Effects of freezing and thawing cycles on the quality of Nile tilapia fillets

Effects of Different Freezing Rate and Frozen Storage Temperature on the Quality of Large-Mouth Bass (Micropterus salmoides)

In order to clarify the individual role of freezing and frozen storage on the quality of fish, fillets of large-mouth bass (Micropterus salmoides) were subjected to different freezing rates (freezing with -18 °C (A), -60 °C (B), and -60 °C with forced air circulation at 2 m/s (C), respectively) followed by frozen storage at -18 °C for 30 and 90 days. Another two groups were frozen at -60 °C, followed by storage at -40 °C (D) and -60 °C (E), respectively. Results showed that water-holding and TVBN were mainly affected by storage time. No significant changes were found in free thiol content among treatments. A greater freezing rate and lower storage temperature generally led to lower TBARS. GC × GC-TOFMS revealed a total of 66 volatile compounds, which were related to lipid oxidation. PLS-DA showed that fresh samples were separated from the frozen-thawed ones, and fillets in groups D and E were relatively close to fresh fillets in the composition of oxidation-related volatiles. In conclusion, freezing rate and storage temperature had a significant impact on lipid oxidation and protein denaturation in the fillets of large-mouth bass, while protein oxidation was more affected by freezing rate.

Analysis of the water state and microfractal dimension of tilapia fillets during freezing and thawing

To study the effects of freezing and thawing times and freezing temperatures on the water state and microstructure of tilapia fillets, experiments on tilapia fillets were carried out at -4 and -18°C with one to four freezing and thawing cycles (FTCs). Low-field nuclear magnetic resonance (LF-NMR) and nuclear magnetic resonance imaging were used to observe the water state after different treatments, and scanning electron microscopy (SEM) and frozen sections were used to observe the microstructure changes. Fractal dimension (FD) was used to quantitatively characterize the microstructure of the fish tissue, and the correlation between FD and fish fillet quality parameters was studied by principal component analysis (PCA). The findings showed that with the increase of FTCs, the thawing loss increased, and the water holding capacity (WHC) fell. FTCs cause a decrease in immobilized water and an increase in free water in the fillet. This indicates the migration of immobilized water to free water. SEM and frozen slice images showed that the growth of ice crystals led to the destruction of myogenic fibers. A decrease in freezing temperature inhibited ice crystal growth. The FD value dropped in accordance with an increase in FTCs. PCA demonstrated that the WHC, NMR data, and FD value had a strong correlation with the quality changes in the tilapia fillets. Therefore, FD and water state can reflect the quality characteristics of tilapia fillets. PRACTICAL APPLICATION: The water migration in tilapia fillets is detected with LF-NMR, and the microscopic image may be quantified using the FD value. Both approaches can offer fresh perspectives on how to assess the quality of tilapia fillets and reflect changes in their quality.

Interrelationship among protein structure, protein oxidation, lipid oxidation and quality of grass carp surimi during multiple freeze-thaw cycles with different pork backfat contents

Storing at -18°C, the surimi was processed from fresh grass carp, supplemented with pork backfat of 50, 100, or 150 g kg^-1 (groups F₀ , F₅₀ , F₁₀₀ , and F₁₅₀ , respectively). The surimi was thawed and refrozen weekly afterward, with changes in surimi protein structure (primary structure, secondary structure, and tertiary structure), protein oxidation index (carbonyl and sulfhydryl), lipid oxidation index (TBARS), protein solubility, as well as surimi quality (texture characteristics and whiteness), were determined. The results showed that the texture characteristics of surimi increased after the first freezing and thawing (F-T) cycle and then decreased. The whiteness of the surimi decreased with the increase of F-T cycle times, while the whiteness of the surimi increased with the increase in fat content. With the F-T cycle times and fat content rising, the relative content of α-helix structures, sulfhydryl content, and protein solubility decreased. In contrast, carbonyl content was in reverse, being an increasing trend. The free amino content showed a decrease after an increase at the early stage with the F-T cycles growing, while it showed a decrease with an increase in fat content. Meanwhile, the redshift in the maximum absorption peak of the tryptophan fluorescence was caused by the F-T cycles, so was the decline in fluorescence intensity; however, there was little difference in tryptophan spectra with the same fat content. The association among protein structure, protein oxidation, lipid oxidation and surimi quality were elucidated by Pearson's two-tailed correlation. The lipid oxidation in the repeated freezing and thawing process of surimi led to the decrease in whiteness. The protein oxidation led to the change of protein structure and the decrease of protein solubility. Lipid oxidation caused by the increase of fat content led to the decrease of free amino content and protein solubility, and finally led to the increase of whiteness and the deterioration of texture characteristics. PRACTICAL APPLICATIONS: In this study, changes in various components of surimi were measured to examine the interrelationship among protein structure, protein oxidation, lipid oxidation and surimi quality. Freeze-thaw cycle and high-fat content are easy to cause lipid oxidation, protein oxidation, a decrease in protein solubility, and deterioration of surimi quality. Therefore, temperature fluctuations must be avoided to extend the shelf life of such products. An appropriate fat content level must be selected to prevent protein and lipid oxidation and maintain quality.

The investigation of storage situation of fish muscle via the analysis of its exudate by MALDI-TOF MS

The potential of MALDI-TOF MS was investigated in terms of its capability to determine the change of exudates of fish muscle with different freeze-thaw cycles (0, 1 and 2), and frozen storage periods. The exudates were collected from dead chilled marine fish species, large yellow croacker (Larimichthys crocea, LC) and freshly slaughtered freshwater fish species, Japanese seabass (Lateolabrax japonicus) to be studied as models. 109 proteins, in which, 32 are extracellular proteins, and 15 are intracellular proteins, were identified by analyzing exudate of LC using MALDI-TOF MS and HPLC-MS/MS. The results show that the present method may be able to determine the change of fish muscle foods in a more sensitive mode than K value indicated quality control. The feasibility of verifying the storage situation of fish sample was performed by analyzing fish samples obtained from the local market. It is promising to estimate the storage situation of fishery products or other animal muscle foods by analyzing their muscle exudates based on the presently developed strategy.

Effect of Multiple Freeze-Thaw Cycles on Lipid Degradation and Lipid Oxidation of Grass Carp Surimi Containing Different Amounts of Pork Back Fat

Fresh grass carp was used to produce surimi samples that were supplemented with 50 g/kg, 100 g/kg, or 150 g/kg pork back fat. The lipid composition, lipase activity, lipid oxidation index, and lipoxygenase activity of samples subjected to repeated freeze-thaw process were determined to assess the effects of the added fat on lipolysis and lipid oxidation of grass carp surimi. Freeze-thaw treatment increased free fatty acid content, mainly due to the decomposition of phospholipids and some neutral lipids by lipase. With repeated freeze-thaw treatment, the levels of free fatty acids and phospholipids were correlated with the lipid oxidation indexes and lipoxygenase activity, indicating that lipid degradation can promote lipid oxidation. In the same freeze-thaw cycle, surimi products with high fat content are more vulnerable to oxidative damage, neutral lipids are the main source of free fatty acids in the early stage of freeze-thaw, and phospholipids are the main source of free fatty acids in the late stage.