Study On Flavor And Quality Variation During Process Of Dried Grass Carp With Low-salt And High-water Content

The grass carp(Ctenopharyngodon idella) is a freshwater fish species of the order Cypriniformes, family Cyprinidae, and the only species of the genus Ctenopharyngodon. To date, mainly of the grass carps are sold as fresh fish and less are consumed as processed product such as drunk sliced fish, dried fish, smoked fish. In recent times, some low salt content (less than 6%) and high water content (around 60%) dried fish product have been appeared in the market in Zhejiang Province. These kinds of products are welcomed by the consumers as its low salt content, which is considered much healthier compared with the traditional high salt content dried fish product. However, few research has been focused on the procession technique, physical and chemical properties, nutritional quality and flavor composition of these new products. In this study, the variations of the physical and chemical properties, nutritional quality during the pickled and heat pump drying processes were investigated initially, in order to give related data for process improvement and obtaining products with unique flavor and stable quality.In this paper, the optimum condition for volatile components extraction from the grass carp and its products were determined by the single factor experiment as follow:DVB/CAR/PDMS solid-phase microextraction conditions (SPME),6g sample, extraction temperature 70℃, extraction time 40 min, desorption time 5 min, GC programmed temperature rate 6℃/min. Comparison of the volatile components in the fresh grass carp and dried grass carp based on the GC-MS analysis indicated that the total volatile components and its category of above two kind samples were 923.57μg/100g with 65 species and 1086.73μg/100g with 69 species, respectively. The main volatile components of fresh grass carp and dried products were aldehydes, ketones, alcohols and hydrocarbon compounds. The aldehydes and ketones content of the dried grass carp increased 246% and 89%, while the alcohol compositions decreased 68.6% comparing with those in the fresh grass carp.Based on the matching mass spectras and retention indices analysis, the species of the volatiles were identified. According to the odor active value (OAV),16,17 and 22 odor active compounds of the fresh fish, pickled fish and heat pump dried fish with 15℃ were determined. The same key odor compounds (OAV≥1) of them were composed of Hexanal, Heptanal, Nonanal, Octanal, (E)-2-nonenal, (Z)-4-Heptenal, 1-Octen-3-ol, hexanol, and so on.Moreover, the nutritional composition, non-volatile flavor substances, including amino acid composition, fatty acid composition, ATP and its related compound in the fresh fish and sample collected in pickled and dried process were investigated. The proportion of the amino acid content of the dry weight in pickled and dried grass carp decreased in a slight degree compared with those in fresh fish, but there’s no significant variation in the proportion of essential amino acids. The results of amino acid nutrition evaluation revealed that there’s a bit decrease in the amino acid nutrition of the grass carp during the processing. Through analysis of flavor substances content and its flavor intensity, free amino acids which have great contributions to the product taste were identified as arginine, lysine, glutamate and histidine. Except the above amino acid, IMP was also a main contributor to the final product taste. Succinate was the main organic acid which plays a important role in the product taste formation.Finally, as to analyze the influence of different drying methods on lipid oxidation and volatile flavor quality of the products, samples were collected at the key point in the process, meanwhile, lipid oxidation index including the acid value, peroxide value and thiobarbituric acid value, as well as variation of the fatty acid composition and content were studied. The results revealed that the fat dry content decreased significantly, but there’s no obvious difference between samples dried with different temperature. Acid value, peroxide value and TBARs were all increased gradually in the processing, which indicated the fat oxidation has been happened but it still could meet the requirements of quality standards. During the processing, contents of saturated fatty acids and monounsaturated fatty acid in the grass carp were all increased, however, the polyunsaturated fatty acid content was reduced. Lower heat pump drying temperature and longer drying time leaded to maximum reduction in polyunsaturated fatty acids. The ration of ω-6 to ω-3 was stable and close to the ideal healthy index. There’s obvious negative correlation between fat content and salt content of the fish samples. A significant positive correlation between the TBARs and salt content in the fish samples was presented, meanwhile, monounsaturated fatty acids content and the polyunsaturated fatty acids content negative correlated significantly.