Combined Effect of Alternating Magnetic Field and Cryoprotectants on the Microstructure and Quality of Yellowfin Tuna During Assisted Re-Freezing

To address quality deterioration in yellowfin tuna during traditional freezing, this study systematically investigated the combined effects of alternating magnetic fields (AMFs) and two cryoprotectants (carboxylated cellulose nanofibers (CCNF); konjac glucomannan (KGM)). Twelve groups were established by combining four magnetic intensities (0, 5, 10, 15 mT) with three treatments (no cryoprotectant, 10 mg/mL CCNF, or KGM). The results show that AMF significantly shortened the freezing phase transition time and mitigated the phase transition delay induced by cryoprotectants. The optimal preservation was achieved by combining a 5 mT AMF with KGM (AMF5-KGM), which significantly reduced thawing, cooking, and centrifugation losses. Mechanistically, the 5 mT AMF and KGM synergistically stabilized the secondary and tertiary structures of myofibrillar proteins, whereas high-intensity AMF (>10 mT) and CCNF exacerbated protein conformational damage. Furthermore, the AMF5-KGM group exhibited maximal structural integrity, with the smallest ice crystal pore area (36.6%). Ultimately, this study demonstrates that a 5 mT, 50 Hz AMF combined with 10 mg/mL KGM synergistically improves the preservation quality of frozen yellowfin tuna, providing a theoretical basis for high-value pelagic fish product.