Dietary Theabrownins Improve Freeze–Thaw Quality of Grass Carp (Ctenopharyngodon idella) Muscle by Inhibiting Ice Crystal Growth and Modulating Metabolism

Freezing is the most commonly used preservation method for aquatic products, but the freeze–thaw cycle leads to the formation and growth of ice crystals, which seriously affects the quality of fish. This study evaluated the effect of dietary supplementation with theabrownins (TBs) on the quality attributes of grass carp (Ctenopharyngodon idella) muscle following freeze–thaw treatment. We examined changes in textural properties, ice crystal morphology and metabolomic profiles in response to TBs inclusion in feed. The results indicated that feeding TBs at 0.02% and 0.06% levels significantly improved the chewiness and cohesiveness of grass carp muscle. Histological analysis revealed that TB-containing feed effectively inhibited ice crystal growth, leading to smaller and more uniform ice crystals, thereby mitigating structural damage to muscle tissue. Metabolomic analysis identified distinct metabolite profile differences between the treatment groups and the control group, with both LTB (0.02% TBs) and HTB (0.06% TBs) groups showing significant upregulation of esters and aromatic compounds compared to the control group. The present study demonstrates that TBs, as a natural feed additive, can enhance the freeze–thaw tolerance of grass carp muscle by modulating ice crystal formation and influencing muscle metabolism. This study provides important insights and practical implications for developing novel strategies to improve the quality of frozen aquatic products.