| Layu is the traditional name for the dry-cured grass carp (Ctenopharyngodon idellus) produced in China. It is popular not only for its special chewiness and flavor but also longer shelf life. The flavour and edible safety is the main quality of fermented fish food. While the processing, preservation and cooking methods determined those properties. In this study provided the theory basis for processing, preservation and cooking of Layu products. The main results were as follows:1. Optimization of dry-salting technology for grass carp using response surface analysisUsing the content of sodium chloride and salt-soluble protein in Grass carp (Ctenpharyngodon idellus) muscle and the protein content in brine as indexes, the effect of different salting condition (time, temperature and concentration of brine) on the quality of grass carp muscle was studied by single factor test and response surface analysis method. A multiple regression model was set up between the results of sensory evaluation and determined indexes in different dry-salting condition. The results showed as follows:1) There were positive correlation between dry-salting factors such as time, temperature and concentration of brine and the content of sodium chloride in grass carp muscle as well as the protein content in brine. The dry-salting time and temperature affected the content of salt-soluble protein in grass carp meat.2) The model set up by response surface method showed regression equation fit well with experimental data, and the content of sodium chloride extremely significantly affects the sensory evaluation, followed by fermentation temperature and time.3) When the dry-salting time 6.02 days, temperatue 9.29℃and the concentration of sodium chloride added to 11.70%, Grass carp dried was best with the sensory evaluation. The results were basically correspond to model prediction.2. The influence of different drying processes on quality of LayuFive different processing conditions, Sun-drying (5-15℃, RS), low-temperature drying (5℃, DW5), low-temperature drying (15℃, DW15), combination drying (15℃, YZ15) and hot-air drying (45℃, RF45) of curing Layu were studied, and the nutrient components, physicochemical properties and sensory quality of Layu were compared. The order of the different processing time listed here:RF45< DW15< DW5< YZ15< RS. Results showed that drying process had a great effect on the nutrient components and they increased the contents of protein, oil and ash, but there is none significant differences between different drying processes. The drying process has significant influence on the dydrolysis of proteins in fish meat showed by SDS-PAGE result, and significantly increased the content of free amino acids in the Layu products, especially RS and YZ15 drying methods. The content of biogenic amines, Nitrite, Nitrates and the TVB-N value increased in the drying process. Overall, sensory evaluation showed that the combionation dried Layu was more acceptable than the other drying methods products. Based on the safety and sensory quality comprehensive evaluation, the combination drying process is the best choice for drying the curing Layu.3. Effect of spice on the lipid oxidation and fatty acid composition of Layu during processingThe Chinese traditional dry-cured grass carp fish (Layu) was processed without (A) and with (B,2%, W/W) spice, Zamthoxylum Bungeanum Maxim, and the lipid oxidation and fatty acid composition of Layu was investigated. All treatments reduced the moisture contents and increased the relative ratio of protein, lipids and ash, and there were non differences between A and B. Zanthoxylum Bungeanum Maxim has the ability of delaying oxidation of oils. The oil of Layu A had a higher peroxide value, thiobarbituric acid (TBA) index value, the content of polar lipids and free fatty acids than Layu B during the manufacturing process. Layu B had a higher content of DHA and EPA than Layu B. Zanthoxylum Bungeanum Maxim can improve the quality of Layu.4. Spice-sucrose treatments to improving the quality of LayuThe effect of spice and sucrose treatments on the quality of Layu was evaluated. Under the same processing, the Grass carp meat was marinate with A (salt), B (salt and Zanthoxylum Bungeanum Maxim), and C (salt, Zanthoxylum Bungeanum Maxim and sucrose). The pH value of C significantly lower than those of A and B. B and C affected the population of total viable counts, but C had a higher population of lactic acid count than those of A and B. Zanthoxylum Bungeanum Maxim has the ability of decreased the formation of total volatile basic nitrogen (TVB-N) in Layu. Sucrose increased the content of water soluble nitrogen (WSN), none protein nitrogen (NPN) and free amino acids (FAAs) of Layu. Zanthoxylum Bungeanum Maxim combined with sucrose improve the color, flavor and umami taste of Layu. Product C has the best quality.5. Effect of sucrose on the non-volatile taste compounds of LayuThe non-volatile taste active compounds, including soluble sugars, succinic acid, free amino acids and flavour 5'-nucleotides in the meat of Layu (A:fermented with 1% sucrose; B:fermented without sucrose), and the effect of sucrose on the non-volatile taste active compounds in Layu during processing were analyzed. Higher levels of sugars, NaCl and free amino acids, and lower flavour 5'-nucleotide were detected in the final products when compared to the fresh fish. In the presence of sucrose, Layu A had higher levels of sugars (461.88±57.83 mg/100g), free amino acids (2566.86±87.21 mg/100g) and 5'-nucleotide (31.97±3.32 mg/100 g) than those of Layu B (57.20±9.85 mg/100g,2401.19±85.27 mg/100g, and 19.42±1.87 mg/100 g, respectively) after ageing. In Layu A,6 sugars (ribose, glucose, fructose, phosphorylated ribose, phosphorylated glucose and sucrose) were detected while only 4 sugars (glucose, fructose, phosphorylated ribose and phosphorylated glucose) were detected in Layu B. There were 11 and 9 free amino acids were of higher taste active values (TAV) greater than on in Layu A and Layu B, respectively. The total content of sugars in aged Layu A is eight times higher than that of Layu B after ageing. The total content of flavour 5'-nucleotide in Layu A (27.94±3.05 mg/100 g) is higher than that of Layu B (15.40±1.87 mg/100 g). As the TAVs of soluble sugars, succinic acid and flavour 5'-nucleotide were lower than one, those compounds are likely to have insignificant impact on the taste of Layu. For the effect of sucrose on the content of free amino acids and flavour 5'-nucleotide in the production of Layu, the equivalent umami concentration (EUC) of Layu A (5.085 g MSG/100g meat) is higher than that of Layu B (2.645 g MSG/100g meat), which meant that the umami taste of the Layu meat was very intense, and the addition of sucrose in the processing, can increase the taste of Layu, especially the umami.6. Biogenic amines in Layu preserved by four different packing methodsA study was carried out to determine the effect of vacuum packed (VP) and modified atmosphere (MAP) (100% CO2) packing, chitosan film (CF) and cling film (C) wrapping on the microorganisms, and the formation of biogenic amines and trimethylamine (TMA) during storage of Layu at 4℃. The pH value generally decreased, while the Total viable counts, Lactic acid bacteria, biogenic amine and TMA contents generally increased in all treatments with increasing storage period. Lactic acid bacteria were the predominant flora in Layu, and some enterobacteriaceae were detected in samples C and VP. Levels of putrescine, cadaverine, histamine, tyramine, spermidine and spermine in Layu samples were found to be in the range of 0-52.53 mg/kg whose concentration changed little for a period of 20 days at 4℃. While 9 biogenic amines were detected at the end of storage, and putrescine, cadaverine, histamine and tyramine were the main biogenic amines. The treatments of VP. MAP and CF possess different inhibitory effect on the increasing of biogenic amines. The content of spermiidine and spermine isn't influenced by the treatments, while MAP and CF can significantly suppressed the increasing of phenylethylamine and tryptamine. The treatment of C, VP, MAP and CF extended the shelf life of Layu to 60 d,120 d,150 d and 180 d, respectively.7. Effect of two cooking methods on sensory quality and biogemic amines of LayuThe effect of cooking methods on the content of sugars, free amino acids and biogemic amines were studied in two kinds of Layu (Layu A:fermented with 1% sucrose; Layu B: fermented without sucrose). The optimum cooking time for Steaming and microwaving is 9 min and 4 min, respectively, getting the best sensory quality. Thermal processes indurced the different changes in sugars (reducing sugars, phosphorylated reducing sugars and sucrose), free amino acids and biogenic amines through Maillard reaction. After the application of steaming 9 min, the content of total sugars in Layu A and Layu B were decreased respectively by 113.58 and 7.52 mg/100g DW. After the application of steaming 9 min, the content of total sugars in Layu A and Layu B were decreased respectively by 186.12 and 75.26 mg/100g DW. The treatment of steaming increased the content of free amino acids, while microwaving decreased that in Layu A and Layu B. Steaming treatment decreased the content of total biogenic amines in Layu A (-2.24%), while increased a little amount of that in Layu B (2.06%). Microwaving treatment decreased the content of putrescine, cadaverine and histamine 28.26%,43.81% and 59.16%, respectively in Layu A; by 15.51%,20.63% and 21.15% in Layu B. Microwaving treatment decreased the total biogenic amines by 46.49% and 18.85% in Layu A and Layu B, respectively. It could be concluded that Layu fermented with sucrose, and with the microwaving method can decrease the content of biogenic amine to the minimum.
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