Mechanical (ball milling) activation to regulate microstructure of κ-carrageenan and improve freeze-thaw stability of κ-carrageenan gels and κ-carrageenan-based emulsion gels

To expand the wide range of applications of κ-carrageenan in the food and biomedical fields, we have physically modified the carrageenan and focused on its freeze-thaw stability. In this study, κ-carrageenan (KC) was modified by mechanically activated to prepare wet ball milling κ-carrageenan (WMKC), and its properties were determined. The research results indicate that mechanical activation helps improve the freeze-thaw stability of KC gels and enables KC to better maintain its gel network structure during the freeze-thaw process. These phenomena are related to the effects of mechanical activation on the particle morphology, molecular structure, molecular interactions, and moisture migration of κ-carrageenan. The particle morphology images demonstrated that the WMKC particles exhibited a more irregular surface, which facilitated greater penetration of water molecules into the interior of the κ-carrageenan particles. Furthermore, the results of molecular structure and molecular interaction analyses showed that mechanical activation resulted in the removal of sulfate groups from the κ-carrageenan molecular chain. This resulted in more free -OH being exposed on the molecular chain, which in turn provided more binding sites for KC molecules with water molecules. Low-field nuclear magnetic resonance (LF-NMR) results also proved that WMKC could better maintain the binding with water molecules during the freeze-thaw process. A better combination of WMKC molecules and water molecules can significantly diminish the irreversible damage to the gel network structure of WMKC during the freeze-thaw process, thereby effectively enhancing the freeze-thaw stability of WMKC. In addition, we further used WMKC as the gel matrix and investigated that the freeze-thaw stability of the WMKC-based emulsion gel was optimal when the mechanical activation duration was 3h, which well meets the demand of κ-carrageenan emulsion gels in the frozen food industry.