Anhydride esterification to regulate water migration and reduce ice crystal formation in κ-carrageenan gel during freezing

In this study, κ-carrageenan was chemically modified by acid anhydride esterification to produce acid anhydride-esterified κ-carrageenan, and its properties were subsequently determined. Acid anhydride-esterified κ-carrageenan can improve the water retention properties of gels and limit the deterioration of gel textural performances during the freezing process. Scanning electron microscopy results before and after freezing showed that the frozen esterified κ-carrageenans had a more regular and less fibrous network structure, which suggests that the esterified κ-carrageenans better maintained the gel network structure. These phenomena were studied in relation to intermolecular forces, water movement, and ice crystal formation. The intermolecular force analysis showed that the esterification of anhydride disrupted the hydrogen bond formed between κ-carrageenan molecules, enhanced hydrophobic interactions among κ-carrageenan molecules, and resulted in stronger electrostatic repulsion. Low-field nuclear magnetic resonance analysis showed that acid anhydride-esterified κ-carrageenan more effectively bound to water molecules during freezing. Differential scanning calorimetry results showed that acid anhydride-esterified κ-carrageenan reduced the amount of freezing water and lowered the eutectic point of the gel. The results showed that hydrogen bonding, electrostatic interactions, and hydrophobic interactions modified the structure of the acid anhydride-esterified κ-carrageenan gel network and its binding to water molecules. This phenomenon allowed acid anhydride-esterified κ-carrageenan to better maintain binding with water molecules during the freezing process, which in turn delayed and reduced the formation of ice crystals and greatly decreased the effect of ice crystals on the structure of the gel network.