Effect of packaging and storage temperature on water holding capacity of catfish fillets during storage based on low field NMR

Abstract: The effect of packaging and storage temperature on the water holding capacity (WHC) of catfish fillet (Clarias gariepinus) was investigated using the low-field nuclear magnetic resonance (LF-NMR). The catfish fillets were stored at 4°C or -0.7°C with air-packaging (AP), vacuum-packaging (VP) or modified atmosphere-packaging (MAP, 60% CO2 and 40% N2). The fillets were sampled on the 0, 1, 4, 7, 10, 15, 20, 30 day since storage to analyze the pH value, cooking loss and water loss after centrifugation. The WHC changes of catfish fillets were illustrated by relaxation time, relaxation area and proton density in LF-NMR, and muscle fiber structure changes were visualized by the scanning electronic microscopy. The present study aimed to give a comprehensive judgment on the process of WHC changes of the catfish fillet during the storage at different temperatures and with different packaging styles, and aimed to reveal the underlying mechanism. The results demonstrated that the WHC of the catfish fillet decreased during the storage period. The pH value significantly declined in each treatment group during the first 4 days; later, the pH value of the AP and VP groups stored at both 4 ℃ and -0.7 ℃ rose significantly (P<0.05), while that of MAP groups (4 ℃ and -0.7 ℃) was maintained at low levels of 6.51-6.88. The highest cooking loss occurred in the 4 ℃ VP group which increased from 11.54% (the initial day) to 23.70% (the 4th day). Water loss after centrifugation significantly rose from 15.98% (the initial day) to 31.35% (the 15th day) in the 4 ℃ VP group. Therefore it can be concluded that the VP is not an effective package style to maintain the WHC. The relaxation time showed a downward tendency from 63.67 (the initial day) to 54.82 (4 ℃ AP group, the 10th day) and 57.10 ms (0.7 ℃ AP group, the 15th day), which was because the binding force of water increased, and the degree of freedom and the mobility of water decreased. The relaxation area increased from 85.49 %(the initial day) to 88.06%-94.77% (the 1st day), and later showed a downward tendency. On the 30th day in storage, the relaxation area of the -0.7 ℃ MAP group reduced to 86.38%, and that of the -0.7 ℃ VP group decreased to 81.11%. The reason was that the contraction of muscle fiber caused the water within the muscle fiber cells to extrude and flow outside of the cell, and part of the immobilized water was transformed to free water. Through the comparison of the WHC among all the 6 groups, the -0.7 ℃ MAP is the best storage way for catfish fillet. Pearson correlation coefficient analysis indicated that the content of immobilized water was significantly correlated with the storage time (P<0.05), pH value (P<0.05), as well as cooking loss and water loss (P<0.01). The results of MRI (magnetic resonance imaging) indicated that the water migrated out of the myofibril cell and was aggregated outside the muscle cell membrane during the storage. The scanning electronic microscopy demonstrated that the muscle fiber of fresh catfish was arranged closely with intact connective tissue and clear muscle fiber boundaries, but during the storage muscle distortion, fracture, voids, damaged membrane of connective tissue, and ambiguous boundaries of muscle fiber were visible. Hence such changes of myofibril cell structure were also the reason causing the decrease of WHC. In summary, LF-NMR is efficient for rapid evaluation of WHC in catfish fillets. The results of the study can provide the storage and package guidance for the manufacture of high quality catfish fillet.