| Hairtail (Trichiurus haumela) is one the most important marine commercial fishes in China which has great nutritive value and is very popular to consumers. However, because of its special habitation and unavailability to cultivation, it has to be frozen for a prolonged shelf-life before processing. Despite the microbial spoilage and enzyme activity being effectively inhibited, quality deterioration, particularly the critical factors for evaluating the shelf-life and consumer acceptability, such as the water-holding capacity, color and texture characteristics of fish muscle cannot be avoided during freezing and frozen storage.Lipid and protein oxidation are among the major reasons of quality deterioration in meat and meat products during processing and storage. The purpose of this study was to evaluate the oxidation of lipid and protein caused by an iron-catalyzed oxidation system, different thawing methods and freeze-thaw cycles and its impact on the muscle quality changes of hairtail. Oxidation of lipid and protein, water holding capacity, color and texture characteristics of hairtail muscle were measured combined with organoleptic evaluation in order to provide theoretical instructions for quality control and further utilization of hairtail. The main results were as follows:(1) Nutritional evaluation of hairtail muscle The general nutritional components and amino acid composition in fresh hairtail were investigated and the nutritional evaluation was conducted based on the results. The fresh hairtail muscle contained81.45±0.29%water,16.36±0.28%crude protein,0.43±0.04%lipid and0.94±0.03%ash. Crude protein accounted for88.19%of dry weight, and the essential amino acid index was1.64with high amino acid scores in fresh hairtail, indicating that hairtail was an ideal source of animal proteins.(2) Effects of a hydroxyl radical-generating system on the oxidation of lipid and protein from hairtail Oxidative damage of lipid and protein from hairtail induced by an iron-catalyzed oxidation system was investigated. Thiobarbituric acid-reactive substances (TBARS) formation was used to evaluate the oxidative damage of lipid, while carbonyl content and sulfhydryl content were measured as the index of protein oxidation. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) pattern, protein solubility and digestibility were also determined with different incubation time. With the prolonged oxidation time, the TBARS value and carbonyl content of myofibrillar proteins (MP) significantly increased (p<0.05), while the sulfhydryl content increased initially and dropped after1h of incubation. Afer5h of oxidation, the TBARS value and carbonyl content increased from1.13mg malonaldehyde (MDA)/kg to6.44mgMDA/kg and from4.13nmol/mg MP to5.16nmol/mg MP respectively, while the sulfhydryl content dropped from29.58nmol/mg MP to26.27nmol/mg MP. Aggregation of the myosin heavy chains during the process was shown by the SDS-PAGE pattern, which might illustrate the declined protein solubility of hairtail. With the increasing of oxidation time, the protein solubiity decreased significantly (p<0.05). The protein digestibility in vitro of hairtail reached the highest value of92.09%after1h of oxidation and then decreased gradually with prolonged incubation. The results indicated that the iron catalyzed hydroxyl radical-generating system promoted lipid and protein oxidation in hairtail, and caused significant decline in protein solubility, but proper protein oxidation appeared to enhance the protein digestibility in vitro.(3) Effects of thawing methods on the oxidation of lipid and protein as well as quality changes of hairtail The oxidation of lipid and protein and quality changes of hairtail thawed by low temperature refrigerator of4℃, microwave oven, flowing water or at ambient air temperature were studied by measuring TBARS value, protein carbonyl content, sulfhydryl content, SDS-PAGE patern, protein solubility, protein digestibility, water holding capacity, color and texture characteristics of hairtail. The results obtained just after thawing and after24h of storage at4℃were compared statistically. Hairtail thawed by low temperature refrigerator had the lowest lipid and protein oxidation degree as indicated by the lowest TBARS value and carbonyl content, and highest sulfhydryl content. After24h of refrigeration, the TBARS values increased significantly (p<0.05) and the microwave thawed samples had the highest TBARS value, increasing from0.46mg MDA/kg to4.40mg MDA/kg. A decrease in myosin band after24h of storage was shown in the SDS-PAGE pattern of the air thawed samples with reduced protein digestibility while the digestibility of those thawed by the other methods increased significantly (p<0.05). The protein solubility was higher in the microwave thawed samples than those thawed by the other methods just after thawing, but it decreased fast after24h of refrigeration (p<0.05). Samples thawed by low temperature and flowing water had lower muscle hydration and b*value, while hairtail thawed in water and air had lower protein salt-solubility, product yield and L*value (p<0.05). After24h of refrigeration, the L*value and W value decreased markedly while the a*value and b*value showed the opposite trend and the texture characteristics deteriorated simultaneously (p<0.05). The sensory scores were relatively higher in the refrigerator and microwave thawed samples (p<0.05). As a result, thawing in low temperature refrigerator was suitable for hairtail which exerted the least damage to hairtail muscle and storage after thawing should be avoided to reduce its adverse impact on the muscle quality.(4) Effects of freeze-thaw cycles on the oxidation of lipid and protein as well as quality changes of hairtail Effects of different freeze-thaw cycles (0,1,3,5,7and9times) on lipid and protein oxidation as well as muscle quality of hairtail were studied to provide a theoretical basis for the preservation and utilization of aquatic products. The TBARS value and protein carbonyl content of hairtail muscle significantly increased (p<0.05) with the increasing of freeze-thaw cycles. After9freeze-thaw cycles, the TBARS value increased from1.01mg MDA/kg to12.45mg MDA/kg and the protein carbonyl content increased from1.47nmol/mg MP to2.90nmol/mg MP. However, the total sulfhydryl content of myofibrillar proteins exhibited the opposite trend, decreasing from20.58nmol/mg MP to17.03nmol/mg MP. The protein solubility dropped to92.83mg/g after3freeze-thaw cycles and then went gently until the9freeze-thaw cycles being72.92%of that of the control which might be partially caused by the aggregation of myosin during the process according to the SDS-PAGE pattern. After3freeze-thaw cycles, the protein digestibility reached the highest value of89.48%, but the values decreased significantly with more freeze-thaw cycles (p<0.05). With the increasing of freeze-thaw cycles, the thawing loss increased, and the product yield, whiteness, hardness, springiness and sensory scores of the muscle dropped markedly (p<0.05). As a result, repeated freeze-thaw cycles promoted the oxidation of protein as well as lipid, resulting in a detrimental effect on the overall quality of hairtail muscle. |
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