Effects Of Freezing And Heating On Physicochemical Properties Of Crisp Grass Carp Muscle And Its Mechanisms

Crisp grass carp(Ctenopharyngodon idellus C. et V) is commercial fish which is a kind of grass carp (Ctenopharyngodon idellus) fed with broad bean for long time. Crisp grass carp was deeply loved by consumers because its very tight, high viscoelasticity, and delicious taste after cooking. Processing strongly influenced the texture of crisp grass carp muscle. For instance, crispness of muscle increased after cooking at 100℃, and decreased or lost by freezing or high-temperature heating. However, there were few reports about the effect and mechanisms of freezing and heating on physicochemical properties of crisp grass carp muscle. Therefore, the influence on microscopic structure, textural properties, water holding capacity, viscoelasticity, flavor and protein function were studied during freezing and heating using TPA, LF-NMR, SEM, dynamic rheometer, electronic nose and electronic tongue, GC-MS,2-DE and other modern analysis techniques. And the main differential expression protein was analyzed. The aim was to reveale the mechanisms behind the changes in physicochemical properties of crisp grass carp muscle during freezing and heating. This study could provide theoretical basis for crisp grass carp muscle preservation and processing. The main results were as follows:(1) Effects of freezing on the microstructure and texture of crisp grass carp muscleThe microstructure and texture of crisp grass carp and grass carp muscle were significantly affected by freezing. Ice crystal formed inside frozen muscle bundles, and muscle fibers contracted, the distance between fiber bundles gradually increased, and perimysium burst after frozen. The cooking loss of crisp grass carp and grass carp muscle increased after freezing, and were 8.0%-8.5%,6.4%-6.8% respectively. The shear force, hardness, chewiness and elasticity of frozen crisp grass carp and grass carp muscle decreased to 195.06g and 467.55g,543.37g and 1100.36g,827.28 and 759.87,0.13 and 0.20 respectively. The texture of grass carp muscle was more influenced by freezing than that of crisp grass carp. The brittleness of crisp grass carp muscle declined because perimysium burst and fiber bundles broke by freezing.(2) Effects of heating temperature on structure and texture of crisp grass carpThe structure and texture of crisp grass carp and grass carp muscle were significantly affected by heating temperature. As the temperature increased, muscle gradually contracted, muscle fiber bundles expanded, and the distance between fiber bundles gradually increased, and fiber bundle membrane and endomysium broke. Subsequently, plasma protein escaped from muscle cells and filled in the spacing. Additionally, heating resulted in an increase in cooking loss and pH value of fish muscle, cooking loss increased 17.89%,19.50% respectively. In addition, pH value increased 0.68,0.84 when crisp grass carp and grass carp muscle were heated for 10min at 90 ℃. The water-soluble protein and salt-soluble protein content decreased with increasing temperature. As temperature increased, shear force, hardness, chewiness and elasticity of crisp grass carp and grass carp muscle gradually reduced, textural parameters of grass carp muscle changed more than those of crisp grass carp. Textural properties of crisp grass carp muscle was changed at 50℃, and there was no visible change at 50℃-90℃. But textural properties of grass carp muscle was changed at 50℃-70℃, and decreased sharply with the temperature increased.(3) Effect of heating temperature on flavor of crisp grass carp meatFlavor and taste of crisp grass carp and grass carp were influenced obviously by heating temperature. Heating temperature and methods (such as steaming, boiling and baking) significantly influenced flavor and taste of crisp grass carp and grass carp muscle. The electronic nose and electronic tongue could distinguish crisp grass carp and grass carp muscle, and distinguish the heating methods of crisp grass carp and grass carp muscle. And the electronic tongue was more sensitive than electronic nose. The volatile components of crisp grass carp muscle were mainly alcohols and aldehydes by SPME-GC-MS. The volatile flavor components in cooked muscle were more than those of fresh muscle. The main flavour compounds of crisp grass carp muscle (fresh, cooking) were nonyl aldehyde, hexanal, 1-octene-3-alcohol.(4) Effect of heating temperature on water holding capacity of crisp grass carp muscleThe distribution of water in muscle was influenced by heating temperature. The water in fresh muscle was mainly not-easy-flow water (T21), and T21 was transferred into free water T22 and T23. Heating 10min at 90℃ induced the content of T21 decreasing from 95.88% and 94.46% to 59.42% and 79.02% for crisp grass carp and grass carp muscle respectively. Crisp grass carp muscle exhibited higher free water content and lower water holding capacity compared with grass carp muscle. Correlation analysis showed that shear force and chewiness of crisp grass carp muscle were positive correlave with T21, and negative correlation with the T22+T23 and T2. In addition, both shear force and chewiness were significantly positive correlation with water holding capacity.(5) Effect of heating on the viscoelasticity of crisp grass carp surimiThe microstructure of surimi was significantly influenced by heating temperature. Muscle fibers and muscle fiber bundle were clearly visible for raw surimi, and small "particles" began to appear on the surface at 44 ℃. Fibers expanded and blurred, "particles" became bigger and more when temperature further increased to 66 ℃. "Particles" filled in the fibers surface and the fiber spacing at 90℃. "Particles" in grass carp surimi were much more than those in crisp grass carp surimi. The viscoelasticity of surimi was influenced by fish species, moisture content, salt content, heating and cooling rate. The values G’ of surimi slightly declined with increasing temperature, then fell sharply at 40℃-45℃, but increased rapidly when temperature was over 45 ℃, and reached the peak at 90 ℃, and continued to increase remarkably when temperature decreased. G’ of crisp grass carp surimi was significantly lower than that of grass carp surimi during heating-cooling.(6) Studies on skeletal muscle proteomics of crisp grass carp and grass carpThe differences in skeletal muscle proteomics of crisp grass carp and grass carp were studied using two-dimensional electrophoresis, and the different proteins were identified using mass spectrometry. There were 38 different points between crisp grass carp and grass carp muscles,14 high express points in crisp grass carp muscle, and 24 high express points in grass carp muscle. The different proteins mainly distributed in 16.7KDa to 79.2KDa, and isoelectric point in pH 4.6-6.4. It was found that the different proteins in proteomics were mostly associated with muscle fibril structure of protein.