thermoreversible Carrageenan Gels Formed Through Crosslinking with Napin

Abstract

Aqueous mixtures of napin, a protein extracted from rapeseed, and κ-carrageenan (κ-car), a polysaccharide extracted from red algae, were investigated. Dynamic mechanical measurements show that when aqueous κ-car and napin solutions are mixed at temperatures above 60°C, the mixture remains liquid. However, a gel forms upon cooling, which melts again during heating. The gelling and melting temperatures, as well as the elastic modulus of the gel, depend on the napin-to-carrageenan weight ratio and the total biopolymer concentration. Opposite charge interactions and hydrogen bonding both play significant roles in this process. Confocal laser scanning microscopy revealed that opposite charge interactions lead to the formation of dense napin/κ-car complexes that become more prominent as the napin fraction increases. This interaction can be reduced or eliminated by increasing the pH toward napin's isoelectric point (pI = 9.5) or by adding NaCl. During cooling, hydrogen bonding between napin and κ-car becomes important, causing the formation of a network where κ-car chains are crosslinked by napin. The balance between these interactions determines the elastic modulus and microstructure of the gel.