Effect of the number of freeze-thaw cycles number on the quality of Pacific white shrimp (Litopenaeus vannamei): An emphasis on moisture migration and microstructure by LF-NMR and SEM

Abstract

The objective of this study was to evaluate the effects of the number of freeze-thaw (F-T) cycles on moisture migration, protein degradation, microstructure and quality in Litopenaeus vannamei. The quality of samples with different F-T cycles were determined by low-field nuclear magnetic resonance (LF-NMR), magnetic resonance imaging (MRI) and scanning electron microscopy (SEM), combined with sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), texture properties analysis (TPA), color difference, polyphenol oxidase (PPO) activity, total volatile basic nitrogen (TVB-N), total viable count (TVC), and sensory evaluation. The results showed that F-T cycles caused a significantly increase in transverse relaxation time in T22 and T23 and a decreased brightness of pseudo-color diagram after 4 F-T cycles, indicating that water mobility increased as immobilized water was shifted to free water. The texture of samples declined as well, especially after 4 F-T cycles. The rapid growth of PPO activity caused a decrease in brightness (L*) and an increase in redness (a∗) after 3 F-T cycles. The increase of TVB-N, TVC, and sensory score explained the changes in shrimp quality which became obvious after 3 F-T cycles and unacceptable after 6 cycles. Meanwhile, fewer than 4 F-T cycles accelerated protein aggregation, while denaturation occurred after 4 cycles. Therefore, repeated F-T cycles could accelerate the changes of protein, microstructure, water distribution, and quality deterioration especially after 3 F-T cycles, with a threshold was 6 F-T cycles.